Air Pollution May Contribute to Severe COVID-19 Outcomes

Pam Harrison

May 26, 2022

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Patients exposed to common air pollutants are at risk for more severe outcomes after they become infected with the SARS-CoV-2 virus, data suggest.

In a prospective study of more than 150,000 patients with COVID-19, incremental increases in exposure to common pollutants were associated with increases in the risks for hospitalization, admission to an intensive care unit (ICU), or death that ranged from 6% to 30%.

Dr Hong Chen

"This study reinforces the idea that air pollution is a pervasive and silent killer and that the association of air pollution and COVID-19 severity reflects the tip of an iceberg of the wide-ranging impacts air pollution has on human health," study author Hong Chen, PhD, MSc, researcher at Public Health Ontario and assistant professor of occupational and environmental health at the University of Toronto, Canada, told Medscape Medical News.

The study was published online May 24 in the Canadian Medical Association Journal.

Focus on Ontario

COVID-19 is associated with a tremendous global health burden. Understanding the factors that influence disease severity is important for prevention and treatment, said the investigators.

Exposure to common air pollutants such as fine particulate matter (PM2.5), nitrogen dioxide (NO2), and ground-level ozone (O3) contributes to the global burden of disease. The current investigators evaluated the associations between exposure to these pollutants and indicators of COVID-19 severity.

To identify patients in Ontario with COVID-19, they examined hospital discharge data from the Canadian Institute for Health Information Discharge Abstract Database and physician service claims in the Ontario Health Insurance Plan database. They used annual exposure surfaces of the three pollutants to calculate participants' long-term exposures.

A total of 151,105 people became infected with COVID-19 in Ontario in 2020. Of this cohort, 5.7% required a COVID-19-related hospital admission, 1.3% required admission to an ICU, and 1.4% died, according to the authors.

The median of long-term exposure to air pollutants was 7.64 µg/m3 for PM2.5, 7.75 ppb for NO2, and 44.80 ppb for O3. In the final, adjusted model, each interquartile range (IQR) increase of 1.70 µg/m3 in exposure to PM2.5 was associated with an odds ratio (OR) of 1.06 for hospital admission and an OR of 1.09 for ICU admission. While death was positively associated with PM2.5, "we did not observe an effect in the fully adjusted model," said the authors.

In the same model, each IQR increase of 2.50 ppb in exposure to NO2 was associated with an OR of 1.09 for ICU admission. The researchers did not observe any effect of exposure on risk for hospital admission or death.

Each IQR increase of 5.14 ppb in exposure to O3 was associated with an OR of 1.15 for hospital admission, an OR of 1.30 for ICU admission, and an OR of 1.18 for death.

Public Health Implications

"We observed that people with SARS-CoV-2 infection who lived in areas of Ontario with higher levels of common air pollutants…were at elevated risk of being admitted to the ICU after we adjusted for individual and contextual confounding factors, even when the air pollution level was relatively low," said the authors.

"Given the ongoing pandemic, our findings that underscore the link between chronic exposure to air pollution and more severe COVID-19 could have important implications for public health and health systems," they added.

Research conducted over the past several decades has identified no threshold level of air pollution that is not associated with adverse health effects, said Chen. "This finding holds true even in Canada, where air pollution levels are among the lowest worldwide," he noted, adding that air quality has progressively improved in Canada over the past few decades.

Several mechanisms could explain the association between long-term exposure to air pollution and COVID-19 outcomes. First, air pollutants can reduce patients' pulmonary immune responses and antimicrobial activities, boosting viral loads. Pollutants also can induce chronic inflammation and overexpression of the alveolar angiotensin-converting enzyme 2 (ACE) receptor, which facilitates the entry of SARS-CoV-2 into cells. Evidence also has shown that exposure to air pollution increases susceptibility to respiratory viral infections and pneumonia, said Chen.

"It has been suggested that people with chronic conditions and socioeconomically disadvantaged subgroups are more likely to have poorer prognosis after the infection, [because] air pollution has long been known to contribute to poor health among these subgroups," said Chen.

"Given that so much was unknown about COVID-19 at the [beginning], there was an unprecedented urgency in understanding who are the most vulnerable to COVID-19 and which factors give rise to severe illness after the infection," he added.

These results support targeted public health actions to protect residents from COVID-19 in heavily polluted regions with historically high NO2 levels, according to the authors.

"Rigorous Statistical Modeling"

Commenting on the findings for Medscape Medical News, Donghai Liang, PhD, MPH, assistant professor of environmental health, Emory University, Atlanta, Georgia, said that this prospective cohort study is important and underscores the link between chronic exposure to air pollution and more severe COVID-19. "The study is well-designed and conducted with rigorous statistical modeling approaches," he added.

Dr Donghai Liang

The impact of ambient air pollution on excess morbidity and mortality has been well established over the past several decades, said Liang. "In particular, major ubiquitous ambient air pollutants, including PM2.5, NO2, and O3 may have both a direct and an indirect systemic impact on the body by enhancing oxidative stress, inflammation, and respiratory infection risk, eventually leading to numerous adverse health effects."

It is therefore possible that long-term exposures to air pollution may have a detrimental effect on COVID-19 outcomes, Liang added. He and his colleagues previously conducted a nationwide study that investigated associations between the same key ambient air pollutants and COVID-19 case fatality and mortality rates in the United States. They reported that long-term exposure to NO2, which largely arises from urban combustion sources such as traffic, may enhance susceptibility to severe COVID-19 outcomes, independent of long-term PM2.5 and O3 exposure.

The study was funded by Health Canada. Chen and Liang have disclosed no relevant financial relationships.

CMAJ. Published online May 24, 2022. Full text

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