Study finds long-term exposure to common air pollutants is associated with COVID-19 severity

As of May 29, 2022, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has infected over 525 million worldwide and caused over 6.2 million deaths. SARS-CoV-2 infection can cause a variety of symptoms, which can include multisystem organ failure and death in a subset of patients.

Study: Association between long-term exposure to ambient air pollution and COVID-19 severity: a prospective cohort study. Image Credit: Lemberg Vector studio /


In order to determine which populations should be prioritized for COVID-19 screening, immunization, and treatment, it is critical to determine which risk factors contribute to COVID-19 severity. To date, male sex, advanced age, the presence of underlying comorbidities, and certain ethnic populations have been considered risk factors for COVID-19 severity.

Prolonged exposure to ambient air pollution, which contributes significantly to the global disease burden, may also increase the likelihood of severe COVID-19 outcomes through various mechanisms. For example, the individuals' pulmonary immune responses and antimicrobial activity are reduced by air pollution, which increases virus loads.

Chronic inflammation and upregulation of the alveolar angiotensin-converting enzyme 2 (ACE-2) receptor, which increases SARS-CoV-2 entrance into cells, can also be caused by air pollution. Due to ongoing immunological activation and excessive amplification of cytokine release, air pollution also contributes to chronic diseases such as cardiovascular disease, which are also associated with a poor COVID-19 prognosis. 

In a recent prospective study published in the CMAJ, researchers evaluate the relationship between long-term exposure to three common air pollutants and key indicators of COVID-19 severity.

About the study

In 2020, the authors of the current study created a population-based cohort that included all individuals with verified SARS-CoV-2 infection who were 20 years or older and did not live in a long-term care institution in Ontario, Canada. Residents of long-term care homes were excluded from the current study, as their fragility and exposure to air pollution differed from the general population.

The researchers used information from Ontario's Case and Contact Management System and Ontario Laboratories Information System, which included information on the date of specimen collection, demographics, and socioeconomic status of people with SARS-CoV-2 infection, as well as the frequency of COVID-19-related hospital admission, intensive care unit (ICU) admission, and death.

Study findings

A total of 8,630 hospital admissions, 1,912 ICU admissions, and 2,137 deaths attributable to COVID-19 were reported among 151,105 patients infected with SARS-CoV-2 in Ontario, Canada in 2020. The median period from the initial diagnosis and admission to the hospital, ICU hospitalization, and mortality was five, eight, and 15 days, respectively.

For fine particulate matter (PM2.5), nitrogen dioxide (NO2), and ground-level ozone (O3) air pollutants, the median long-term exposure was 7.64 g/m3, 7.75 parts per billion (ppb), and 44.80 ppb, respectively. When contextual factors associated with air pollution and susceptibility to testing positive for COVID-19 were adjusted, odds ratios (ORs) of 1.06 and 1.09 for PM2.5 were obtained for hospital admission and ICU admission, respectively. Death was positively associated with PM2.5 levels, except in the fully adjusted model.

Sequential results were obtained for NO2 exposure, wherein an OR of 1.09 per interquartile range (IQR) increase of 2.50 ppb NO2 was obtained for ICU admission in the fully adjusted model. NO2 levels did not have a significant influence on hospital admission or mortality.

In the partially adjusted models, no association between O3 levels and COVID-19 outcomes was identified. However, in the fully adjusted model, O3 exposure was associated with an elevated risk for hospital admission, ICU admission, and mortality, with ORs of 1.15, 1.30, and 1.18, respectively, per IQR increase of 5.14 ppb.


After adjusting for individual and contextual confounding factors, the study findings indicate that people with SARS-CoV-2 infection who lived in areas of Ontario with higher levels of common air pollutants had an increased risk of being admitted to the ICU, even while the air pollution level was minimal. The authors also discovered that persistent exposure to PM2.5 and O3 increased the risk of COVID-19-related hospitalization and death.

Taken together, these findings demonstrate that chronic air pollution exposure before SARS-CoV-2 infection, particularly chronic O3 exposure, may contribute to COVID-19 severity.

Journal reference:
Colin Lightfoot

Written by

Colin Lightfoot

Colin graduated from the University of Chester with a B.Sc. in Biomedical Science in 2020. Since completing his undergraduate degree, he worked for NHS England as an Associate Practitioner, responsible for testing inpatients for COVID-19 on admission.


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