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Is there a link between air pollution and COVID-19 severity?


A new study investigates whether long-term air pollution exposure is associated with increased COVID-19 severity.

It’s no secret that air pollution is an issue.  Although rates of greenhouse gas emissions are declining with the implementation of new technologies and environmental regulations, air pollution levels are still significant, particularly in urban cities.1 

Notably, a growing body of evidence suggests that exposure to this air pollution may pose a health risk for citizens.  Several studies have found potential associations between air pollution and asthma, lung cancer, and cardiovascular disease.2,3,4,5  This may be relevant given that about 32% of the Canadian population reside in air pollution exposure zones.2,6 

Air pollution and COVID-19 can both impact the respiratory system; however, it is unknown whether air pollution is associated with increased severity of COVID-19 symptoms or outcomes.7,8  To investigate this, a study was conducted across the province of Ontario, Canada, and the findings were published in the Canadian Medical Association Journal.8 

Researchers investigated all patients in Ontario over the age of 19 with confirmed SARS-CoV-2 infections throughout 2020 by using the Ontario Case and Contact Management System and the Ontario Laboratories Information System.8  To determine COVID-19 severity and outcomes, researchers used rates of COVID-19 hospital admissions, ICU admissions, and related deaths up until May 2021.8 

Researchers investigated participants’ postal codes to determine their geographical locations in the past five years.8  Air pollution exposure, respectively, was measured by the levels of three major components of air pollution in participants’ geographical locations.  The first pollutant was NO2 (nitrogen dioxide), which comes from fuel emissions of cars and other machinery.9  The second pollutant was O3 (ozone), which is beneficial when located very high up in the stratospheric ozone layer.  However, ground-level ozone is a harmful pollutant that can exert negative impacts on human health and the environment.10 

The third pollutant was particulate matter 2.5 (PM2.5), which refers to inhalable airborne particles with individual diameters of less than 2.5 micrometers, and PM2.5 can make the air appear hazy at high concentrations.11  The two variables were examined for a potential correlation.

Overall, the study identified 151,105 confirmed COVID-19 cases in Ontario in 2020.  Within this group, there were 8630 COVID-19 related hospital admissions, 1912 COVID-19 related ICU admissions, and 2137 COVID-19 related deaths.8  One limiting factor of the study was that only confirmed COVID-19 cases were analyzed, so this data does not account for COVID-19 cases without a formal diagnosis.  Additionally, the study did not take into account peoples’ workplaces, which limits accuracy because people may be exposed to different levels of pollutants at their jobs compared to their residential addresses.

In the study group, after adjusting for confounding variables, nitrogen dioxide exposure was associated with an increased risk of ICU admission, but not hospital admission or death.  PM2.5 exposure was associated with increased risks of both hospital and ICU admissions, but not COVID-19-related deaths.   Finally, ozone exposure was associated with an increased risk of all three adverse outcomes. 

The results of this study suggest that elevated exposures to all three pollutants were associated with an increased risk of COVID-19-related ICU admissions.  Moreover, the study suggests potential associations between the risk of hospital admissions and exposures to PM2.5 and ozone, as well as the risk of COVID-19 related death and ozone exposure.8  More research is needed to confirm the validity of these findings, and more research is needed to determine whether these same findings would take place outside of Ontario.


  1. Statistics Canada (2022 May 31). Air and climate – Key indicators. Government of Canada. Accessed 2022, May 31, from https://www150.statcan.gc.ca/n1/en/subjects/environment/air_and_climate
  2. Traffic-related air pollution: a critical review of the literature on emissions, exposure, and health effects. Final Version of Special Report No.17. Boston (MA): Health Effects Institute; 2010. Available: http://pubs.healtheffects.org/view.php?id=334
  3. Anderson, H., Favarato, G., Atkinson, R.W. (2013). Long-term exposure to air pollution and the incidence of asthma: meta-analysis of cohort studies. Air Qual Atmos Health 1: 47-56
  4. Benbrahim-Tallaa L., Baan, R.A., Grosse, Y., et al; International Agency for Research on Cancer Monograph Working Group (2012). Carcinogenicity of diesel-engine and gasoline-engine exhausts and some nitroarenes. Lancet Oncol 13: 663-664.
  5. Brook, R.D., Rajagopalan, S., Pope, C.A., et al; American Heart Association Council on Epidemiology and prevention, Council on the Kidney in Cardiovascular Disease, and Council on Nutrition, Physical Activity, and Metabolism. (2010). Particulate matter air pollution and cardiovascular disease: an update to the scientific statement from the American Heart Association. Circulation 121: 2331-2378.
  6. Brauer, M., Reynolds, C.C.O., Hystad, P. (2012). Traffic-related air pollution and health: a Canadian Perspective on scientific evidence and potential exposure-mitigation strategies. Vancouver (BC): University of British Columbia.
  7. Bourdrel, T., Annesi-Maesano, I., Alahmad, B., et al (2021). The impact of outdoor air pollution on COVID-19: a review of evidence from in vitro, animal, and human studies. European Respiratory Review 30: 200242. Doi: 10.1183/16000617.0242-2020
  8. Chen, C., Wang, J., Kwong, J., et al (2022, May 24). Association between long-term exposure to ambient air pollution and COVID-19 severity: a prospective cohort study. CMAJ 194(20): E693-E700. Doi: 10.1503/cmaj.220068
  9. United States Environmental Protection Agency (2021, June 7). Nitrogen Dioxide (NO2) Pollution. Accessed 2022, June 1, from https://www.epa.gov/no2-pollution/basic-information-about-no2#:~:text=Nitrogen%20Dioxide%20(NO2)%20is,from%20the%20burning%20of%20fuel.
  10. United States Environmental Protection Agency (2021, May 5). Ground-level Ozone Pollution. Accessed 2022, June 6, from https://www.epa.gov/ground-level-ozone-pollution/ground-level-ozone-basics
  11. United States Environmental Protection Agency (2021, May 26). Particulate Matter (PM) Pollution. Accessed 2022, June 6, from

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