Effects of indoor environmental parameters related to building heating, ventilation, and air conditioning systems on patients' medical outcomes: A review of scientific research on hospital buildings

Abstract

The indoor environment of a mechanically ventilated hospital building controls infection rates as well as influences patients’ healing processes and overall medical outcomes. This review covers the scientific research that has assessed patients’ medical outcomes concerning at least one indoor environmental parameter related to building heating, ventilation, and air conditioning (HVAC) systems, such as indoor air temperature, relative humidity, and indoor air ventilation parameters. Research related to the naturally ventilated hospital buildings was outside the scope of this review article. After 1998, a total of 899 papers were identified that fit the inclusion criteria of this study. Of these, 176 papers have been included in this review to understand the relationship between the health outcomes of a patient and the indoor environment of a mechanically ventilated hospital building. The purpose of this literature review was to summarize how indoor environmental parameters related to mechanical ventilation systems of a hospital building are impacting patients. This review suggests that there is a need for future interdisciplinary collaborative research to quantify the optimum range for HVAC parameters considering airborne exposures and patients’ positive medical outcomes.

Practical Implications.

• The indoor environment of a mechanically ventilated hospital building controls infectious disease transmission and influences patients’ outcomes.

• A summary of the recommended optimum ranges of temperature, relative humidity, ventilation rates, air filtration, differential pressure control, and ventilation strategies will be beneficial for the patients’ wellbeing.

• These findings from the published scientific literature will be helpful for researchers, policy‐makers, healthcare and building design professionals in enhancing standards related to the HVAC systems considering patients’ outcomes.

• Future interdisciplinary collaborative research has been identified and can lead to specific optimum ranges for indoor environmental parameters in different spaces for hospital buildings.

1. INTRODUCTION

Surgical site infections (SSIs) are infections that occur after surgery,1 and they are the most common nosocomial infections.2, 3 According to a Centers for Disease Control (CDC) and Prevention report, 31% of hospitalized patients get infected by SSIs among all other health care‐associated infections (HAI).3 This nosocomial infection can cause morbidity and even contribute to mortality.4, 5 SSI incidence is dependent on surgical sterilization procedures, mechanical ventilation systems, high‐efficiency particulate absorbing (HEPA) filters, ultraviolet radiation, air renewal, humidity control, temperature, differential air pressure, particle count, surface colony count, and antibiotic prophylaxis.6, 7, 8, 9

The importance of the indoor environment on patients’ healing processes was initially mentioned by Florence Nightingale10 and is one of the major issues to which healthcare professionals, environmental psychologists, consultants, and architects are giving priority.11, 12, 13, 14, 15, 16, 17, 18, 19 The indoor environment controls infection rates and influences the overall patient outcome.19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34 Appropriately designed building heating, ventilation, and air conditioning (HVAC) systems can enhance patients’ recovery process,35 can reduce the length of hospital stay,15, 18, 25, 36, 37 can lessen medical errors and infection rates,20, 23, 38 and can improve the indoor air quality (IAQ) and minimize HAI.39 An improved indoor environment of a hospital building can reduce costs associated with airborne illnesses by 9%‐20%.40

To establish the relationship between health outcomes and the physical environment, Rubin et al30 identified 85 relevant studies where parameters including room size, room privacy, interior design of a room, patient control of his/her environment, music, lighting, exposure to sunlight, window view of nature, ventilation system contaminants, humidity, and temperature have been reviewed. Among these studies on the effect of healthcare environment on patient outcomes, seven were on humidity; four on air filtration system; four on ventilation system; two on temperature; one on (increase outside air changes, improve filter efficiency, maintain constant temperature and humidity, increase positive pressure of operating room air). They30 concluded that there is convincing evidence linking patients’ clinical outcomes and built environment parameters. Zimring et al32 identified a connection between the hospital indoor environment and patient and staff outcomes with respect to four sectors: staff stress and fatigue reduction and increased effectiveness in delivering care; patient safety improvement; stress reduction and improvement in patient outcomes; and overall healthcare quality improvement. Dijkstra et al23 summarized literature on environmental stimuli (eg, furniture, art, color, nature, plants, gardens, carpeting, room size, spatial layout, private rooms, noise, music, odor, television/video, light, windows, and view from a window) and their impact on patients’ psychological outcomes rather than on direct physiological outcomes. Huisman et al25 reviewed hospital interior layouts and their relationship with medical staff error, patient falls, infection rates, indoor quality, comfort, building materials, visual comfort, acoustics, view, and privacy. They covered the indoor environment (eg, ambient temperature, humidity, ventilation strategies, and air quality) under the subtopic of safety and security, not patients’ medical outcomes.25

Previous studies related to the hospital building's indoor environment discretely focused on patients’ thermal comfort,35, 41, 42, 43, 44, 45 acoustic comfort,46 visual comfort, and IAQ.47, 48, 49, 50 The impacts of single or multiple indoor environmental parameters related to the mechanical ventilation system on overall patient outcomes have not been summarized yet. Hence, this review covers the findings from the published scientific literature on the associations of temperature, relative humidity, ventilation rate, air filter, differential pressure, and ventilation strategy, with patient outcomes. This literature review considered articles published after 1998 and was restricted to codes, guidelines, and standards published by professional societies, licensing agencies, and regulatory organizations. Studies related to natural ventilation for infection control are outside of the scope of this review. Additionally, parameters related to building design, architecture, interior design, noise, aroma, and lighting are being excluded from the scope of this literature review, since this review exclusively focuses on parameters related to the HVAC systems.

This review will help researchers, policymakers, healthcare, and building design professionals to understand the importance of indoor environmental parameters and provide information for enhancing standards related to the HVAC systems in attaining positive medical outcomes for patients. The review also identified avenues for future interdisciplinary collaborative research to quantify the optimum range for indoor environmental parameters considering patients’ positive medical outcomes.

2. METHODOLOGY OF THE REVIEW PROCESS

A multidisciplinary reviewing process was adopted to find out both quantitative and qualitative academic research evidence on indoor environmental parameters and their impact on patients’ medical outcomes. PubMed [Medline], JSTOR, ScienceDirect, Scopus bibliographic databases, Google Scholar, and Texas A&M University Library databases were searched for 58 keywords. Combination keywords, such as “ambient temperature AND patient outcomes,” “mechanical ventilation system AND patient outcomes,” “indoor air quality AND patient outcomes,” “airflow AND patient,” “mechanical ventilation AND infection,” “physical environment AND patient outcomes,” and others were used as well to confine the search area. Scientific publications in the fields of both hospital buildings and parameters related to the HVAC system of a mechanically ventilated building were reviewed. Also, the citations in each study found during the main search were reviewed for potential relevance. This paper includes relevant articles that were published after the review done by Rubin et al.30

Through a systematic review process, a total of 1871 abstracts were screened, with a total of 899 papers being identified as relevant to the scope of this study. These articles went through a full‐text review process and were excluded if the patient outcomes were not biological and physiological, or if the built environment parameters are not related to the building HVAC system, variables such as noise, aroma, light, and other building layouts and interior design. Additionally, research on naturally ventilated hospital buildings was excluded as well since this review article solely focuses on the parameters related to the mechanical ventilation system of a hospital building. As a result, 176 articles have been included in this review paper. However, articles that partially fulfilled the objective of this review were included. For example, papers concentrating on both patient and staff outcomes were evaluated, though only patients’ outcomes related study were included in this review paper. Among 176 papers in this review, 133 investigated patients’ outcomes as a function of at least one indoor environmental parameter related to building HVAC systems.

The article selection process is shown in Figure 1. All the references of these articles were verified and crosschecked.

Figure 1.

Diagram showing the methodology of the review process

3. INDOOR ENVIRONMENTAL PARAMETERS IMPACTING PATIENT OUTCOMES

Patient outcomes are dependent on the indoor environment of a hospital building.19, 33, 51, 52, 53 Previous research on evidence‐based design for healthcare facilities has established that hospital‐acquired infection rates are directly related to IAQ.17, 25, 45, 54, 55, 56, 57 Patients’ psychological health is affected by poor indoor environment.58, 59 Studies have demonstrated an association between environmental variables and “Sick building syndrome” (SBS).60, 61, 62 Patients and elderly in hospitals and nursing homes are sensitive to these specific building‐related illness caused by SBS; they are hypersensitivity pneumonitis; building‐related asthma; and legionellosis.61 Environmental parameters can be modified to improve the physical environment and promote patients’ positive health outcomes.15, 16, 32, 33

Kameel and Khalil63 have mentioned that HVAC supply air temperature and RH can inhibit the growth of bacteria and activate or deactivate viruses. Lutz et al64 identified a positive correlation between infection rates and parameters related to the indoor environment (eg, type of air filter, the direction of airflow and air pressure, air changes per hour (ACH) in a room, humidity, and ventilation system cleaning and maintenance). Codinhoto et al22 summarized the built environment parameters relating to health outcomes of both patient and healthcare professionals by proposing a framework and grouped these environmental parameters to “ergonomics,” “fabric and ambient,” “art and esthetics,” and “services.” To propose a framework based on a cause‐effect relationship, they categorized patient health outcomes under three sections: psychological, physical, and physiological.22 They concluded that the indoor environment of a healthcare facility has a considerable impact on patients’ health outcomes.22 Rashid and Zimring65 have developed a framework relating indoor environment with patient stress as an outcome. Ulrich et al66 analyzed nine built environment parameters (eg, audio environment, visual environment, safety enhancement, way‐finding system, sustainability, patient room, family support spaces, staff support spaces, and physician support spaces) with respect to patient recovery.

This review focuses on summarizing published literature that investigates the impact of each or multiple indoor environmental parameters on patient outcomes. The following section will categorize studies related to patient medical outcomes under indoor air temperature, relative humidity, indoor air ventilation rate, air filtration system, differential pressure control, and mechanical ventilation strategies.

3.1. Indoor air temperature

3.1.1. Indoor air temperature and nosocomial pathogens

Potential airborne pathogens such as bacteria, viruses, and fungi can pose severe health effects.67 The susceptibility of patients to nosocomial pathogens depends on the pathogen's survivability on various surfaces.68, 69 Temperature is one of the major factors that influence the transmission and survivability of these microorganisms.70, 71, 72

Low ambient temperature increases influenza virus transmission since the survivability of infectious agents rises.73, 74 The optimum temperature to control the survival of airborne influenza viruses is as high as 30°C (86°F) at 50% relative humidity,75 which will create an uncomfortable indoor environment as per ASHRAE1 Standard 55.76 Through an experimental study, Lowen et al73, 77 concluded that at 20°C (68°F) influenza virus transmission is dependent on humidity, but at the higher temperature (30°C; 86°F), the transmission was eliminated regardless of relative humidity. Low temperature is associated with longer persistence of most viruses, such as the astrovirus,78 adenovirus, poliovirus, herpes simplex virus, and hepatitis A virus.68, 69 Most bacteria, fungi, and viruses are more stable and persist longer at low temperatures, such as 4°C (39.2°F) or 6°C (42.8°F).69 Tang75 focusing on the disease‐oriented evidence reviewed the survival of airborne infectious bacteria in relation to indoor air temperature. He concluded that temperatures above about 24°C decrease the survival of gram‐negative, gram‐positive, and intracellular bacteria.75 Clinically relevant airborne fungi, potentially life‐threatening for immunocompromised patients are Aspergillus species (Aspergillus flavus and Aspergillus fumigatus)75, 79; Blastomyces; Coccidioides; Cryptococcus; and Histoplasma species.80 Unlike the laboratory‐based testing for viruses and bacteria, air sampling testing to identify the presence or absence of fungi and their spores in natural settings revealed higher spore counts at a higher temperature,75, 81, 82 although, based on the literature review, Kramer et al69, 71 concluded opposite findings.

3.1.2. Indoor air temperature and thermal comfort perception

Indoor air temperature is important for patients’ thermal comfort perception.83 Thermal comfort has an impact on patients’ healing processes,35, 84 satisfaction with surgical care,85 well‐being, and safety.35, 86, 87, 88 Due to medication and drug use, a patient's thermoregulatory system affects the overall perception of thermal comfort.89 Uncomfortable environments have negative effects on patients, such as sleeplessness and restlessness,86 and can cause shivering, inattentiveness, and muscular and joint tension.63, 85, 90, 91, 92, 93, 94 Maintaining thermal comfort in an operating room (OR) is a challenge since the situation varies with the surgery types, various patient requirements, various activity levels of hospital staff, different interior settings of lights and equipment, and the total number of occupied people at a certain time.43, 95 It is recommended to modulate the OR temperature according to the need of each surgery type for optimum comfort level.19, 35, 43

3.1.3. Optimum temperature and patient's thermal risks

In hospitals, researchers recommended separate thermal zones to address the different needs of patients, and their separate thermal preferences43, 45, 96, 97, 98 summarize the recommended optimum temperatures to prevent the thermal risk of patients (Table 1). Patients’ thermal status99, 100 and a low ambient OR temperature101, 102, 103, 104, 105 are the main reasons that cause patients’ intraoperative hypothermia. Studies have identified the correlation between low ambient room temperature and hypothermia among patients during the perianesthesia or perioperative period.101, 106, 107, 108 Since high ambient temperature (>23°C or 73.4°F) is required to avoid perioperative hypothermia, it may be found uncomfortable for the OR personnel.109, 110

Table 1.

Recommended optimum temperature for different spaces in a hospital building

Spaces in a hospital building	Optimum temperature set point	References
Operating room	>26°C (78.8°F)	De Witte and Sessler235
	24‐26°C (74.2‐78.8°F)	Balaras et al,128 Sadrizadeh, and Loomans137
	≥21°C (69.8°F)	Melhado et al,43 Khodakarami, and Nasrollahi35
Postoperative care area/room	≥24°C (75°F)	Hooper et al236
Intraoperative area/room	20‐25°C (68‐77°F)	Hooper et al,236 Association of Perioperative Registered Nurses,237 Morris,104 Morris and Wilkey,105 Wang et al238
Delivery room	≥26°C (78.8°F)	Knobel et al,239 Knobel and Holditch240
Nursery (for infants)	Around 28°C (82.4°F)	Lyon and Freer125

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Surgical site infection is one of the leading effects of even mild hypothermia, where a 1.9°C (3.42°F) reduction in core body temperature increases the chance of SSI three times in a patient after colorectal surgery.90 Mild perioperative core hypothermia may increase the risk of wound infection,90, 111 increase the length of hospital stay,90, 112, 113 increase blood loss,114 cause cardiac complications,112 and cause a prolonged post‐anesthetic recovery.113 Perioperative hypothermia poses a relative risk of severe complications, such as cardiac events,115, 116, 117, 118 blood loss,114 impaired wound healing,115, 116, 117 wound infections,119 an increased rate of morbidity and mortality,114, 115, 120, 121 length of hospital stay, and the cost of treatment.115, 116, 117

Higher ambient temperature is recommended during anesthetic induction and surgical skin preparation; conversely, a lower ambient temperature is recommended before surgical incision.19 However, this method has limited effectiveness among adult patients because of the time interval for warming the room is relatively brief and requires wide swings in temperature to have a significant clinical effect.122 A study on critically ill trauma patients confirmed that there is no correlation between the decrease of ambient OR temperature and patient core body temperature with effective use of active warming strategies on patients.123, 124 Controlling indoor air temperature is crucial for other severely ill patients (eg, burn victims) where the application of active warming strategy is difficult.19, 86 Thermal stability is important for preterm infants to reduce harmful side effects, such as delayed adaptation to extrauterine life, hypoglycemia, respiratory distress, hypoxia, metabolic acidosis, coagulation defects, acute renal failure, necrotizing enterocolitis, and failure to gain weight or weight loss and morbidity.125, 126, 127

3.2. Relative humidity (RH)

3.2.1. Influence of relative humidity on infectious disease transmission

In hospital buildings, evidence has confirmed that RH affects infection control because it is related to the growth and transfer of airborne bacteria,128 some strains of viruses, and fungi.35, 83, 129 A strong correlation has been found between the transmission of viruses and absolute humidity,74 and viruses outbreak when the vapor content of the air decrease.130 Several controlled studies concluded that both RH and humidity ratio values influence the survival of viruses and bacteria.69, 74, 131, 132, 133, 134

An RH level higher than 45%‐50% promotes fungal growth indoors75, 83, 135, 136 and affects the concentrations of allergens, bacteria,98, 137 and increase the settling rate of aerosols.136 Most fungal species cannot grow when RH is below 60%.69 High humidity levels support microbial growth due to moisture absorption by building materials.136 Pathogenic microorganisms can adhere quickly to moist and slick or damaged walls and ceilings138 which can affect patient and staff well‐being.139, 140, 141, 142

Relative humidity has an impact on the viability of both airborne and droplet transmission of viruses.75, 143 However, this relationship is quite complex. Both lipid‐enveloped and non‐lipid‐enveloped viruses are less stable at relative humidities between 40% and 70%,143 while an ideal range for the airborne influenza survival is 23%‐81%.73 A study131 found that at 23% RH, the infectivity of influenza virus is as high as 71% to 77%, whereas at higher RH level (43% RH) the infectivity found 16% to 22%. Based on an experimental study, Lowen et al73 concluded that airborne transmission of influenza virus was maximum at 20%‐35% RH, and poor at 50% RH. Tang et al134 summarized that higher RH (>50%) is favorable to viruses without a lipid envelope, for example, poliovirus.144 However, lipid‐enveloped viruses, for example, influenza,73, 74 Lassa fever virus, and human coronavirus survive longer in low RH (<50%) and their persistence eliminate at RH >80%.134 Findings on the effects of RH on the survival of airborne bacteria appear to be more complicated than with viruses.75, 143, 144 A literature review69 on the persistence of nosocomial pathogens on any intimate surfaces concluded that at higher humidity most types of bacteria persist longer and spore concentrations of most fungi were higher. However, conflicting results were found on the on the persistence of clinically relevant pathogens. Table 2 summarizes research findings on favorable RH ranges for the growth and survivability of various nosocomial pathogens and microorganisms.

Table 2.

Favorable relative humidity (RH) ranges for the survivability of nosocomial pathogens and microorganisms

	Example	Relative humidity (RH)	References
Virus
Viruses (with lipid envelops)	Respiratory viruses (Influenza virus, Para‐Influenza virus, Corona virus, Respiratory syncytial virus, Herpes simplex virus, Measles virus, Rubella virus, and Varicella zoster virus)	Lower RH (20%‐30% RH)	Tang,75 Kramer et al69
	Influenza, Lassa fever virus, and Human coronavirus (hCV)	Lower RH (<50% RH)	Tang et al,134 Noti et al131
	Influenza	20%‐35% RH	Lowen et al73
	Influenza	23%‐43% RH	Noti et al131
Viruses (non‐lipid enveloped)	Adenovirus, Enterovirus, and Rhinoviruses	Higher RH (70%‐90% RH)	Tang,75 Kramer et al69
	Poliovirus	Higher RH (>50% RH)	Tang et al134
Bacteria
Gram‐negative bacteria	Pseudomonas spp., Enterobacter spp., Klebsiella spp., Salmonella seftenberg, Pseudomonas aeruginosa, Chlamydia trachomatis	Higher RH	Tang,75 Tang et al134
	Serratia marcescens, Escherichia coli, Klebsiella pneumoniae, and Proteus vulgaris	Lower RH (<50% RH)	Tang,75 Kramer et al,69 Tang et al,134 Cox144
Airborne gram‐positive bacteria	Staphylococcus epidermidis, Streptococcus haemolyticus, Bacillus subtilis, and Streptococcus pneumoniae	Lower RH (<50% RH)	Tang,75 Tang et al134
	Enterococcus faecalis	Lower RH	Robine et al241
	Listeria monocytogenes	Higher RH	Kramer et al,69 Tang et al134

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3.2.2. Impact on indoor comfort

In a hospital building, RH levels are related to patients’ indoor thermal comfort and hygiene of spaces.35, 128 Low humidity levels can affect patients' indoor comfort perception145, 146 and can cause irritations,63, 146 dry skin and nose,35, 63 and throat irritation.35, 63, 145, 146 Dryness can promote blood coagulation, which is undesirable for patients during surgery.35, 128, 137 Research confirmed that for preterm infants, higher humidity levels along with a warm temperature are recommended to control evaporative heat loss.147, 148 A study confirmed that if the humidity of the incubator decreased below 60%, then the infant body temperature decreased by as much as 1°C (33.8°F) within 5 minutes.148 To reduce evaporative heat loss among newborns, increasing the humidity level is the most effective option.125 Balaras et al128 suggested the recommended RH range for a hospital building should be 30% to 60%.

3.3. Indoor air ventilation

Pathogens and other respiratory viruses, such as influenza,134, 149, 150, 151 SARS‐associated coronavirus (SARS‐CoV),134, 152, 153 tuberculosis,134, 152, 154 Q‐fever, and measles152 can be transmitted through an airborne route. There is sufficient evidence to support that indoor air ventilation can contribute to the spread of airborne infectious disease in hospitals.152, 155, 156, 157, 158, 159, 160, 161, 162 Several literature reviews covering both naturally and mechanically ventilated buildings have examined the impact of ventilation on health outcomes.152, 159, 163, 164, 165, 166 Some of these covered other building types along with hospitals.152, 163, 164, 165

The following subsections summarize suggested ventilation requirements considering airborne pathogen transmission. Primarily, based on previous research findings, this will review the optimum range of ventilation rates or flow rates for positive patient outcomes. Additionally, ventilation strategies, air filtration systems, and desirable room pressurization with respect to the adjacent areas in a hospital building will be reviewed. This review excluded all published standards and guidelines (eg, ANSI/ASHRAE/ASHE Standard 170,167 FGI,168 AIA,169 ASHRAE Handbook,170 2013 ASHRAE HVAC Design Manual for Hospitals and Clinics171). that specify mandatory or recommended requirements including ventilation rates, filtration, and pressure relationships. Additionally, studies on naturally ventilated hospital buildings are being excluded since this review exclusively focuses on parameters related to the mechanical ventilation systems.

3.3.1. Ventilation rate

Ventilation rates are measured as ACH, that is, how many times the air in a defined space is replaced per hour. Several comprehensive literature reviews on ACH and infectious disease transmission concluded that there is insufficient evidence to specify the minimum and maximum ventilation requirements in hospitals based on infection control risk to patients.152, 164, 172, 173 English166 reviewed the ventilation guidelines of US hospitals and concluded that the effect of ventilation requirements on general infection rates is still unidentified except in ORs174 and airborne isolation rooms.175 English166 showed a chronological summary of air change rates in hospitals, where establishing the relationship with patient outcomes was outside of his scope.

Although many studies observed that lower ventilation rates could increase the risk of airborne cross‐infection,154, 160, 174, 175, 176 increasing the ACH only may not always be advantageous for the patients’ well‐being from the infection risk perspective.164, 173, 177, 178, 179, 180, 181, 182 Using numerical modeling through computational fluid dynamics (CFD) analysis, Memarzadeh and Xu177 suggested that in an enclosed mechanically ventilated room (eg, an isolation room) increasing the airflow rate (ie, ACH) may not be the major contributing factor to control infections transmission. Instead, the ventilation system design and the distance from the contaminant source are important factors than flow rate.177 They177 suggested uninterrupted path between the contaminant source and the exhaust to control contaminants. Grosskopf and Mousavi183 had a similar conclusion for general patient and isolation patient test room studies. Results from another study conducted in the field environmental chamber concluded that increasing supply ACH might escalate the airborne infection risk transmission under certain circumstances (eg, position of the source and the susceptible person in relation to the supply and return air grills).179 A study in a simulated two‐bed hospital isolation room with mixing air distribution system showed that the elevated ventilation rates might increase the risk of airborne cross‐infection.173 The exposure level depended on the positioning and distance from the source, and posture of the infected patients.173 The recommended 12 ACH in the present standards and guidelines resulted in draft discomfort within the occupied zone due to higher air velocities.173

Conversely, across‐sectional observational study by Menzies et al154 showed a higher tuberculosis infection risk for healthcare workers in non‐isolation rooms (eg, general patient rooms) with ventilation rates of less than 2 ACH. Another study87 of ventilation performance in patient rooms showed that a ventilation rate of 4 ACH with supplemental heating and cooling would be favorable in terms of thermal comfort, uniformity, and ventilation effectiveness. Results also suggested that six ACH is optimum, and similar conclusions have been found for hospital isolation rooms.87 Through a CFD study, Memarzadeh and Jiang175 suggested that total ventilation rates over 10 ACH will not be effective for Airborne Infection Isolation Rooms (AIIR) since this flow rate did not decrease the exposure to infectious disease transmission. Based on the review on aerosol‐transmitted infections in the isolation room, Tang et al134 recommended minimum 12 ACH, so that the air moves from healthcare workers to a patient. They134 also suggested that placing patients close to the exhaust vent will reduce the cross‐contamination risks. A study following airflow modeling and particle tracking methodologies concluded that an OR with ceiling heights between 2.74 m (9 ft) and 3.66 m (12 ft) should maintain 20‐25 ACH for contamination control.184 A study on 4‐bed patient rooms showed a minor reduction in infectious disease transmission through hand colonization when ventilation rates change from four ACH to six ACH.185 The results of a CFD simulation in the general wards of Hong Kong hospitals showed that a flow rate of nine ACH effectively minimized infection risk of three respiratory viruses.186 Table 3 below summarizes the recommended minimum total ACH of the supply air and outside air comparison between Ninomura and Bartley187 and English.166

Table 3.

Comparison of total ACH of the supply air and outside air recommendation in different hospital building spaces

Different spaces in a hospital building	Minimum total ACH/outside ACH
	Ninomura and Bartley187	English166
Patient room	6‐4 ACH/2 ACH	4 ACH/2 ACH
Labor/Delivery/Recovery/Postpartum	6‐4 ACH/2 ACH	6 ACH/2 ACH
AIIR	12 ACH/2 ACH	–
Emergency rooms and radiology—waiting and triage rooms	12 ACH/2 ACH	–
Procedure rooms/operating rooms	15 ACH/3 ACH	6 ACH/2 ACH
Nursery	–	6 ACH/2 ACH
Anesthetic storage	–	8 ACH/–
Patient corridor	–	2 ACH/–

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Based on results discussed above, it is evident that the current air distribution methods practiced today are insufficient in order to control the spread of infectious disease within a hospital environment. Along with ACH, the contamination risk depends on the (a) positioning and distance of the susceptible person (eg, caregiver; patient) from the infected source; (b) position of both susceptible person and infected source in relation to the supply and return air grills; (c) posture of the infected source; (d) air velocities; and (e) air distribution pattern.

3.3.2. Ventilation strategies

The ventilation strategy and air distribution pattern in a hospital building are correlated with the airborne transmission of infectious agents.152, 158, 188, 189, 190 This section will evaluate the role of various ventilation strategies in removing airborne pathogens from different spaces in hospitals.

CFD analysis found that in patient rooms, displacement ventilation made larger bioaerosols (>10 μm) suspend in the air for longer periods, whereas smaller particles were able to escape the space.191 Another experimental study concluded that in multiple bed patient rooms, the spacing between beds should be farther apart with the displacement ventilation strategy compared with mixing the air.192 Qian et al192 also concluded that the exhaled nuclei droplet from infected patients penetrates long distances during displacement ventilation, and takes longer to dissipate than mixing ventilation strategies.

A comparative experimental study for hospital wards showed that the displacement ventilation system would have higher contaminant concentration than a mixing ventilation system if the auxiliary exhaust is located to the lower part of the wall.193 However, when the exhaust was relocated at the upper part of the wall, the displacement ventilation at 4 ACH showed lower contaminant concentration than traditional mixing ventilation at six ACH.193 Another study with a similar conclusion added that, for better performance, supply air diffusers should be unobstructed and located at a lower level and toilet transfer grilles at a high level.194 Compared with pure mixing ventilation, the displacement ventilation increases the cross‐infection risk 12 times when two persons (source and target) are at face‐to‐face and face‐to‐side position at 0.35 m distance.181 The contaminant concentration profile showed that the displacement ventilation showed higher risk of transmission close to the contaminant source and exhaust, compared with a mixing ventilation system.193, 194, 195 Guity et al194 also concluded that the displacement ventilation delivers much lower contaminant concentration in areas further away from the patient.

Conversely, a study on two floor‐supply‐type ventilation flow patterns showed that unidirectional–upward system was more efficient in removing the smallest droplet nuclei (<1.5 μm), but the single‐side‐floor system was effective at removing large droplets and droplet nuclei.188 Another study on supply air inlet locations showed that the underfloor air distribution system performs better in reducing bioaerosol concentration than the ceiling type and side wall supply systems.196

Experimental test chamber results and Eulerian‐Lagrangian computations revealed that the mixing ventilation system has a positive influence on bioaerosol dispersion.176 A simulation of the hospital ward with a ceiling‐mixing type ventilation system showed that the dispersions of airborne contaminants were significantly affected by the location of the exhaust air vents.197 They also concluded that the decay rate of contaminant concentration is exponential with a complete mixing ventilation system.197 Using an engineering computational technique for the isolation room, researchers have found that the parallel‐directional airflow pattern and staggered air‐supply and exhaust vents positioning can efficiently control infectious disease contamination.198 This study also concluded that the ceiling to floor level ventilation airflow resulted in poor infection control.198 An analytical CFD study in hospital ward showed that when the air was supplied and extracted through the ceiling, it was more effective in removing airborne pathogens compared with other strategies.189

After reviewing 20 ORs, Balaras et al128 summarized that for quicker dissipation of contaminated air, laminar downward airflow was found to be effective with air changes ranging from 3.2 to 58 ACH. Another simulated study showed that positioning the ventilation grills at the ceiling removes the aerosol more quickly than the wall ventilation system.189 For downward air movement, Khalil199 recommended the supply air outlets need to be located at the ceiling and the exhaust inlets on the opposite walls. The optimum location of supply outlets is crucial to reduce the residence time of pollutants efficiently.95 In ORs, laminar airflow can limit surgical‐site infections by lessening the bacterial air contamination.65, 200, 201, 202 Based on routine surveillance data on hospital ORs, Brandt et al8 concluded that ventilation with vertical laminar airflow was associated with a higher risk for severe SSIs. To control the contamination through microbiological organisms, a laminar airflow system promotes high ACH at low supply air speed.200

Based on above discussion, while designing ventilation strategy in any spaces within a hospital building, the posture and distance between two persons (source and target), the location of diffusers (inlet and outlet vents), and air change rates are important factors in reducing contaminant concentration. It is worth to mention that these test results are either simulation‐based or an outcome from the experimental test chamber, which are usually done in a static environment without considering real‐world scenarios (eg, provider traffic).

3.3.3. Air filtration system

A high‐efficiency particulate air (HEPA) filtration system can reduce the load of bacteria, which is the most common cause of hospital‐associated infections203 and other infectious particles.204, 205, 206, 207, 208, 209, 210, 211 HEPA filters were found to be effective for patients having environmental fungal contamination, such as invasive fungal infections (IFIs) caused by construction near hospitals.207, 211, 212, 213, 214, 215, 216 This filtration system can decrease the airborne concentrations of aerosolized pathogens and viruses.217, 218 Full outside air ventilation along with return air through HEPA filtration is capable of reducing the concentration of droplet nuclei with 30%‐90% effectiveness.219 However, HEPA filter within ducts can become a breeding ground of microorganisms which may significantly contaminate the filtered air.64, 220, 221, 222 Lutz et al64 found that the insulation and filter materials are vulnerable to fungal degradation under high humidity. Due to the ineffectiveness of HEPA filters in reducing environmental fungal spore counts,223 researcher emphasized the importance of terminal filtrations system,64 photocatalytic oxidation application,221 and portable HEPA filtration units223 in areas with vulnerable immunocompromised patients.

HEPA filters can add pressure losses, which results in higher fan power, greater water quality control (for legionella), and moisture control due to the impact of moisture content on the survival of pathogens.83, 167 Memarzadeh70 suggested that for effective patient outcomes, these filtration systems should be paired with higher ACH.

3.3.4. Differential pressure control

Differential pressure is important to control the contamination of airborne infectious agents through airflows between the protective and less protective spaces of a hospital building.157, 224, 225, 226 In a hospital building, pressurization or depressurization relative to its surroundings needs to be maintained in the microbiology laboratories, the anteroom to AIIRs, AIIRs, autopsy suites, bronchoscopy rooms, emergency department and radiology waiting rooms, and ORs or surgical rooms.226 AIIRs need to be maintained at negative differential air pressure (“negative” means that the air pressure of the area is lower than the adjacent spaces) to avoid contamination from patients with highly infectious diseases (ASHRAE 170‐2008 as cited in Aliabadi et al161). Sterilizing spaces and service zones, such as laundry and bathrooms, should be negatively pressurized.161 In contrast, surgery rooms should continuously maintain positive differential air pressure to avoid particle infiltration.161, 227 Protective environment rooms for immunocompromised patients (eg, AIDS) need to be kept at positive differential air pressure.161, 168, 224 To control the IAQ, Ninomura and Bartley187 summarized that the critical care; AIIRs; bronchoscopy rooms; and endoscopy rooms should be negatively pressurized with respect to its adjacent areas, whereas, diagnostic and treatment area; ORs; pharmacy; sterile storage; and clean linen storage should be positively pressurized.187

After reviewing international standards, Kao and Yang198 mentioned that the minimum pressure differential requirements between the isolation and non‐isolation zone varied from 2.5 to 30 Pa. US guidelines recommend the minimum pressure difference for the AIIR should be 2.5 Pa (0.01 in. of H₂O).157, 169, 224, 225, 228 The Curry International Tuberculosis Center229 mentioned that a small negative pressure might not be adequately maintained due to external factors, such as fluctuating air currents caused by elevators, doors, or windows to the outside. They suggested a minimum of 7.5 Pa (0.03 in. of H₂O) differential pressure between the isolation room and the anteroom following the recommendation from the California Division of Occupational Safety and Health (Cal/OSHA).229 Depending on the need and restrictions for surgery rooms and adjacent spaces, 5‐15 Pa (0.02‐0.06 in. of H₂O) differential pressure was recommended,227 while others suggested up to 20‐25 Pa (0.08‐0.1 in. of H₂O).230

Experimental tracer gas studies performed in a test chamber simulating an AIIR showed that the pressure differential −15.0 Pa (0.06 in. of H₂O) could effectively reduce the risk of infectious disease contamination.231 According to Streifel,228 the minimum differential pressure for ORs and protective environment rooms should be 0.25 Pa (0.001 in. of H₂O). A multi‐zone airflow simulation study concluded that the leakage in room pressure control could significantly affect contaminant transfer.232 This emphasizes the importance of considering leakage in achieving design pressure differentials.232 Results from the tracer containment testing of AIIR showed that due to people's movement, differential pressure 15 Pa (0.06 in. of H₂O) might not even effectively prevent the migration of air volume.233 Adams et al234 compared the containment efficiency in an anteroom‐equipped hospital AIIR at varied differential pressure (2.5, 11, and 20 Pa) in the presence or absence of care provider movement. They concluded that the higher pressure differential (>2.5 Pa or 0.01 in. of H₂O) would control the contamination effectively with and without provider traffic.234

Based on the above discussion, it is important to note that in a hospital building critically important spaces need to be pressurized or depressurized with respect to its surroundings. To prevent contamination by infected patients, negative air pressure inside the room (eg, AIIR) relative to the surrounding areas should be maintained by mechanically removing (exhausting) more air than is supplied. Conversely, to protect patients (eg, immunocompromised patients in OR) from airborne infectious agents, the room needs to be positively pressurized. While designing and controlling this inlet and outlet airflows disequilibrium in a mechanically ventilated hospital building, the movement of people and leakages through doors, windows, or cracks need to be considered.

4. CONCLUSION

4.1. Summary of literature findings

In a hospital building, the built environment can have a beneficial impact on patients’ healing processes.12, 22, 23, 25, 29, 30, 32, 34, 35, 38, 43 This review covered the published research that has assessed patients’ medical outcomes with respect to at least one indoor environmental parameter related to the mechanical ventilation system of a building including temperature, relative humidity, and overall IAQ. Scientific publications in the fields of both healthcare and building HVAC systems published after the review paper of Rubin et al30 have been included in this review process. Studies related to the natural ventilation system, building design, architecture, interior design, noise, aroma, and lighting were outside of the scope of this review. This review summarized peer‐reviewed papers on how indoor environmental parameters related to the mechanical ventilation systems of a hospital building impact patient outcomes.

Higher indoor air temperature is recommended to control the survivability and transmission of most bacteria,75 fungi,69, 71 and viruses.68, 69, 73, 74, 75, 78 Low ambient OR temperatures are critical for patients, which lead researchers to recommend the optimum temperature to prevent thermal risk, with a minimum suggested temperature for ORs of 21°C (69.8°F),35, 43 and a maximum of 26°C (78.8°F)128, 137, 235 or higher.235 Based on the existing literature, this recommended range varies depending on the types of surgery, patient demographics (eg, age, gender), and OR personnel.

Most bacteria, fungi, and viruses persist longer at higher humidity (eg, >70%) and infectivity lessens exposure in the RH range between 40% and 70%.69 Higher RH (50% RH or higher) is recommended to control the transmission of lipid‐enveloped viruses, for example, influenza.73, 74, 131 However, lower RH (50% RH or lower) is suggested for viruses without a lipid envelope.75 For most gram‐negative bacteria (eg, Escherichia coli; Klebsiella pneumonia), higher RH (50% RH or higher) is detrimental to their survival,69, 75, 134, 144 except a few (eg, Pseudomonas spp., Enterobacter spp., Klebsiella spp., and Salmonella Seftenberg).75, 134 Similarly, higher RH is recommended to control airborne gram‐positive bacteria.75, 134 Figure 2 summarizes the favorable RH ranges for the survivability of nosocomial pathogens and microorganisms. Higher RH is also suggested for hospitals to avoid thermal discomfort145, 146 and other negative consequences due to dryness.35, 63, 128, 145, 146 It is evident from these findings that maintaining certain RH ranges may be detrimental for some microorganisms but favorable for others. Guidelines for humidity ranges in any hospital areas should consider both survival and infectivity of airborne‐transmitted infectious disease control and thermal comfort.

Figure 2.

Diagram of favorable RH ranges for different microorganisms

This review found that ventilation rates, ventilation strategies, air filtration, and differential pressure control can contribute to the spread of airborne infectious disease in hospitals, although specific recommendations are institution specific. The evidence indicates that higher ventilation rates may reduce the infection rate in several situations. However, the maximum required ventilation rates (above which there is no further reduction of infection risk) at different spaces in hospitals are yet unknown,164 and it depends on other parameters, such as air distribution pattern; position and distance of the susceptible person from both source and air diffusers; and position and posture of infected source.134, 173, 177, 179, 183

Results from both experimental and computational studies confirmed that for patient rooms, the mixing ventilation strategies showed better contamination control and lowered the infectious disease transmission risks.176, 181, 189, 192, 193, 194, 195 For multiple bed patient rooms, displacement ventilation strategies may not be suitable unless beds are placed apart; the exhaust is located at the upper part of the wall; and unobstructed supply air diffusers are located at a lower level of the wall. Additionally, the posture and distance between two persons (source and target) have impacts on the contaminant concentration profile. These results are highly dependent on a subject's position and distance from the source of the contaminant181, 194; location of the vents189, 193, 194, 196, 197, 198, 199; and ventilation rates.128, 176 Along with appropriate ventilation strategies, the HEPA filtration system can effectively reduce the contamination load.204, 205, 206, 207, 208, 209, 210, 211, 217, 218, 219 However, maintaining the differential air pressure with respect to the adjacent spaces is very critical since the HEPA filters within ducts may have limited control over airborne nosocomial infections due to contaminated air from adjacent spaces.220 In order to control airborne contamination through pressurization‐depressurization in critical areas of a hospital building, important considerations are leakage in room pressure control232 and provider traffic.233, 234 The comprehensive ventilation guidelines for different zones within a hospital require patient‐oriented evidence incorporating ACH along with the (a) positioning and distance of the susceptible person (eg, caregiver; patient) from the infected source; (b) position of both susceptible person and contaminated source in relation to the supply and return air grills; (c) posture of the infected source; (d) air velocities; (e) air distribution pattern; and (f) location of the air filtration system.

4.2. Recommendations

Based on this literature review, temperature, humidity, and the indoor air ventilation system in hospital buildings affect various infectious organisms, which then have an effect on patient outcomes. Published results contain contradictory findings, which made the comparative assessment difficult due to inconsistency in experimental design, choice of variables, location and settings, demographics, diseases, patients, and the types of outcome measurements. Hence, it is impossible to make evidence‐based decisions regarding the optimum ranges to improve patient‐oriented outcomes such as symptoms, morbidity, quality of life, or mortality. These contradictory results of the current research suggest that all indoor environmental parameters related to the HVAC system need to be measured or included in the comparative analysis of each study. Additionally, a common set of variables need to be defined for comparative analysis.

A few epidemiological studies have been undertaken specifically to investigate the suitable ranges of multiple indoor environmental parameters (eg, temperature, RH, ACH); there is little patient‐oriented evidence to formulate guidelines for hospitals. While extensive simulation‐based research has been performed, very little patient‐oriented evidence has been produced. For validation, simulations and experiments need to correlate by physical measurements. Additional multidisciplinary studies including researchers, patients, building owners, facility managers, and maintenance staffs studies are needed, which would address evidence‐based decisions regarding the optimum ranges to improve patient‐oriented outcomes.

Studies that look at nosocomial infection rates, the spread of infection within hospitals, and associated costs are potential avenues of research. A multidisciplinary study combining available molecular biology testing, advanced computer modeling, experimental testing, and on‐site experimental designs could provide evidence to identify optimum ranges for temperature, humidity, and ACH along with appropriate ventilation design strategies. It is also necessary to address these variables as a function of spaces within a hospital since each zone has unique occupants and different functionality. Additionally, the structural variation of infectious agents (ie, viruses, bacteria, and fungi) may need to be considered separately when investigating airborne survival since each will have differing conditions under which they may be optimally suppressed. Finally, the relationship between IEQ variables, thermal comfort perception of patients, and airborne contamination need to be investigated. The health effects of ventilation in locations with highly polluted outdoor air and other diverse outdoor conditions present an important area of future research.

Shajahan A, Culp CH, Williamson B. Effects of indoor environmental parameters related to building heating, ventilation, and air conditioning systems on patients' medical outcomes: A review of scientific research on hospital buildings. Indoor Air. 2019;29:161–176. 10.1111/ina.12531