Examining the Impact of Air pollution, Climate change, and Social Determinants of Health on Asthma and Environmental Justice

Abstract

Purpose of review:

In this review, we discuss the current literature examining the impact air pollution and climate change has on asthma onset, control, and exacerbation. This review also addresses the risk of exposure to specific disproportionately affected communities, highlighting health disparities in exposure and asthma outcomes.

Recent Findings:

Recent studies have shifted from highlighting the associations between asthma exacerbations and indoor and outdoor air pollution. Studies are now focused on confirming the association of asthma incidence from these same exposures. Many studies have linked particulate matter to adverse asthma outcomes, however, the pollutant exposures that pose the greatest risk and the effect of natural disasters fueled by climate change are under current study. Some studies have observed that the true burden that pollutant exposures have on asthma outcomes occurs at the intersection of exposure and vulnerability. Future studies in this area will address social determinants of health, societal factors such as redlining and other systemic racism practices.

Summary:

Although decades of research support the causal link between air pollution (gaseous and particulate) and the exacerbation of pre-existing asthma, recent studies suggest air pollution can cause incident (new onset) asthma. Studies have started to focus on the underlying drivers of poor outcomes in asthma. Many of the structural impediments to high quality asthma care at the society level (e.g. poverty, redlining, systemic racism) also are risk factors for worsened climate events and air pollution exposure. The individuals in these disproportionately affected groups are doubly affected by worsened exposure and worsened access to care for the resultant asthma exacerbations or incident asthma. More research is needed to understand the specific climate and air pollution mitigation efforts where disproportionately affected communities would derive the most benefit.

Introduction:

In the setting of steadily accelerating climate change, there has been an increase in asthma prevalence and healthcare burden from asthma exacerbations and other asthma related care [1^◼] With more than 262 million individuals around the world affected by asthma, the personal financial cost burden of worsened asthma can lead to personal hardship, particularly in low- and middle-income countries [2,3,4^◼]. This financial burden is one social determinant of health which can prevent individuals from achieving their full health potential. In addition, due to perpetual social inequity, many of these families reside in heavily polluted communities, largely urban, as a result of long-standing redlining and other discriminatory practices. Environmental air pollution has gained significant recognition in recent years due to its effect on long-term morbidity and mortality [4^◼]. Considering the impact of air pollution on asthma is particularly timely as roughly 99% of the world’s population are exposed to air pollution at concentrations above the recommended guidelines set by the World Health Organization (WHO)(with the majority living in urban areas) [3]. In the setting of the now well-established link between air pollution and asthma exacerbations and the growing understanding of air pollution and asthma incidence, advancing the understanding of how climate change and environmental injustice can negatively impact asthma is an essential next step. In this review, we will summarize the current literature from July 2022-December 2023 on air pollution, climate and asthma with a particular emphasis on environmental justice.

Air Pollution and Asthma

Asthma exacerbations and asthma incidence

Air pollution can trigger exacerbations of existing obstructive lung disease including asthma and COPD and even potentially cause both asthma and COPD over time [5]. For patients with existing asthma, air pollution can lead to exacerbations directly through airway inflammation caused by the oxidative potential of particulate matter and other factors [6^◼]. The link between air pollution exposure and asthma incidence was long suspected, but required birth cohorts with longitudinal monitoring of air quality and lung function to become firmly established [6^◼]. In a recent review, PM_2.5, NO₂, O₃, SO₂, and CO were shown to impair lung development and increase the risk of early onset asthma [6^◼]. Reducing the ambient concentrations of air pollution has the potential to decrease the incidence of asthma. In a longitudinal study of NO₂ and incident asthma by Garcia et al in 4,140 children, they observed an incidence rate ratio (IRR) for asthma of 0.80 (95% CI, 0.71–0.90) for a median reduction of 4.3–parts-per-billion, with an absolute incidence rate of 0.83 cases per 100 person-years. For reductions in PM2.5, the study observed an IRR of 0.81 (95% CI, 0.67–0.98) for a median reduction of 8.1-μg/m³ with an absolute incidence rate decrease of 1.53 cases per 100 person-years [7]. The link between air pollution and asthma incidence is thought to be driven by excessive activation of type 2 immune responses to aeroallergens, as well as adverse effects of air pollution on lung development leading to alterations in airway size and structure [8] Consequently, these perturbations result in airway inflammation, airway remodeling, and subsequently reduced lung function [6^◼,8]

Outdoor Air Pollution Components and Asthma:

Recent studies have highlighted the respiratory health risk of multiple ambient pollutants and the effect of multiple additional factors including location, individual habits, genetic characteristics and climate change [4^◼]. Over 80% of individuals who reside in urban areas (particularly within communities of color) are exposed to air pollutant levels exceeding the recommended limits [9^◼]. Of the major pollutants regulated worldwide, the primary pollutants that pose the greatest risk to respiratory health include particulate matter (PM), (including black carbon (BC) and ultrafine particles) and gaseous substances. These gaseous substances include nitrogen (NOx) and sulfur (SOx) oxides, ozone (O₃), carbon monoxide (CO), and volatile compounds. PM and gaseous substances induce oxidative stress and subsequently produce reactive oxygen species severely impacting respiratory health [6^◼]. Specifically, a recent study in France observed worsening of asthma symptom score associated with annual exposure to PM2.5, BC and NO2.¹⁰ The worsened asthma control related to air pollution exposure leads to increased healthcare utilization in children and adults for asthma [6^◼]. In urban areas, traffic related air pollution (TRAP) is a particular focus. A recent systematic review and meta-analysis evaluated ambient air pollutants and the risk of immunoglobulin E-mediated allergic disease [11]. This study by Wang et al (2022) observed a significant association between long term exposure to PM_2.5, NO₂, and CO and the development of allergic rhinitis (AR) and atopic dermatitis (AD) [11]. The observation that an increase in NO₂ and CO exposure was associated with a significant elevation in IgE and subsequent allergic diseases, suggested a potential association with allergic asthma as well [11]. The potential risk of outdoor air pollution is enhanced given the significant contribution of outdoor air pollution to indoor air quality.

Indoor Air Pollution and Asthma

As individuals remain indoors for roughly 80–90% of their day in the United States, indoor air pollution exposure becomes increasingly relevant. It is estimated that more than 3 billion people are exposed to high levels of household air pollution, with a significant number being found within communities of color [9^◼]. Indoor air pollutants typically include those generated from the burning of solid fuels, incense, household mold, dust and allergens, as well as tobacco combustion. Additionally, the risk of adopting a sedentary lifestyle by staying indoors can increase the risk of developing obesity and subsequent obesity associated asthma [8^◼]. The use of household cleaners, paints, fragrances and intrusion of outdoor particulates are major sources of indoor volatile organic compounds (VOC) [12]. VOCs are chemicals that lead to airway irritation, wheezing, breathlessness, and asthma exacerbation. In a recent article by Wang et al [12], the authors studied the associations between VOCs in dwellings and adult asthma symptoms and lung function in three North European cities (Reykjavik, Uppsala and Tartu) [12]. In this study, indoor concentrations of nine VOC compounds were associated with adult asthma symptoms or airway obstruction as evident by reductions in FEV₁ and the FEV₁/FVC ratio [12]. Gas stoves have also been studied as a potential factor for poor indoor air quality. The Gruenwald et al (2022) study estimated that 12.7% of childhood asthma could be attributable to gas stove exposure [13]. While NO₂ from gas stove use is an airway irritant that can exacerbate asthma and could potentially lead to incident asthma, there are many factors that could modify the strength of this association including quality of the gas stove, home quality and ventilation along with the underlying severity of asthma [14].

It is widely accepted that tobacco exposure via second-hand smoking in the early years of life increases the incidence of early asthma development. A study conducted by Coogan and colleagues examined active and passive smoking and the incidence of asthma from 1995 to 2011 [15]. In this study 1,523 persons reported incident asthma with multivariate Hazard Ratios (HR) for former active smoking, current active smoking, and passive smoking of 1.36 (95% CI, 1.11–1.67), 1.43 (95% CI, 1.15–1.77), and 1.21 (95% CI, 1.00–1.45) compared with never active/passive smoking [15]. Another study examined second-hand smoke exposure and its associated risk of respiratory symptoms, asthma, and COPD [16]. Individuals who had been exposed to second-hand smoking lifelong, in adulthood only, or in childhood only versus non-exposed had increased odds ratios (OR) for asthma of 1.36 (1.14–1.63), 1.49 (1.09–2.05), and 1.13 (0.99–1.30) [16]. The potential risk of incident asthma associated with the increase in youth use of electronic combustible cigarettes in the past decade is currently an area of active research. In addition to primary use, the health effects of secondhand exposure to aerosols in the indoor environment is not fully understood. While these studies are centered in high income countries, patients in low- and middle-income countries suffer similar, if not enhanced, risk from outdoor and indoor air quality [17, 18]. Screening for potential threats to indoor air quality is an important area for clinicians to both screen and counsel their patients.

Climate change and Asthma:

Due to a dramatic increase in industrial and anthropogenic activities, there has been a significant increase in outdoor air pollution. The burning of fossil fuels generates CO₂, PM, NO₂, SO₂, and ozone [19]. As pollutants such as carbon dioxide, methane and nitrous oxide are also greenhouse gases, the increase in air pollution has also led to an increase in global temperatures. The increase in global temperatures has led to an increase in the frequency and intensity of extreme weather events such as heat waves and natural disasters like hurricanes and storms [5]. In addition to the particulate matter and gaseous exposures, extreme heat is a particularly important exposure that threatens respiratory health.

Heat:

Climate change has led to a steady increase in the global atmospheric temperature. In the last 10 years, the number of record warm days and nights have increased, with the last 7 years being marked as the hottest years in recorded history [5]. Research has shown that breathing extreme heat and humidity increases bronchoconstriction and cough in patients with asthma [19]. Increasing temperatures also have been linked to earlier and longer pollen seasons as well as increased frequency and intensity of hurricanes and thunderstorms, which can precipitate thunderstorm related asthma [5]. Urban areas are particularly susceptible to these extreme changes due in part to increased ambient heat. The urban heat island effect is caused by a combination of building density, proximity to roads, lack of tree canopy, and human related air pollution and leads to an increased all-cause mortality and worsened asthma outcomes in the populations living in these areas [19]. It is shown that communities most at risk are those with high airway sensitivity (i.e. asthma, reactive airway disease) and low adaptive capacity (determined by social determinants of health) [19]. Many of the communities of color and low income communities are located in urban environments as a result of current societal inequity.

Wildfires:

The increase in frequency and intensity of wildfires is a direct result of climate change [20].The emissions from wildland fires are a complex mixture of toxic particulates and gases, owing the differences in fuel including woodlands and residential or industrial areas consumed in the fire [21^◼◼, 22]. As a result of these exposures, there have been reported increases in emergency department visits and hospitalizations related to respiratory illness, with the greatest increases seen in children and elderly with asthma [19]. In a recent study evaluating the impact of the June 2023 New York City episode of wildfire smoke exposure on asthma, that the authors observed PM_2.5 concentrations greatly exceeding the daily US ambient air quality standard [23]. On the day of the PM_2.5 peak, it was noted to be the highest asthma ED visit day up to that point in the year, with the effect most pronounced in adults 18–64 years old [23]. More surprisingly, however, the authors note the visits were only 10% higher than earlier high pollen period days [23]. Higher temperatures in urban areas, longer pollen seasons, along with common (and exceptional) pollutant exposures pose a threat to asthma control in populations who are disproportionately affected by climate change and air pollution [24].

Environmental Justice:

The Problem:

One of the most important recent changes in the climate, air pollution and asthma literature is the recognition of environmental injustice. Specifically, one important environmental justice issue is the disproportionate exposure of communities of color and low-income communities to air pollution and the harmful effects of climate change. Environmental justice is defined as “the fair treatment and meaningful involvement of all people regardless of race, color, national origin, or income, with respect to the development, implementation, and enforcement of environmental laws, regulations, and policies” [25]. To date, communities of color and low-income populations experience significant deterioration in their health and well being due to structural inequality and systemic racism [26^◼]. The individual contributions from the physical and social environments to health inequity has been an understudied and underappreciated threat to respiratory health. Thus, understanding the impact of environmental injustice on asthma morbidity deserves particular consideration.

Structural Racism: Redlining

Residential segregation remains a leading driver in the perpetuation of respiratory health disparity. Through the implementation and continued practice of historical redlining, racial and ethnic minority groups generally reside in neighborhoods that have been neglected by society, leading to concentrated poverty, isolation, and high crime rates [24]. In addition, these neighborhoods are more likely to have industrial zones resulting in an increased exposure to environmental hazards in a population already facing reduced access to asthma services [24]. In a recent study evaluating redlining practices, air pollution, and asthma in Pittsburgh, PA, researchers found that the proportion of Black families in each neighborhood was inversely related to superior Home Owners Loan Corporation (HOLC) grades, with rates of uncontrolled and/or severe asthma becoming progressively higher as the ‘desirability’ of the housing grades decreased [9^◼]. Unfortunately, despite this increase in uncontrolled asthma, there was no increase in asthma specialist care and there was lower use of asthma biologic therapy in these areas [9^◼]. This suggests not only suboptimal management but also discriminatory practices that are deeply embedded in society.

Environmental Injustice:

Over the years there have been different policies enacted to improve low-income and impoverished areas. However, gentrification and the subsequent property tax increases that follow have to led to the displacement of predominantly families of color from their existing homes into neighborhoods with more exposure to environmental hazards. According to Burbank et al (2023), people of color and low-income families are more likely to live in poor housing compared with White and higher-income families [27]. These poor living conditions are associated with a higher prevalence rate of allergic diseases due to in part to exposure to higher levels of indoor allergens and pests [27]. In addition, in a recent study evaluating air pollution and mortality in the context of race and social class by Josey et al (2023), the authors observed that lowering PM_2.5 exposure from 12 µg/m³ to 6µg/m³ was associated with a decreased hazard ratio for death [28^◼◼]. Thus, it was concluded that both high and low income Black patients and low-income White patients may benefit more from lower PM than more affluent White communities. Thus, improving air quality has the potential to improve mortality in low income and Black communities and help mitigate environmental injustice.

Conclusion:

Air Pollution and climate change are important factors that have negative impacts on respiratory health. Through this review, we have highlighted the impact of air pollution, climate change and environmental injustice on asthma, including increase in asthma incidence, poor symptom control, exacerbations and mortality. As climate change and air pollution are in a worsening cycle, the strategies needed to address these risks are desperately needed. Disproportionately affected populations including communities of color and low-income communities are at a higher risk for exposure to environmental hazards and subsequent poor health outcomes. Environmental injustice is a threat to health and multidisciplinary solutions are needed to address the structural inequities in society and healthcare that perpetuate these disparities.

Key Points.

• Climate change and air pollution are in a worsening feedback loop.

• In addition to the well-established risk of exacerbating existing asthma, a growing body of literature supports the risk of incident asthma from particulate and gaseous air pollution exposure.

• While outdoor air pollution is an established risk to asthma, the evidence base for the risk of indoor air pollution to asthma outcomes is growing.

• Increased ambient temperatures and wildfires are two risks for asthma exacerbation that climate change is expected to worsen.

• Further research focused on environmental justice is needed to mitigate the impact of social determinants of health on asthma outcomes.