Crosscutting of the pollutants and building ventilation systems: a literature review

Abstract

Considering the time spent in enclosed environments, it is essential to study the relationship between pollutants and building ventilation systems to find whether the types and levels of pollutants and greenhouse gasses, which are expected to be exhaled through ventilation systems into the atmosphere, have been adequately evaluated. We propose the hypothesis that the exhaled air from residential buildings contains pollutants that may become another source of contamination affecting urban air quality and potentially contributing to climate drivers. Thus, the main goal of this article is to present a cross-review of the identification of pollutants expected to be exhaled through ventilation systems in residential buildings. This approach has created the concept of “exhalation of buildings” a new concept enclosed within the research project in which this article is included. We analyze the studies related to the most significant pollutants found in buildings and the studies about the relation of buildings' ventilation systems with such pollutants. Our results show that, on the one hand, the increase in the use of mechanical ventilation systems in residential buildings has been demonstrated to enhance the ventilation rate and generally improve the indoor air quality conditions. But no knowledge could be extracted about the corresponding environmental cost of this improvement, as no systematic data were found about the total mass of contaminants exhaled by those ventilation systems. At the same time, no projects were found that showed a quantitative study on exhalation from buildings, contrary to the existence of studies on pollutants in indoor air.

Graphical Abstract

Background and motivation

In the 2015 Paris Agreement, the General Assembly of the United Nations approved the 2030 Agenda for Sustainable Development with 17 global scope and universal application goals, including the specific objective of taking immediate actions to combat climate change and its impacts (objective 13) (United Nations 2015). This objective could be placed the current trend in designing and constructing buildings that aim to be off-grid, carbon–neutral, energy-efficient, and generally environmentally friendly, which leads to conceiving buildings as relatively autonomous units (Le et al. 2019).

In this context, the pollutants from numerous sources that affect the quality of the air surrounding us can be positioned. Studies dealing with indoor atmospheres are generally limited to the quality of incoming air and the definition and fulfillment of indoor air quality (IAQ) requirements. Air pollutants generated by the regular functioning have been much less studied. However, it is necessary to consider that most human activities are performed in an enclosed space, characterized by a diverse and chemically complex air quality (Gil Hormazábal and Adonis 1998).

The World Health Organization (WHO) published a report in 2016 entitled “Monitoring Health for the Sustainable Development Goals,” which defines a 2030 target of reducing mortality rate attributed to household and ambient pollution. It remarks that the major obstacles to reducing the levels of mortality caused by air pollution include “Lack of integration of health considerations into decision-making in sectors, and lack of monitoring of air pollution levels, sources and consequences on public health to help direct action by the health sector and other sectors to improve health and health equity” (WHO 2016).

The Spanish National Climate Change Adaptation Plan 2021–2030 states that climate change encompasses urban metabolism, understood as the energy flows and matter cycles that circulate and nourish cities and the territories where they are located. Consequently, climate change is a factor that aggravates air pollution in cities and areas where air quality is affected (Gobierno de España 2020).

Regarding the importance of pollution source identification, the European Joint Research Center (JRC) and the WHO have driven a global study specifically about the airborne particulate matter (Karagulian et al. 2015). They conclude that even though traffic is an essential contributor to ambient pollution in cities, there is a considerable ratio of “unspecified sources of human origin” mainly attributed to secondary particle formation. All those unknown sources lead us to question whether the buildings might be considered an additional source of aerial pollutants in urban environments. In that case, the building’s ventilation system would be the focal point of this scenario.

In this context, the proposed research has the goal of quantifying the pollutants exhaled by residential buildings through the ventilation systems, and it should be encapsulated in the global concern about the unknown pollutants source and the need of defining the complete impact of buildings as inhabited units in cities.

The European Joint Research Center (JRC) and the WHO have driven a global study regarding the ambient source contributions of airborne particulate matter (Karagulian et al. 2015). They conclude that even though traffic is targeted as an important contributor to ambient pollution in cities, there is a considerable ratio of “unspecified sources of human origin” (up to 62% in Canada) that is mostly attributed to secondary particle formation. However, should buildings be considered as well? Are buildings an additional source of pollutants in cities?

While there are hundreds of studies, of different scales and methodologies, referring to the outside air that go in buildings (outdoor air, ODA), to the air inside buildings (indoor air, IDA) and how good is that indoor air (indoor air quality, IAQ) that provide a convenient framework for this research, there is little, if any, information on what happens to the inside air when vented out. Many studies try to determine the most adequate ventilation system to eliminate such pollutants as found in IAQ, but all those therefore assume that ventilation systems become the technological solution to improve IAQ. This research may set the foundations of a new technological paradigm, by looking at the fate of IDA (some of which becomes ODA only to return as IDA, conserving its pollutants). Departing from the simple “throw away” solution, by characterizing outgoing IDA, this research seeks to foster much needed technological developments more akin to recycling. Are we not in the urban preamble of what is currently happening to the sea, where the plastics we throw end up coming back to us through the food chain?

This research aims at breaking this vicious loop. We assume that by returning spent IDA back to the atmosphere we have eliminated the problem, but it is not so. Accepted solutions that affect millions of people carry a tremendous inertia that is difficult to overcome. For instance, the way we still design buildings and cities for our society assumes that ventilation is the standard procedure to refresh IDA. Moving to modern buildings of almost zero energy consumption involve a change of direction that requires technological developments also to deal with the IDA lifecycle and effective removal of contaminants, but not by dumping them to ODA only for them to cycle back.

Do building ventilation systems, whether dedicated to controlled mechanical ventilation systems, kitchen extraction, garage ventilation or drain waste ventilation, expel those contaminants outside, to the city, only to be then reintroduced into the buildings? Are we facing a similar evil loop as with the people’s generated plastic waste spilling into the ocean that has run full circle and is now re-entering people through the food chain?

Thus, the repercussion of the household-generated pollution and air quality on the local scale environment is the target of the project. We intend to address a detected research gap since no studies have been found that analyze what happens to the air that is exhaled from the building, which pollutants does this air contain, and how much of them are thus spilled out.

This background has led us to the following approach: in buildings, for instance, it is well known that the entry of water to buildings that exits from them as black or stormwater and the accumulation of the latter, or the greywater management are studied and in a constant process of improvement. Similarly, studies related to the outdoor air of cities and the parameterization of IAQ are increasing but still under development. Related to this last point, studies carried out in different countries and different types of buildings show that the population is now exposed to higher levels of PM or NO_x.

Thus, unlike the abovementioned water management, which already might be considered a cycle, in the case of air pollution, multiple sources and destinations of air pollutants are not yet defined. Hence, several questions arise, such as what the outdoor air pollution generated by a building is or, in other words, what the pollutants exhaled by buildings are.

The core of the project is the quantification of air pollution exhaled from the building as a whole to the atmosphere. While similar studies deal with the analysis and measurement of gases existing, or being generated, in indoor spaces, as of the time of writing no study or figures have been found in the literature about the rate at which these gases make out of the building and are dispersed to the external atmosphere, or reinvested to the building, during its normal (“metabolic”) function, other than the specific emissions from certain point sources such as boilers. In any case, these studies will be taken as the initial basis from which to set out the measurement and analysis strategy.

Methodology

In scientific review articles, it is common for authors to set some keywords to parameterize their bibliographic searches in specific databases, such as Web of Science^©, Scopus^©, or Science Direct^©. However, in our paper, this bibliographic search methodology was not possible because the proposed subject has a particular innovative character, and the issue arouses a gap in scientific discussion. Thus, the process of collecting scientific articles referring to the proposed topic followed a slightly different approach.

The keywords were related to the chapters of the article, i.e., “building service,” “ventilation,” “ventilation system”; “indoor air pollution”; “pollutant in residential buildings”; “vent stack”; “dwelling ventilation”; “garage ventilation”; “kitchen ventilation.” Although practically none of the articles found had a direct relationship with the subject proposed in the paper, the scientific works referenced here, in a way, provided important content to base the discussion on the topic of air pollutants “exhaled” from residential buildings.

The article pretends to be the seed and lay the foundation of the “building exhalation” concept, which, as the results presented here, should be studied in the future since, among other reasons, it represents a topic encapsulated in the global need of defining the complete impact of buildings as inhabited units in cities. It must be clarified that the concept is accompanied by necessary intellectual speculation, given the lack of literature found directly related to building exhalation. The explanation of this process can be represented as a crossflow configuration where the references have been organized regarding the primary pollutants exhaled from buildings (“Pollutants in residential buildings” section) and by considering the building ventilation systems (“Ventilation systems involved in the air exhalation of residential buildings” section).

In this context, under the hypothesis of whether the “exhaled” air from residential buildings contains pollutants that become another source of contamination that affect urban air quality, the main goal of this article is to present a cross-review of the identification of pollutants expected to be exhaled through ventilation systems in residential buildings. To do that, 170 references have been analyzed. The sources of the mentioned documents are international journals, international edited books, international standards, and reports from international governmental organizations.

It is a study between architecture, engineering, and biology in its approach to biomimicry. This makes the same reality, exposed from different points of view, complicated to find homogeneously in different bibliographic sources.

Scientific context

Green buildings and ventilation systems

As Steinemann et al. (2017) describe, the health and well-being of residential occupants are not fully attributable to green buildings and improved IAQ, even if they promote energy efficiency and other sustainable aspects.

In fact, the “green” term does not necessarily guarantee a good IAQ. The following points address the research gaps or priorities identified in the study: first, the study seeks to understand the properties of green buildings that can improve air quality and, conversely, the measures that can make a building healthier in relation to air quality. Secondly, the evaluation of the relationship between the labels or credits awarded and the measurable impacts on air quality. Finally, whether air quality improvements in green buildings are due to green products and materials, in conjunction with research on their air emissions (Steinemann et al. 2017).

Moreover, recent trends in the technology of HVAC are focused on the re-circulation of the air instead of refreshing it. This might reduce air quality, understanding how the contaminants, the increased population density in cities, the use of new synthetic materials, and the traffic pollution contribute and interact to increase adverse effects on humans and deteriorate IAQ (Spiru and Simona 2017). Although ventilation is thought to be the main measure to mitigate contaminants, sometimes it may not be enough, such as in areas with high outdoor pollution. This measure cannot prevent the introduction of VOCs into buildings (Spiru and Simona 2017).

Additionally, the same study concludes that these mechanically ventilated buildings reduce indoor PM_2.5 concentrations while increasing the concentrations of indoor NO₂ compared to the naturally ventilated ones. Thus, in this context, since IAQ varies seasonally and spatially, it is influenced by season, outdoor conditions, installed HVAC system type, and building characteristics (Spiru and Simona 2017).

It can also be added that there is a relationship between green building, energy performance, and air pollution through different certification systems such as LEED or WELL certifications. However, these procedures aim to eliminate pollutants from inside the building. In contrast, this review seeks to spotlight those air pollutants that the different ventilation systems emit to the outside.

Indoor air pollution

According to WHO, air pollution may be the most significant environmental risk to health in the world (WHO 2016). A similar conclusion is achieved in Cohen et al. (2017), where the interpretation of the results states that “ambient air pollution contributed substantially to the global burden of disease in 2015, which increased over the past 25 years, due to population aging, changes in non-communicable disease rates, and increasing air pollution in low-income and middle-income countries.” And a study developed by the Forum of International Respiratory Societies’ Environmental Committee suggests that air pollution affects many organs beyond the lungs (Schraufnagel et al. 2019) and “…although air pollution affects people of all regions, ages, and social groups, it is likely to cause greater illness in those with heavy exposure and greater susceptibility”. Thus, it becomes a transversal matter affecting society, emphasizing inequalities.

Although studies point to increased outdoor air pollution in urban areas, creating the belief that cleaner air is being breathed inside buildings, the Environmental Protection Agency (EPA) maintains that indoor air is more polluted than outdoor air and may cause considerable health problems (EPA 2021). Indicators of indoor air pollution include strange odors, stale or stuffy air, lack of air movement, dirty or faulty central heating or air conditioning, excessive humidity, mold, after-effects of remodeling, or the feeling of being healthier outside (Spiru and Simona 2017).

With these starting data, however, the concern of this review is not the origin but to try to identify how much of that polluted air leaves the building to the city. One of the fundamental factors that provoked this air quality decline in air quality is the construction of buildings designed to be more airtight, with a reduced regeneration of the indoor atmosphere by a smaller proportion of fresh air coming from the outside to improve energy efficiency (Gil Hormazábal and Adonis 1998). This type of construction might also contribute to the unpredicted disparity in developed countries. It has been recently found that families with higher socioeconomic status are at greater risk of exposure to chemical contaminants (Montazeri et al. 2019).

In indoor spaces, inadequate ventilation favors increased pollutant levels, which depend on the emitting source and the dilution caused by the ventilation (Gallego et al. 2013). These indoor air contaminants have prompted numerous epidemiological research related to building uses and typologies, such as the relation between children’s health and exposure to pollutants in schools. The health impact is due to air quality (indoor/outdoor), especially respiratory diseases (Annesi-Maesano et al. 2013).

The most studied indoor air pollutants due to their high impact on health care formaldehyde (CH₂O), tobacco smoke, VOC, NO₂, CO, PM, radon, and biological agents, but also parameters such as PM, VOC, bacteria, mold, CO₂ and its relation to the temperature and the relative humidity, which exhibits seasonal and spatial variations with higher levels of VOC, CO₂, and PMs in winter, and an increase in bacteria and mold concentration in the summer (Mentese et al. 2009).

The review about the energy performance of buildings, outdoor and indoor air quality carried out by Spiru and Simona considered the relationships between building properties (type of dwelling, period of construction, location of the dwelling, type of ventilation system, construction material) and outdoor air quality (Spiru and Simona 2017).

Spiru and Simona (2017) determined that formaldehyde concentrations were 30% higher in newly constructed buildings than in older ones. They also found that mechanically ventilated dwellings and concrete buildings had higher concentrations. On the other, those buildings located in highly populated areas have a strong connection between the NO₂ concentration in the outdoor and indoor air, and mechanical ventilation emphasizes this (Spiru and Simona 2017).

In summary, mechanical ventilation systems in residential buildings enabled better monitoring and control of indoor air quality. In this sense, the increase in the efficiency of mechanical ventilation systems for cleaning indoor air added to the reduced air infiltration factor (due to better insulation and airtightness) increased the rate of used air carried out to the atmosphere through building services (Niculita-Hirzel 2022).

Pollutants in residential buildings

The pollutants inside an indoor space can arise from various sources: materials used for the construction, improper or excessive use of cleaning and hygiene products, combustion gases, standing water, and the environment itself, where outside pollutants come in through the air renewal system or by infiltration. Considering that most people spend 85–90% of their time indoors, it is necessary to understand that there is a relationship between indoor and outdoor particulate matter. This is coupled with epidemiological evidence indicating a relationship between exposure to outdoor particulate matter and adverse health effects. Chen and Zhao (2011) base this on the relationship of three different parameters (indoor/outdoor (I/O) ratio, infiltration factor, and penetration factor) to analyze the relationship between indoor and outdoor particles.

These authors consider the penetration factor to be the most relevant parameter for the mechanism of particle penetration through cracks and leaks in the building envelope, in contrast to other authors who point out the importance of real-time monitoring and that data feedback is essential for the monitoring of air pollutants (Xu et al. 2022).

In the following sections, the literature reported contribution to indoor/outdoor building contamination of 6 pollutants, carbon monoxide (CO), carbon dioxide (CO₂) methane (CH₄), nitrogen oxides (NO_x), particulate matter (PM), and volatile organic compounds (VOC), is reviewed. These six pollutants have been selected as the most significant in building exhalation, attending the US Environment Protection Agency (USEPA) and the studies presented in the reviewed literature. In Table 1, the recommended concentrations and regulated limit values in Europe of the selected pollutants are gathered.

Table 1.

Recommended and/or regulated concentration values of the pollutants considered in this review

	WHO global air quality guidelines (WHO 2021)	EU Ambient Air Quality Directive (European Parliament 2008)
CO	4 mg/m3 24-h mean	10 mg/m3 8-h mean
NO2	10 µg/m3 annual mean 25 µg/m3 24-h mean 200 µg/m3 hourly mean	40 µg/m3 annual mean - 200 µg/m3 hourly mean
PM2.5	5 μg/m3 annual mean 15 µg/m3 24-h mean	25 μg/m3 annual mean
PM10	15 µg/m3 annual mean 45 µg/m3 24-h mean	40 μg/m3 annual mean 50 μg/m3 24-h mean
	ISO 17772 (ISO 17772:1,2017), EN 16,798 (EN 16, 798–1,2019)
VOC	TVOC < 1000 μg/m3 for low polluting buildings
CO2	550 ppm above outdoor level
CH4	No guidelines have been found related to CH4 concentrations in buildings

Carbon monoxide (CO)

Carbon monoxide is colorless, odorless, and at elevated concentration levels, a highly toxic gas emitted as a product of incomplete combustion of hydrocarbon-based fuels. Its most frequent origin is oxygen starvation in engines, boilers, fireplaces, cigarettes, area heaters, or other appliances, arising from design limitations, improper adjustments, or misuse(EPA 2020).

When inhaled, CO binds reversibly with blood hemoglobin to form carboxy-hemoglobin, impairing the oxygen-transport of the blood, as well as the oxygen’s release to body tissues, causing therefore severe and even fatal asphyxiation. Therefore, regulation of CO concentrations in urban atmospheres is necessary due to its toxicity and its implications for human health (Buchelli Ramirez et al. 2014).

The burning of coal or wood in stoves has been a historically insidious problem in many homes. Its incidence has been decreasing in high-income countries (Roca-Barceló et al. 2020). However, it is still significant (Rose et al. 2017) in countries or population groups where several factors combine, e.g., high rurality and economic deprivation (Ralston and Hampson 2000; Roca-Barceló et al. 2020).

As for other gaseous oxides where the primary sources are traffic, industry, or urban air pollution, in the absence of different sources, indoor CO level changes should closely mimic outdoor dynamics. In most Europe and North America, outdoor CO levels are generally well below safety standards (Penney et al. 2010), and indoor levels should remain at or below outdoor concentration. However, point sources may exist, e.g., malfunctioning or poorly regulated gas heaters and stoves, tobacco smoking, garages, or even incense burning (Jetter et al. 2002).

However, this is not the case in many countries and environments. CO tends to vary less than outdoor changes (Lawrence et al. 2005), indicating indoor-prevalent sources such as open-fire stoves or tobacco (Naeher et al. 2000). As CO is relatively unreactive and is not absorbed by building materials or ventilation system filters (Penney et al. 2010), ventilation airflow is about the only way to remove CO from indoor environments once generated, provided that outdoor concentration is less, at the cost of energy waste. High levels of CO may require forced ventilation, as natural convection or window opening may not dilute the gas readily enough (NIOSH 1996). Alternate technologies imported from other areas are being explored, but remain somewhat experimental, e.g., catalytic oxidation (Abbasi et al. 2019).

The relatively few choices for excess CO removal, especially in a context of compromised ventilation imposed by energy-saving regulations (González-Martín et al. 2020), call for proper monitoring of indoor areas with predictable risk of high CO levels such as garages, facility rooms, or kitchens with fuel-burning appliances. CO detectors and alarms are now commonplace and, indeed, mandatory under specific regulations. For example, CO detectors must be installed in garages exceeding 100 m² in Spain, governing forced ventilation if the concentration reaches 50 ppm (Gobierno de España 2019).

Aside from its medical aspects, from the architectural point of view, indoor CO research seeks to characterize and classify its sources, levels, and dynamics, including prevention, mitigation, and removal. Conditionals are building types, activities, and social, environmental, and economic envelope often summarized as a country. However, research is not homogeneous across all aspects. A review of 20 papers published in the last 2 years about indoor CO found that 60% of them establish CO levels arising from cooking in households across a range of developing countries, while only 20% dealt with any form of CO removal or mitigation, and only one on nonresidential buildings (Limchantra et al. 2019; Liu et al. 2019; Luo et al. 2019; Naghizadeh et al. 2019; Piedrahita et al. 2019; Seraj 2019; Weaver et al. 2019; Abbasi et al. 2019; Adefeso et al. 2020; Alves et al. 2020b; Fatmi et al. 2020; Rastogi and Lohani 2020; Shen et al. 2020; Tabinda et al. 2020; Tadevosyan et al. 2020; KC et al. 2020). Additional research on CO dynamics across a range of building typologies and a spectrum of activities to balance CO removal and energy conservation could undoubtedly be helpful.

Carbon dioxide (CO₂)

People spend more than 90% of their time on indoor activities—home, work, school, or commuting—and because humans produce and exhale carbon dioxide, this lifestyle leads to a higher concentration of CO₂ indoors than in outdoor spaces (Bonnefoy et al. 2004; Satish et al. 2012; Ortiz et al. 2020). In residential buildings, CO₂ generation is associated with human respiration, combustion fuel, cooking, and animal and plant metabolism. However, outdoor sources may increase the CO₂ indoor level through natural airflow rate (Gall et al. 2016).

In this regard, it is emphasized that CO₂ generation rates can be derived from well-established concepts within human metabolism and exercise physiology, which relate these rates to body size and composition, diet, and level of physical activity (Persily and de Jonge 2017).

Even though in normal indoor environmental conditions, where CO₂ concentration does not have an immediate harmful effect in low amount, it is considered an indicator of the indoor air quality level (Jokl 1998; Seppänen et al. 1999a; Sundell et al. 2011; Richardson et al. 2014; Dai et al. 2018; Leivo et al. 2018). Therefore, some standards usually specify levels of CO₂ concentration relating them with the minimum ventilation rate, which limits the concentration of pollutants in the air (Emmerich and Persily 2001; Schell and Int-Hout 2001; Bekö et al. 2010; Fan et al. 2014; Ramalho et al. 2015; Gall et al. 2016; Cheng and Li 2018; Altomonte et al. 2020; Khovalyg et al. 2020). Although, the airflow rate depends on the pollution content and exposure level, which directly affects the human (Carrer et al. 2015, 2018).

Ventilation rate based on perceived air quality is the most common building standard method to control air quality. Airflow rates for diluting emissions from occupants are specified for adapted and non-adapted occupants (Khovalyg et al. 2020). According to Persily (1997) and Carrer et al. (2018), a CO₂ concentration of about 1000 ppm could be taken as the maximum level indoor, and higher concentrations can lead to occupant sensibility and discomfort (Satish et al. 2012; Allen et al. 2016). An experimental study demonstrated that a CO₂ concentration of about 1000 ppm and other controlled pollutants influenced the healthy individuals during computer-based tests, decreasing their decision-making action (Satish et al. 2012).

Recently, and due to the pandemic COVID-19, green building certification schemes may prescribe additional technologies for recognizing the required IAQ and new patterns of ventilation programs to minimize the risk of exposure to infected aerosols (Afroz et al. 2020). Moreover, strict control of IAQ with higher rates of ventilation and air renewal is essential since such measures can prevent the transmission of the virus through the air, especially in crowded spaces, even though they contradict the objective of energy efficiency (Franco and Leccese 2020). Anyway, the indirect estimation of the occupation of the internal opening using the CO₂ concentration monitoring seems to be relevant, mainly in times of pandemic where it is evident the vertiginous increase of the homework pattern and the more significant amount of time in which people remain inside home (Afroz et al. 2020; Franco and Leccese 2020).

Perceptions of poor air quality in standard indoor settings provoked by high levels of CO₂ increased the prevalence of acute health symptoms (e.g., headache, mucosal irritation), slower work performance, and expanded absence (Seppänen et al. 1999b; Milton et al. 2000; Wargocki et al. 2000; Shendell et al. 2004; Erdmann and Apte 2004; Federspiel et al. 2004).

Some authors have assumed that CO₂ concentrations up to 5000 ppm, which is in the range of concentrations found in buildings, have no direct impacts on individuals’ perception, health, or work performance (Mudarri 1997; Persily 1997). However, other authors have questioned this with subtle adverse effects on controlled human exposures to CO₂ between 2000 and 5000 ppm (Kajtar et al. 2006). These values are relevant if we consider that the summary of CO₂ levels and ventilation conditions in studies examining the effect of bedroom ventilation and IAQ on sleep quality provides discounts over 2000 ppm (Sekhar et al. 2020).

Methane (CH₄)

Methane has a much greater potential for global warming than CO₂ and has been linked to global climate change in the past (Ruddiman 2007). Even though it has a short residence time of 9 years in the atmosphere, it is an important atmospheric pollutant: the second largest contributor to global.

A total of 50–65% of global CH₄ emissions are of anthropogenic origin, with a flux of approximately 330Tg CH₄ per year (Kirschke et al. 2013). A reduction in anthropogenic CH₄ emissions requires data on the location and sectoral contributions of individual emitters, especially at the scales at which reduction policies can be applied. Observations of elevated CH₄ levels in cities show that a significant part of anthropogenic CH₄ emissions come from urban areas (Blake et al. 1984; Wunch et al. 2009).

Although urban areas are increasingly recognized as a global potential source of methane to the atmosphere, the location of methane sources and relative contributions of source sectors are not well known (Hopkins et al. 2016a). There is a lack of basic knowledge of CH₄ emissions (Hopkins et al. 2016b). So, there is a vast quantity of papers in the literature analyzing CH₄ emissions in animal buildings (Van der Heyden et al. 2015), but not in residential buildings. However, the concentrations of CO₂, CO, and total volatile organic compounds (TVOCs) are associated with CH₄ (Veld 2000)_.

It has been estimated that 35% of the anthropogenic CH₄ in North America is emitted from urban regions (Marcotullio et al. 2013). However, recent atmospheric studies at California’s state and city levels suggest a 30–80% underestimation of CH₄ emissions in the state’s greenhouse gas inventory (Wunch et al. 2009; Hsu et al. 2010; Wennberg et al. 2012; Peischl et al. 2013; Jeong et al. 2013; Wong et al. 2015).

In the context of the building’s exhaled air, no special attention has been paid to the measurement of CH₄ emissions, as shown in the following sections, where it is stated that hardly any records of CH₄ measurements have been found.

Nitrogen oxides (NO_x)

The WHO (2010) enumerates nitrogen oxides among compounds used to evaluate IAQ. In Europe, excesses of this group of pollutants were observed, leading to the need for more research to precisely estimate the risks for human health (EEA 2018).

Nitrogen oxides are compounds from a mixture of oxygen and nitrogen and may cause acid rains and oxidation of certain materials. Furthermore, three of the most known NO_x derivatives are nitric oxide (NO), nitrogen dioxide (NO₂), and nitrous oxide (N₂O), which NO and NO₂ are listed as critical atmospheric pollutants (World Health Organization 2010), and N₂O is considered a greenhouse gas is contributing to climate change (Aste et al. 2013).

Studies pointed out that NO₂ has a strong health impact independent of other pollutants, including all-cause mortality, respiratory and allergic diseases, including asthma, and hospital admissions (Shubhankar and Ambade 2016; Deng et al. 2016; Perera 2017; He et al. 2019).

Among the sources of NO₂ in multifamily residences are stoves, ovens, cookers, water heaters, i.e., gas-fired appliances, and stove and fireplaces in general, all these fueled by coal, wood, oil, biomass, or liquefied petroleum gas (LPG). Other two NO₂ sources could be essential to raise the level of indoor concentration: tobacco smoking and attached garage (Belanger and Triche 2008; Kalimeri et al. 2019).

Building furniture may also be one more source of NO₂. It affects indoor concentration through chemical reactions because their surface characteristics accelerate the depletion of NO₂ or contribute to gas-phase indoor chemistry by forming the nitrate radical from the ozone NO₂ (Weschler et al. 1994; World Health Organization 2010; Waring and Wells 2015; Arata et al. 2018).

Gómez Alvarez et al. (2014) described the potential of increasing NO₂ concentration from the material surfaces in the building spaces where household chemicals generate nitrous acid with more evidence on the window’s material. In the same way, Arata et al. (2018) found an increase of 8 μg/m³ of indoor NO₂ for wood-framed windows.

Unflued indoor gas heating contributes to indoor NO₂ levels in many residences. Nevertheless, in houses with no unflued gas heating, one of the most critical determinants of indoor NO₂ is likely to be gas cooktops, and that is an additional concern (Nagda et al. 1989; Gillespie-Bennett et al. 2008; Mullen et al. 2016). The complete combustion of natural gas cooking burners directly produces water vapor and carbon dioxide, but the high flame temperatures also produce NO_x and, consequently, NO₂ (Singer et al. 2017)_.

Zota et al. (2005) associated the increased gas stove use during the winter for space heating with decreased air exchange rates. In this direction, Chen et al. (2020) verified the decrease of IAQ with burning of fuel to heating of spaces, and Noris et al. (2013) attributed the reduction of NO₂ concentrations mainly to additional ventilation, installation of range hoods, and replacing the stoves with models that did not have pilot lights.

Because pollutants decrease the IAQ, strategies for mitigating exposure from NO₂ generated in the kitchen must be taken. One of the primary strategies is exhaust ventilation through a range hood (Stratton and Singer 2014). This kitchen exhaust air device may reduce pollutant concentration, improving the global IAQ by removing the surrounding air nearby the stove (Dobbin et al. 2018).

Particulate matter (PM_2.5 and PM₁₀)

Particulate matter is one of the essential ambient air pollutants involved in several adverse health effects (Obaidullah et al. 2012). Exposure to concentrations of PM is considered a leading global health concern, higher than the Global Burden of Disease (GBD) predicted in 2015 (Burnett et al. 2018). Besides, the buildings as a PM source in cities have not been studied.

Several experimental works about the emissions and concentrations of PM inside homes and residential buildings. Jones et al. (2000) carried out one of the first studies of indoor and outdoor sources of PM in urban and rural homes. They determined the contribution of indoor activities to PM₁₀, PM_2.5, and PM₁ mass concentrations and analyzed the particles chemically to identify the gift of the outdoor sources to the indoor particles.

More recently, Suryawanshi et al. (2016) presented the indoor concentrations of PM_0.6 and identified sources leading to indoor air pollution. The average PM_0.6 indoor concentration was about 94.44 μg/m³. Five possible sources of indoor pollutants were identified by factor analysis: wall dust (25.7%), wooden furniture and paper products (25.2%), coal combustion (21.8%), soil particles (17.5%), and tobacco smoking (9.8%).

The effect of cooking on indoor air quality through the on-site measurements of cooking-generated fine particles (PM_2.5 and PM₁₀) in 30 residential buildings was analyzed by Kang et al. (2019). The results indicated that the concentration of fine particles exceeded the values recommended by the standards. They also observed that the fume hood system could not effectively reduce the concentration of particulate matter emitted from cooking during the cooking period. In fact, the particles emitted by the cooking were rapidly eliminated when the natural ventilation and the fume hood system operated simultaneously. The factor that most influenced the particulate matter concentration was the type of cooking.

The study of Canha et al. (2017) was based on observing the indoor air quality during the sleep period of 10 couples in Lisbon dwellings, using a multi-pollutant analysis, to understand how compliance with legislation and guidelines ensures excellent indoor air quality. In 70% of the cases, PM_2.5 (15.3 ± 9.1 mg/m³) exceeded the WHO guideline of 10 mg/m³ and, irrespective of ventilation condition, particulate matter levels (PM_2.5 and PM₁₀) were always high.

Vuković et al. (2013) have introduced active biomonitoring of mosses as a complementary procedure to the usual measurements of trace element content in semi-enclosed spaces, obtaining elevated mass concentrations of PM₁₀ and a wider range of carcinogenic heavy metals. Li and Xiang (2013) investigated PM₁₀ and PM_2.5 pollution in an underground car park in Wuhan using a gravimetric method. The results showed that PM pollution was worse at the exit than at the entrance.

Some studies have been done in commercial and office buildings. PM_2.5 and NO₂ concentrations simultaneously indoors and outdoors for ten different shops and office buildings in Dublin were measured (Challoner and Gill 2014). The I/O PM_2.5 concentrations were all found to be close to or above 1, indicating that neither naturally nor mechanically ventilation was performing any significant function to reduce particulate concentrations from outdoors. I/O ratios of both components, particularly NO₂, increased significantly overnight while outdoor concentrations were reduced much more than indoors. This would indicate that promoting a greater air exchange between indoors and outdoors during nighttime might be beneficial for pollutant removal.

Finally, we can mention some papers that analyze PM urban air pollution. Karagulian et al. (2015) established the shares of the typical sources of PM_2.5 and PM₁₀ by examining the available literature studies: 25% was contributed by traffic, 15% by industrial activities, 20% by domestic fuel burning, 22% from unknown sources of human origin, and 18% from natural dust and salt. Blanchard et al. (2019) analyzed 20-year records of emissions and ambient air pollutant concentrations to understand the relative contributions of different emission source types of fine particles (PM_2.5). And according to the research conducted by Alves et al. (2020a, b) about aerial pollutants from kitchens, the PM_2.5 concentrations are related to the cooking/fuel or energy source.

Volatile organic compounds (VOCs)

Among the indoor pollutants, the VOC is the one that originated mainly indoors. The internal source of VOC is composite products, construction materials, adhesives, flooring, painting, and furniture. A study was performed in Japan to measure the VOC in a new house before occupants and furniture are settled to evaluate whether it is possible to design homes with low TVOC by considering the construction materials used (Suzuki et al. 2019). It concluded that low-emission materials are carefully selected, VOC concentrations can be sufficiently low.

The VOC source contributions are highly dependent on ventilation conditions (Plaisance et al. 2017; Harčárová et al. 2020). While generally, the studies suggest that low ventilation rates might conclude in potential health risks, some other studies indicate that high ventilation rates in residential houses may also have a negative impact, possibly due to the infiltration of outdoor pollutants (Carlton et al. 2019).

Residential garages have high concentrations of VOCs due to emissions from vehicles, storage containers, and items stored in the garage (Batterman et al. 2006). In cases where the garage is attached to the building, these VOC emissions can be transported into the ambient air and into living spaces. VOC concentrations and emissions in 15 Michigan residential garages that varied in type, size, use, and other characteristics resulted in 36 types of VOCs found in the garage and 20 in the ambient air (Brown 2002). Based on a literature review of 50 IAQ studies, gasoline-related VOC levels found in garages were considerably higher than levels found in residences. Garages showed the highest VOC concentrations in non-occupational or environmental settings, including some short-term measurements at busy traffic intersections and on high-traffic roads (Batterman et al. 2006, 2007).

Although there are usually numerous sources of VOCs in indoor air, it is imperative to consider that the migration of air from the garage to living spaces can carry potentially significant levels of contaminants (Batterman et al. 2006). These studies suggest that homes with attached garages have higher levels of VOCs, and quantitative analysis shows that attached garages are the primary source of many compounds found in residences (Fugler et al. 2002; Emmerich et al. 2003; Batterman et al. 2005).

Concerning indoor experimental analysis, Zhou et al. (2019) studied the emission of VOC in dry building materials and the effect of environmental conditions on their concentration with time. They concluded that an increase in ambient temperature and relative humidity would promote the release of TVOC, and the effect on formaldehyde release will be more significant.

We can also mention Järnström et al. (2006), who analyzed the evolution of the concentration of different VOC in new buildings in Finland at 6 and 12 months from the finish of the construction. They obtained that the lowest concentration levels were measured in buildings with mechanical ventilation systems. They detected the change in the type of VOC emitted from the ones generated in the construction to new ones. The mean values of VOC concentration after 12 months were less than 15 μg·m⁻³.

Holøs et al. (2019) fit an equation for the volumetric emission rate of TVOC [μg/(m³·h)] as a function of the uniform airborne concentration of TVOC and the ventilation air change rate with data from the literature and concluded that although the ventilation rate is key to control airborne concentrations, it does not noticeably influence TVOC emission rates. The effect of airtightness and ventilation on VOC concentration in newly furnished unoccupied residential houses is examined in Hernandez et al. (2020) The beneficial effect of mechanical ventilation was gauged as measured TVOC were 90% higher in its absence.

Lizana et al. (2020) analyze indoor volatile organic compounds and aldehydes using passive techniques in 18 indoor microenvironments associated with schools and homes located in southern Spain. They conclude the need to take particular care in bedrooms, whose air pollutant concentrations are responsible for approximately 60% of the total dose of VOCs and aldehydes inhaled by children. Finally, attention may be drawn to a study that describes mobile source multi-pollutant concentrations within a parking garage to support exposure and risk assessments and epidemiology (Kim et al. 2007).

Ventilation systems involved in the air exhalation of residential buildings

As it will be shown, four identified localized air exhalation sources layout the pollutant measurement strategies: vent stack, dwelling ventilation, garage ventilation, and kitchen exhaust ducts. The approach represents the classification of ventilation according to the Spanish regulations. Specifically, the reference is made to the following sections of the CTE: CTE DB HS: Healthiness; HS3 Indoor Air Quality; HS5 Water Evacuation (Gobierno de España 2019).

Vent stack

This ventilation system ensures the building properly removes blackwater, allowing air to enter the plumbing system. According to the Spanish regulations, the buildings must have ventilation subsystems depending on the height, all of them extended up to the roof (Gobierno de España 2019).

The drainage and the vent pipe systems of the building work simultaneously: drainage pipes carry waste out of the building to the sewer system (public system or local/particular system), and vent pipes supply fresh air to each plumbing set in the building to help drainage pipes moving water through the maintenance of the proper atmospheric pressure in the water system (Fig. 1).

Fig. 1.

Example of the vent stack outlet of the fecal sanitation system (the outlet on the right) on the roof of a residential building in use (Spain)

Plumbing air vents also prevent sewer gases from remaining in the building, allowing wastewater gas and odor to escape. Thus, vent pipes, also known as vent stacks, are located on roofs and must be away from windows or air conditioning units, where the exhaust gases can quickly dissipate. Special attention is paid to the presence of methane in these systems.

The vents can be operated individually or combined into more vents located on the roof. Where buildings exceed ten floors in height, drainage stacks require relief vent connections at specified intervals from the top and connected to a vent stack terminating above the roof (WHO 2006).

Several authors alert about the return of the exhausted gases to the buildings through the apertures and suggest recommendations about vent stack installation. For example, Kaushal et al. (2004) commented that all air intakes must be away from vehicle exhaust, vent stacks, and any other contaminant source, at a distance no less than 9 m. However, there could be different difficulties in achieving correct air intake distances due to many barriers. For instance, a relatively small roof area installed a rooftop HVAC unit and vent stack pipes (ASHRAE 2007).

In vast and complex buildings one-pipe drainage system is commonly used to permit waste gas ventilation. However, vent pipes are sometimes omitted in the single-pipe system, known as single-stack systems. In this system, the stack and branches must be carefully designed to provide adequate ventilation (WHO 2006). According to WHO (2006), the pipes through waste gases from the building are exhausted to open-air may have a diameter between 25 and 40 mm.

Morey and Shattuck, cited by Ahearn et al. (2004), identified adverse air quality for building inhabitants when outdoor air intakes were incorrectly placed near the plumbing vents so that they admitted exhausts or microbial-laden aerosols.

In some individual sewer septic systems, typically used in single-family houses, the pipe vent is responsible for extracting the gases from the wastewater chamber to release them into the atmosphere. The climatic condition of the surrounding is affected, causing air pollution (Lasisi et al. 2018).

Dwelling ventilation

Hygienic ventilation supplies fresh and clean or cleaned air into an indoor space to reduce exposure to hazardous pollutants by diluting and removing air pollutants. Thus, ventilation is one of the strategies used to control indoor air quality (Carrer et al. 2018).

Two primary standards determine the ventilation rates and design criteria, EN 16, 798–1 (2019) and ASHRAE 62.1 (2016) Even though the ventilation requirements to guarantee the indoor air quality vary depending on the country, most of them, among the developed countries, is based on those two standards mentioned.

This dwelling ventilation is the mechanical ventilation that ensures specific air changes per unit time within the dwelling apartments. The current regulation for residential buildings in Spain specifies air changes in indoor spaces. CTE-HS3 stipulates that those buildings will have the means to adequately ventilate their indoor spaces, eliminating the pollutants that occur regularly during everyday use so that enough flow of outside air can be provided. Their extraction and expulsion can be guaranteed (Fig. 2A, B).

Fig. 2.

Example of dwelling ventilation execution: A “white ducts”; B at the roof of a residential building (Spain)

Regarding the ventilation systems generally found in new buildings, two ventilation systems can provide the necessary changes per hour. On the one hand, the single flow controlled mechanical ventilation. In these systems, the air is introduced passively throughout the apartment, e.g., through micro-ventilation slits in windows and infiltration. At the same time, it is extracted mechanically to the roof through the wet rooms. On the other hand, the dual flow controlled mechanical ventilation introduces and removes the air mechanically, usually from the facades, through a heat recovery unit.

Garage ventilation system

Mechanical ventilation systems are generally installed in large, enclosed garages to provide the fresh air needed to remove contaminants from the air in a timely manner to maintain a good level of air quality (Obaidullah et al. 2012).

While the ventilation of a garage could be either natural or mechanical, this review considers only the mechanical system (Fig. 3), given that natural ventilation requires openings in opposed facades.

Fig. 3.

Example of the execution of a garage ventilation system, from the garage to the roof, in a residential building (Spain)

The extraction of the garage exhaust air is made through the rooftop. The regulations require minimum ventilation rates and a carbon monoxide detection system to activate the fans when attaining a threshold concentration. Thus, for example, the Spanish law (CTE) requires a minimum ventilation flow in car parks and garages of 120 l/s per parking space.

Measures to address contaminant levels in residential garages and the transfer of contaminants to living spaces have been established in ventilation standards for new construction (ASHRAE 2013), as well as recommended testing to verify their effectiveness (Batterman et al. 2006). Current results of pollutant migration between residences and garages remain primarily qualitative, despite the expressed need for air exchange and migration studies that can serve to better understand and quantify the impacts of attached garages on residential air quality (Emmerich et al. 2003; Batterman et al. 2007).

Thus, based on this review, parking can be approximated in two ways:

• I.Architecture and systems typology:
• II.The operation of the combined supply and exhaust system is more effective than the exhaust-only system in controlling carbon monoxide levels in inhabited areas.
• III.Studying the influence of basements, garages, and common hallways on indoor residential VOC measurements.
• IV.The results of Lasisi et al. (2018) indicate that single-family homes with cars in attached garages were most affected by parked vehicles, followed by homes with vehicles in garages. Residing in a house with an attached garage could result in benzene exposures that are greater than exposures from car commuting in heavy traffic, with a risk of 17 excess cancers in 1 million population.
• V.To achieve greater energy efficiency, parking lot ventilation systems should be automatically activated based on ambient emission concentrations.
• VI.It is essential to know that the design and dimensions of parking lots are the most important structural factors influencing the fuel consumption of vehicles as a function of movements within parking lots and, consequently, emissions (Demir 2015).
• VII.Pollutant typology, considering that the primary harmful exhaust emissions based on the motor vehicles are gases such as CO, HC (hydrocarbons), and NO_x (Demir 2015), is determined in the “Scientific context” section.

Kitchen exhaust ducts

Kitchens are not just for preparing meals, they are also spaces for socializing. After the bedroom, it is probably the indoor area where the most time is spent. The air quality in a kitchen is influenced by many factors, from the method of food preparation and the ingredients used, the style of cooking, the temperature of the cooking process, the volume of the room, and the number of people using the space (Alves et al. 2020a).

Some studies showed that the level of pollutants in the indoor environment is higher than in the outdoor environment, with pollutants emitted from different sources such as combustion by-products, cooking, construction materials, office equipment, and consumer products (Wang et al. 2007).

Kitchen exhaust air ducts are used and are built to fulfill specific standards. ASHRAE Standard 62.1 (2019) is widely used for this purpose. Regarding kitchens, Spanish CTE establishes the need for additional extraction ventilation in kitchens in the cooking zone to extract vapors and contaminants from food cooking, independently of the general ventilation of the habitable premises (Fig. 4A, B). This was emphasized by the view of the EVIA Working Group that cooking fumes can better be removed with dedicated cooker hoods (Holsteijn et al. 2017). This condition is satisfied if a system is available in the cooking zone for a minimum extracting flow rate of 50 l/s.

Fig. 4.

Example of kitchen exhaust on the roof of a residential building in Pamplona (Spain): A) extraction openings must be connected to exhaust ducts. These ducts are placed in riser shafts that reach the chimney stacks of buildings; B) Three of the rests are closed while one is partially open and the last is entirely open, as a function of the speed selected by the user

Common pollutants coming from the kitchen are mainly PM, CO, NO₂, and VOC. The pollutant quantification depends on cooking fuels and outdoor environmental factors (rural, urban, traffic, etc.). Tan et al. (2013) stated that PM, CO, NO₂ and VOC concentrations were measured in the kitchens, and emission rates were estimated for the cooking periods. Most of the indoor air pollution comes from sources inside the building (Agelopoulos et al. 2000) .

Elemental analysis of PM₁₀ showed high concentrations of metals (Fe, Na, and Zn), while their morphologies indicated that they were present as salts, skin, and particles of biological origin. Gaseous emissions, particularly NO₂ and CO, were more frequent in households with gas appliances (Spiru and Simona 2017).

Tan et al. (2013) have shown that indoor air quality in kitchens using gas stoves is mostly worse than in those using electric ovens, especially for particulate matter, NO₂ and CO. Besides, these pollutants can migrate through the rest of the environment. In addition, the same study stated that indoor VOC concentrations were notably higher than outdoor concentrations because VOC concentrations are rapidly dispersed and diluted in the outdoor environment.

Finally, it is essential to remark that it has been observed that there was no difference in CO, CO₂, and VOC concentrations when measurements were taken from different heights (Shen et al. 2020).

As a resume, building services are the paths through which indoor pollutants are vented to the outside of the residential buildings. On the one hand, blackwaters containing methane are connected to sewage drainage and ventilation systems, which breathe all this gas into the building's roof for further emission to the atmosphere. On the other, gases and particulates from inside the dwellings, including CO, CO₂, CH₄, NO_x, PM, and VOC, are collected by the general ventilation of the housing units and by the ventilation of bathrooms and kitchens. And finally, garages, generally positioned in the basements of these buildings, have a significant amount of CO, PM, and VOC.

Discussion

This review of pollutants confirms that many studies have been carried out to specify the number of pollutants inside buildings. Still, no references or comparative studies have been found that confirm whether these interior quantities are the same as those expelled or if there are leaks in the ducts or other types of uncontrolled losses in the building itself or outside. Additionally, two issues are striking: on the one hand, the heterogeneity of the investigations carried out to measure different pollutants and, on the other the lack of special attention to the measurement of CH₄ emissions that has become a key parameter after the 2021 Glasgow Climate Change Conference UNFCCC.

Additionally, it has been explained the approach to the concept of exhalation applied to buildings. It is a logical derivative of the intellectual stream of biomimicry that has been growing in importance and value (Badarnah 2016; Pedersen Zari 2016; Kolokotsa et al. 2020). It is within this approach that previous works by the authors combining architecture, engineering, and biology (Bermejo-Busto et al. 2016, 2017; Zuazua-Ros et al. 2016, 2017a, 2017b) have led to the concept of exhalation to better understand the pollution that comes out of buildings, in a similar way to the exhalation that occurs in living beings. To this end, a graphical summary is presented showing the conceptual approach to the exhalation of pollutants in residential buildings through the interaction of different ventilation systems and the influence at the urban scale.

In terms of socio-economic impact, if someone could reach at least one of every possible route for the radical change of design paradigms in some building typology, the impact could be very high in the field of building construction and urbanism. Thus, that achievement could improve the technical and economic optimization of the design, construction, and operation of the energy systems and building services.

In this case, it is worth discussing both the general findings and the absence of initially expected results since they are almost equally important. Among the later, it can be highlighted that:

• For the pollutants reviewed, the authors did not find in the scientific literature the air pollution “exhaled” by residential buildings; therefore, there is a knowledge gap that the literature review confirms and validates the need for this type of study.

• The issue of contamination by air leakage from the ducts of residential buildings was not found in the scientific references. The significant variability in the quality of execution of residential buildings can mean critical impacts to adjacent dwellings exactly due to leaks.

On the contrary, some main findings are:

• It should be emphasized the importance that, as some authors pointed out, another approach could be developed in relation to the standards for IAQ in green buildings, taking into account health guidelines when people are exposed to aerial pollutants (Steinemann et al. 2017).

• It should also be mentioned that the concentrations of some indoor pollutants can increase significantly overnight, while outdoor pollutants concentrations decrease (Spiru and Simona 2017).

• It seems to be better to control VOC emissions in garages and contaminant migration through the garage/dwelling interface (Batterman et al. 2007). Although, if the expectations for the development of the electric car are met, in the future it is estimated that pollution in garages of residential buildings will decrease substantially.

Even though scientific studies have not been found regarding pollutants release from buildings, unless otherwise proven, the authors’ hypothesis assumes that the pollutants described in this article, when returned to the external atmosphere, can both be dissipated in neighboring areas and reintroduced into other, or perhaps the same, buildings. This is similar to the vicious loop observed about mankind-generated plastics waste spilling into the ocean that has run full circle and is now re-entering people through the food chain. Besides, it is a fact that the increase in the use of mechanical ventilation systems in new buildings forces the ventilation rate to increase. While this enhanced ventilation is generally assumed to improve indoor air quality, how this impacts outdoor atmospheric contamination is most often neglected.

Thus, this review presents a framework of the facilities of residential buildings as an exit path for pollutant emissions through a complex pipeline network, which are released directly into the atmosphere. As the review has shown, even though it appears that removing all these pollutants into the atmosphere is a good practice in an IAQ perspective, it might not be a pleasant result for OAQ.

From the authors’ point of view, some future actions may be involved:

1. Creating maps of aerial pollution spreading from residential buildings in surrounding areas (Demir 2015). Moreover, once it is known how much contaminated gases are exhaled, it will be necessary to know the pathways of outdoor particles entering in a new indoor environment (Chen and Zhao 2011).

2. Once the pollutants in the exhaled air are also reactants, it is also suitable to characterize the environment where they are mixed through simulation tools. This work could be associated with visual assessment of contaminant impacts in multizone buildings (Parker and Williamson 2016).

3. Procedures must be established to prevent the release of this pollution into the air in urban areas. In an ideal long-term view, waste air from residential buildings must be filtered, retaining the pollutants, and preventing the release into the atmosphere. In addition to classic filter solutions, at the laboratory scale, photocatalysis could be a promising method to eliminate N₂O, CH₄, and CO₂ and other major contributors to global warming such as black carbon, chlorofluorocarbons (CFC), tropospheric O₃, and its precursors: VOC, NO_x, and some other GHG or short-lived climate forcers (de Richter and Caillol 2011). In an ideal long-term view, waste air from residential buildings must be filtered, retaining the pollutants, and preventing the release into the atmosphere.

4. The measurement of methane in the exhaled contaminated air from residential buildings would allow to quantify the methane content flows to the atmosphere. Firstly, in order to evaluate their contribution as a greenhouse gas and, secondly and depending on the results obtained, to gauge the viability of capturing and using exhaled methane as energy source in the building’s energy systems.

5. This context allows us to consider whether it will be convenient in the future to establish a system such as an efficiency label for pollution from buildings or a guideline for maximum exhalation of pollutants per square meter of residential building.

6. To propose an alternative tool, based on biomimicry, that can generate alternative points of view from which to fill this knowledge gap.

7. Finally, this work has referred to dwellings, but it would be desirable to apply its methodology and objectives to other types of buildings.

The research project origin of this review is ongoing and the measurements of contaminants at the duct outlets will continue for several years. Preliminary results have been presented in Dorregaray-Oyaregui et al. (2022) and Manzueta et al. (2022). Although the purpose of this article is not the specific definition of these measurements, it is decided to advance here some of the results obtained in a 9-storey residential building, located in Pamplona (Spain) that was built following the 2006 Spanish Building Code (Fig. 5).

Fig. 5.

Averages (dark color) and deviations (light color) of A CO, B VOC, C CO₂, and D CH₄, concentrations (ppm); 1-month measurements; S1….S12 → Samples from different measurement places (Manzueta et al. 2022)

Our data show that CO and CO₂ average emissions did not exceed the range that is considered to be normal for urban areas. The VOC levels have high deviation values, thus making the researchers consider problems with sampling, so calibration measures were taken and possible problems with sampling are now being evaluated. The mean values of CH₄ concentrations are higher than expected in natural urban air. Researchers are now collecting more samples and analyzing the possible origins and reasons of these results.

Conclusions

This paper presents a cross-sectional review between pollutants (chemistry) and building ventilation systems (building services engineering) to identify the pollutants expected to be exhaled through ventilation systems. The collected studies present the primary pollutants in residential buildings (CO, CO₂, CH₄, NO_x, PM, and VOC) and ventilation measurements in vent stacks, dwelling ventilation, garage ventilation systems, and kitchen exhaust ducts.

With all these concepts in mind, this review is based on residential buildings. This typology has lower air changes per hour and smaller and more airtight spaces than other building typologies such as offices, commercial buildings, or schools. Besides, they present high levels of indoor pollutants that must eventually be released, making them a priority hazard and facilitating the measurements by reducing uncontrolled, diffuse leakage.

From this work, it can be concluded that a research gap is detected since no studies have been found that analyze what happens to the air that is exhaled from the residential buildings and what is the concentration of pollutants in it. In addition, this rapprochement among subjects has led to the creation of the concept of “exhalation of buildings,” something new and which is inserted within the research project in which this article is included.

The authors’ research has confirmed that the consideration of this new approach to building exhalation can also provide new solutions by conceiving building pollution as a unit to work on, not as a collection of activities and pollutants separated from each other. The goal is to put together the outcomes from the previous analysis, to draft strategies to minimize or eliminate the air pollutants before they reach the outdoor urban environment, and to evaluate the technical viability of the proposed solutions.

The lack of references to similar studies prevents us from specifically raising here the magnitude of the mitigation that can be achieved. Yet, the researchers want to underline that the mitigation that occurs, however small it may be, applied to millions of residential buildings, can have an important impact not only regional, but applicable to many urban centers.

In an even more general perspective, characterizing the exhalation of buildings may become necessary to further refine the calculations about environmental drivers. Several exhalation components are greenhouse gasses (CO₂, CH₄), and most require a good accountability for climate change models. Uncertainties in emissions from residential sources such as buildings have been identified and should be reduced to improve models.

The results obtained when seeking answers to the initial questions presented in this cross-review have confirmed the lack of research directly related to the hypothesis. This raise concerns about the depth of our current knowledge and understanding about the role of buildings in the contribution to environmental air pollution and, by extension, to other global phenomena directly related to some of the potential pollutants being released by buildings.

The research project is ongoing and the measurements of contaminants at the duct outlets will continue for several years. Preliminary results have been presented in CLIMA 2022: the 14th REHVA HVAC World Congress 22nd – 25th May in Rotterdam, The Netherlands, with the tittle “The conceptualization of exhalation in buildings” (Dorregaray-Oyaregui et al. 2022).

The next steps in our research project will focus on carrying out empirical studies to help quantifying and characterizing the building exhalation pollutants and confirm or dismiss the need to deeply explore this neglected area of research.

Abbreviations

CH₄

Methane

CO

Carbon monoxide

CO₂

Carbon dioxide

CTE

Código Técnico de la Edificación/Spanish Technical Building Code

CTE-HS3

Código Técnico de la Edificación Salubridad-Calidad del aire interiot/Spanish Technical Building Code salubriousness-Indoor Air Quality

CH₄

Methane

CO

Carbon monoxide

CO₂

Carbon dioxide

CTE

Código Técnico de la Edificación/Spanish Technical Building Code

GHG

Greenhouse gas

HVAC

Heating, ventilation, and air conditioning

I/O

Indoor and outdoor ratio

IAQ

Indoor air quality

NO

Nitric oxide

N₂O

Nitrous oxide

NO₂

Nitrogen dioxide

NO_x

Nitrogen oxides

O₃

Ozone

OAQ

Outdoor air quality

PM

Particulate matter

TVOC

Total volatile organic compounds

VOC

Volatile organic compounds

WHO

World Health Organization

Author contribution

All authors contributed equally in the preparation of this manuscript. Idea for the review article: Arturo H. Ariño; Leonardo de Brito Andrade; César Martín-Gómez. Literature search: Arturo H. Ariño; Leonardo de Brito Andrade; Amaia Zuazua-Ros; Juan Carlos Ramos González; Bruno Sánchez; Sara Dorregaray; César Martín-Gómez; Robiel Manzueta. Data analysis: Leonardo de Brito Andrade; Amaia Zuazua-Ros; Juan Carlos Ramos González; César Martín-Gómez; Robiel Manzueta. Drafted and revised the work: Arturo H. Ariño; Leonardo de Brito Andrade; César Martín-Gómez; Robiel Manzueta.

Funding

The Spanish Ministerio de Ciencia, Innovación y Universidades for funding the research project “Cuantificación de parámetros contaminantes de la exhalación de los edificios en entornos urbanos EXHAL / Quantifying pollutants originated by the exhalation of buildings in urban environments.” Federal University of Santa Catarina for financial support during the postdoctoral fellowship of Dr. Leonardo de Brito Andrade at Escuela Técnica Superior de Arquitectura of Universidad de Navarra. PhD fellowship Ayudas para la Formación de Profesorado Universitario FPU2020/04936 del Ministerio de Universidades (Spain) received by Robiel Manzueta.

Data availability

Nothing to declare.

Code availability

Nothing to declare.

Declarations

Ethics approval

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Consent to participate

Authors consent to participate in accordance with the journal's rules.

Consent for publication

Authors consent to publish the results of their work in accordance with the journal’s rules.

Conflict of interest

The authors declare no competing interests.