Ambient Air Pollution Is Associated with Increased COVID-19 Incidence

By Connor Castillo, USC

The USC study of ambient air pollution during four 2020-2021 case surges found that long-term exposures to PM2.5 and NO2 were associated with increased COVID-19 incidence.

Over the past two years, both COVID-19 and air pollution have had major impacts on the health and livelihoods of Southern California residents. The study, published in the Environmental Research journal,  demonstrates the ever growing need to take into consideration the adverse effect of air pollution on the COVID-19 pandemic. Specifically, there are consistent  associations between ambient air pollution and COVID-19 incidence during four major case surges from 03/01/2020 to 02/28/2021. Key findings demonstrate that both long-term and short-term exposures to fine particles (PM2.5 )and nitrogen dioxide (NO2 )are associated with higher incidence of COVID-19 infections in Southern California. The observed findings were adjusted to ensure that the association was only dependent on air pollution, and not other factors such as socio-demographics, neighborhood transportation use and population density.

This study was built upon  446,440 COVID-19 cases from electronic medical records (EMR) of Kaiser Permanente Southern California which hosts around 20% of the southern California population. What makes this study unique and advantageous is that it is one of the largest studies done in the United States investigating the association between  ambient air pollution in COVID-19 incidence in a large and diverse population with multiethnic and socioeconomic backgrounds. 

This is the first study published assessing effects of both long-term and short-term air pollution exposures across four case surges before February 2021 when intervention policy, vaccination campaign, and air pollution levels changed dynamically. Findings of this study are consistent with results from other studies in Israel and in a small area of Los Angeles county where long-term PM2.5 and NO2 exposures were found to be associated with increased COVID-19 incidence. 

Potential Causes and Future Steps

Ambient air pollution has been known for its adverse effect on lung inflammation, as well as the increased risk of conditions that can cause severe COVID-19 including asthma, cardiovascular disease, diabetes, and obesity. Fine particles in the ambient air help to transport viruses to longer distances. Moreover, results from animal models also suggested that exposures to these air pollutants may increase the expression of the coronavirus binding receptor, angiotensin-converting enzyme 2 (ACE-2), allowing for the virus to become more infectious. Therefore, in this study, researchers hypothesize that ambient air pollution exposure is associated with higher COVID-19 incidence in the large population. As this study focused on the pandemic period before February 2021, future research is needed to confirm the association of ambient air pollution and COVID-19 infections from 2021 to 2022 when new variants such as delta and omicron are highly prevalent. 

In conclusion, “Improving air quality can significantly improve the prevention of COVID-19 infection and incidence in the future,” said Dr. Zhanghua Chen, an environmental epidemiologist, biostatistician, and co-author of this study. “We must continue to reduce air pollution and improve air quality.”

Authors of this study include Margo A. Sidell a, Zhanghua Chen b, Brian z. Huanga,b, Ting Chowa, Sandrah P. Eckelb, Mayra P. Martinez a, Fred Lurmannc , Duncan C. Thomasb, Frank D. Gilliladb, Anny H. Xianga.

aDepartment of Research and Evaluation, Kaiser Permanente Southern California, Pasadena, CA, USA

bDepartment of Preventive Medicine, Keck School of Medicine, University of Southern California, Los Angeles, CA, USA

C Sonoma Technology, Inc., Petaluma, CA, USA

M.A.S., Z.C., F.D.G., and A.H.X. were responsible for the study concept and design. A.H.X. and Z.C. obtained funding. A.H.X., Z.C., M.A.S., B.Z.H., T.C., S.P.E., M.P.M., F.L., D.C.T., and F.D.G. conducted the study. B.Z.H., M.A.S., T.C., M.P.M., and A.H.X., acquired data. B.Z.H, M.A.S., T.C., and A.H.X. analyzed data. M.A.S., Z.C., B.Z.H, and A.H.X. drafted the manuscript. All authors revised the manuscript for important intellectual content and approved the final version to be published.


This study was supported by the National Institute of Environmental Health Sciences (3R01ES029963-01 to AHX and ZC) at the National Institutes of Health, and the Keck School of Medicine Department of Preventive Medicine COVID-19 Pandemic Research Center (CPRC) at the University of Southern California. The funding agencies had no role in the design or conduct of the study; in the analysis or interpretation of the data; or in the preparation, review, or approval of the manuscript.

The project protocol has been reviewed and approved by the Institutional Review Board at Kaiser Permanente Southern California and the University of Southern California.  ©2022