The Potential Effects of Wildfire Smoke on COVID-19 Risk and Severity

There may be interactive health effects between the 2 crises of the COVID-19 pandemic and US wildfires.

As the effects of coronavirus disease 2019 (COVID-19) continued into the summer months of 2020, the western region of the United States was met with a series of wildfires in what has been cited as the West Coast’s worst fire season in recorded history.1 In addition to the various challenges involved in responding both to a pandemic and a natural disaster simultaneously, there may be interactive health effects between the 2 crises.

In a paper published in August 2020 in the American Journal of Public Health, Sarah B. Henderson, PhD, senior environmental health scientist at the British Columbia Centre for Disease Control in Vancouver, Canada, warned about the potential effects of wildfire smoke on the risk of COVID-19 infection and severity.2

Wildfire smoke typically contains elevated levels of fine particulate matter (PM2.5) that can persist for periods ranging from days to months. Consistent associations have been noted between PM2.5 exposure and increased risk of respiratory viral infections, with evidence indicating that such exposure “leads to immunosuppression, inflammation, and decreased inactivation of pathogens by macrophages.”2

An earlier Chinese study linked increased air pollution to higher daily mortality rates because of severe acute respiratory syndrome (SARS), and more recent findings identified biomass burning as the PM2.5 component most closely associated with influenza cases.3,4 It is estimated that a moderate episode of wildfire smoke could potentially increase the effect of a COVID-19 outbreak by approximately 10%.2

Additionally, the authors of a 2019 meta-analysis5 observed an increase (risk ratio, 1.06; 95% CI,  1.02-1.09) in asthma-related hospitalizations “for each 10 micrograms per cubic meter increase in PM2.5 from wildfire smoke, which is larger than the expected effect for the typical ambient PM2.5 mixture,” Dr Henderson commented.2 A range of other studies have linked wildfire smoke exposure to increased emergency department visits for symptoms of dyspnea, asthma, chronic obstructive pulmonary disease (COPD), and bronchitis, as well as increased outpatient visits for respiratory symptoms.6

Considering the limited ability to suppress wildfires, cleaner indoor air should be a priority focus to mitigate the effects of wildfire smoke exposure in general and in terms of COVID-19 risk. The Environmental Protection Agency offers a free course to educate physicians on the effects of wildfire smoke and how to help patients reduce its health impact, and the US Centers for Disease Control provides public guidance on protection from wildfire smoke within the context of COVID-19.7,8

The use of portable air purifiers is recommended, with some experts stating that these devices should be at the forefront of the public health response to smoke exposure from fires.9 For individuals who are unable to keep their entire house sealed and filtered, there are guidelines that physicians can share with patients to help them create a designated “clean room” for this purpose.10

We spoke with the following clinicians to gain further insights regarding this issue: Richard J. Castriotta, MD, FCCP, FAASM, pulmonologist, sleep medicine specialist, and professor of clinical medicine at the Keck School of Medicine at the University of Southern California in Los Angeles; and Reza Ronaghi, MD, interventional pulmonologist and critical care physician at the University of California in Los Angeles.

What is known or suspected about the potential effects of wildfire smoke on the risk of COVID-19 infection or severity? 

Dr Castriotta: The contaminants in wildfire smoke, especially the smaller PM2.5 particles that penetrate deeply into the lungs, may impair our natural defense and clearance mechanisms. For this reason, people exposed to wildfire smoke may be more likely to become infected after exposure and might even become more ill after infection.

Dr Ronaghi: Wildfires alone will not affect the risk of contracting COVID-19 or the prognosis from COVID-19 infection, per se. However, because the particles from the fires can embed themselves deep into the lung and cause irritation and inflammation as well as cough and shortness of breath, they may make a coinfection with COVID-19 more severe. Therefore, to prevent these effects it is best to avoid the outdoors in affected areas during fire season. 

What can patients with chronic lung disease do to decrease the risk of an exacerbation if they are in an area with wildfires and high COVID-19 rates? 

Dr Castriotta: Patients with COPD and other chronic lung diseases such as pulmonary fibrosis should stay inside with a good air conditioning system. The use of a HEPA [high-efficiency particulate air] filter for the air conditioner would give added protection. When going outside, individuals should ideally wear an N95 mask for optimal protection.

Dr Ronaghi: The most important thing patients can do is to stay indoors. These particles are very small and thus going outside can allow them to enter the lungs and cause irritation leading to exacerbation, especially for patients with underlying lung disease.

For those staying indoors, it is important to make sure windows and all openings to the outside are closed, and to make sure any cracks or tiny openings are covered with tape or other covering to prevent the particles from entering. It is also important to not vacuum or do anything that would disrupt the particles and cause them to become airborne. 

What should physicians watch out for in their patients — with or without lung disease —in terms of symptoms related to these concomitant crises?

Dr Castriotta: Physicians should be on the alert for changes in smell or taste, increased exercise intolerance, increased shortness of breath, or fever.

Dr Ronaghi: Physicians should look for worsening of underlying lung disease, worsening shortness of breath, and worsening cough. Patients will commonly talk about shortness of breath that is new or worse, cough that is worse, burning eyes, and sore throat. 

What are physicians doing differently for patients with already diagnosed COVID-19 in areas with heavy wildfire smoke? 

Dr Castriotta: Patients with COVID-19 infection in the wildfire areas are treated similarly to those in other areas. However, precautions about leaving the home or hospital are greater.

Dr Ronaghi: The advice is to stay indoors and prevent being exposed to the fire particles. If these patients must go outside, they should wear N95 protective gear to prevent being exposed to the fires and to prevent exposing others to COVID-19. 

How are shelters on the West Coast handling these issues to keep people safe from COVID-19 infection as well as the fires?

Dr Castriotta: The shelters set up for those displaced by wildfires use social distancing, face masks, COVID-19 testing, and other standard procedures. This means there are fewer places for wildfire refugees close to home.


1. Migliozzi B, Reinhard S, Popovich N, Wallace T, McCann A. Record wildfires on the West Coast are capping a disastrous decade. New York Times. September 24, 2020.

2. Henderson SB. The COVID-19 pandemic and wildfire smoke: potentially concomitant disasters. Am J Public Health. 2020;110(8):1140-1142.

3. Kan H-D, Chen B-H, Fu C-W, Yu S-Z, Mu L-N. Relationship between ambient air pollution and daily mortality of SARS in Beijing. Biomed Environ Sci. 2005;18(1):1-4.

4. Croft DP, Zhang W, Lin S, et al. Associations between source-specific particulate matter and respiratory infections in New York state adults. Environ Sci Technol. 2020;54(2):975-984.

5. Borchers Arriagada N, Horsley JA, Palmer AJ, Morgan GG, Tham R, Johnston FH. Association between fire smoke fine particulate matter and asthma-related outcomes: Systematic review and meta-analysis. Environ Res. 2019;179(Pt A):108777.

6. Black C, Tesfaigzi Y, Bassein JA, Miller LA. Wildfire smoke exposure and human health: Significant gaps in research for a growing public health issue. Environ Toxicol Pharmacol. 2017;55:186-195.

7. US Environmental Protection Agency. Wildfire smoke and your patients’ health. Accessed on October 19, 2020.

8. US Centers for Disease Control and Prevention. Wildfire smoke and COVID-19. Accessed on October 19, 2020.

9. Barn PK, Elliott CT, Allen RW, Kosatsky T, Rideout K, Henderson SB. Portable air cleaners should be at the forefront of the public health response to landscape fire smoke. Environ Health. 2016;15(1):116.

10. US Environmental Protection Agency. Create a clean room to protect indoor air quality during a wildfire. Accessed on October 19, 2020.