California wildfires: impact of air pollution during the covid-19 pandemic

The sky glowed a dark, sinister orange in San Francisco on 9 September; a thick layer of smoke blocking daylight like a solar eclipse (or like a scene from Bladerunner 2049 as some astute twitter users noted). [1] With windows and bathroom vents taped closed, and children at home trying to concentrate on Zoom lessons, we monitored evacuation warnings and prayed that the electricity outages would skip us. We lived in hot, stagnant disbelief as eight days passed until we were able to open a window. 

We are living through the worst wildfires in recorded history. With several months before any expected rainfall, over 4.4 million acres have burned across 10 states, 3.3 million in California alone. [2,3] Dozens have died, over 4,000 structures have been destroyed and almost half a million people have fled their homes. [3] This is a tragedy of staggering proportions. Sadly, it is potentially just the beginning of an even bigger public health crisis.

The risks of chronic exposure to air pollution are well known—with outdoor air pollution estimated to cause 4.2 million premature deaths worldwide each year primarily from lung cancer, cardiovascular disease, and chronic respiratory problems.[4] Exposure to wildfire smoke is associated with respiratory disease including asthma and COPD exacerbations, cardiovascular outcomes including myocardial infarctions, arrhythmias and strokes as well as all-cause mortality in hemodialysis patients. [5-7] 

It is easy to dismiss the risks of smoke as a long-term problem of chronically polluted cities. “I just don’t worry about the air, or things that might affect me in a few decades, like eating red meat and exercise” a friend mentioned during one of these smoky days. But the health effects of wildfire smoke can be immediate, and affect young adults, in addition to older people who are even more susceptible. For example, among California residents exposed to wildfire smoke across 14 counties between 2015-2017, the risk of out-of-hospital sudden cardiac arrest was significantly higher within three days of exposure to heavy smoke. [8] Middle-aged adults (35-64 years) had almost double the risk of cardiac arrest on the day of exposure to high smoke levels, compared to those unexposed. Another recent study demonstrated that ambulance calls for both respiratory and cardiac distress increase within one hour of wildfire smoke exposure. [9] Moreover, pregnant women are more likely to have a preterm birth especially if exposed during their second trimester, whereas exposure during the first trimester is associated with decreased birth weight. [10]

The covid-19 pandemic and the impending flu season, make this particular wildfire season especially worrying, because of possible additive effects. Wildfire smoke decreases our body’s ability to fend off pathogens: it both paralyses respiratory cilia leaving the respiratory system without its first line of defense and causes overall immune dysregulation. For example, exposure to wildfire smoke in healthy teenagers causes an increase in proinflammatory cytokines IL1-B and C-reactive protein, children exposed to wildfire experience increased methylation of the Foxp3 gene, a transcription factor for T regulatory cells, and alveolar macrophages are less able to fight off subsequent viral and bacterial pathogens. [11-13] These immune effects may make subsequent virus exposures, including covid-19, more dangerous. 

Emerging research shows that the death rate from covid-19 is significantly higher in polluted areas. In one study of over 3,000 counties in the United States an increase of only 1 ug/m3 of PM2.5 was associated with an 8% increase in the covid-19 death rate. [14] Another analysis of data from 2009-2018 in the state of Montana showed that higher PM2.5 concentrations from wildfire were associated with a 16-22% increase in influenza rates months later. [15] Biologically, it makes sense that the increase in pollution from our current wildfires will cause a subsequent  increase in covid-19 rates. One model predicts that a wildfire of moderate magnitude has the potential to increase the incidence and deaths from covid-19 by 10%. [16]  

It is impossible to separate this public health crisis from the root causes of climate change and the pervasive inequalities that persist across the United States. Those most vulnerable, including people from ethnic minorities and those of lower socioeconomic status, simultaneously experience higher rates and severity of covid-19 infection, higher exposures to air pollution and wildfires, and less access to healthcare. 2020 is the year when social, environmental and health emergencies have collided and are now impossible to ignore. Doctors and scientists must lend their voices and credibility to broader environmental and social movements and use their vote and political advocacy for necessary change. 2020 must be our year for collective action.

Mary Prunicki, senior research scientist, director of Air Pollution and Health Research, Sean N. Parker Center for Allergy Research at Stanford University, Stanford University School of Medicine

Eleni Linos, professor of dermatology and epidemiology, Stanford University School of Medicine

Vanessa Nava, research assistant, Department of Dermatology, Stanford University School of Medicine

Eric M. Smith, research assistant, Sean N. Parker Center for Allergy Research, Stanford University School of Medicine

Competing interests: None declared. 


1 Emma, Nolan. Bay Area’s Orange Skies From California Fires Compared to ‘Blade Runner 2049’. Newsweek. 2020.

2 National Fire News. National Interagency Fire Center 2020.

3 ​California Statewide Fire Summary September 16, 2020. Cal Fire 2020.

4 Schraufnagel DE, Balmes JR, Cowl CT, et al. Air Pollution and Noncommunicable Diseases. Chest 2019;155:409–16. doi:10.1016/j.chest.2018.10.042

5 Delfino RJ, Brummel S, Wu J, et al. The relationship of respiratory and cardiovascular hospital admissions to the southern California wildfires of 2003. Occupational and Environmental Medicine 2009;66:189–97. doi:10.1136/oem.2008.041376

6 Wettstein ZS, Hoshiko S, Fahimi J, et al. Cardiovascular and Cerebrovascular Emergency Department Visits Associated With Wildfire Smoke Exposure in California in 2015. JAHA 2018;7. doi:10.1161/JAHA.117.007492

7 Xi Y, Kshirsagar AV, Wade TJ, et al. Mortality in US Hemodialysis Patients Following Exposure to Wildfire Smoke. JASN 2020;31:1824–35. doi:10.1681/ASN.2019101066

8 Jones CG, Rappold AG, Vargo J, et al. OutofHospital Cardiac Arrests and WildfireRelated Particulate Matter During 2015–2017 California Wildfires. JAHA 2020;9. doi:10.1161/JAHA.119.014125

9 Yao J, Brauer M, Wei J, et al. Sub-Daily Exposure to Fine Particulate Matter and Ambulance Dispatches during Wildfire Seasons: A Case-Crossover Study in British Columbia, Canada. Environ Health Perspect 2020;128:067006. doi:10.1289/EHP5792

10 Abdo M, Ward I, O’Dell K, et al. Impact of Wildfire Smoke on Adverse Pregnancy Outcomes in Colorado, 2007–2015. IJERPH 2019;16:3720. doi:10.3390/ijerph16193720

11 Prunicki MM, Dant CC, Cao S, et al. Immunologic effects of forest fire exposure show increases in IL1β and CRP. Allergy 2020;:all.14251. doi:10.1111/all.14251

12 Prunicki M, Kelsey R, Lee J, et al. The impact of prescribed fire versus wildfire on the immune and cardiovascular systems of children. Allergy 2019;74:1989–91. doi:10.1111/all.13825

13 Sawyer K, Mundandhara S, Ghio AJ, et al. The Effects of Ambient Particulate Matter on Human Alveolar Macrophage Oxidative and Inflammatory Responses. Journal of Toxicology and Environmental Health, Part A 2009;73:41–57. doi:10.1080/15287390903248901

14 Wu X, Nethery RC, Sabath BM, et al. Exposure to air pollution and COVID-19 mortality in the United States: A nationwide cross-sectional study. Epidemiology 2020. doi:10.1101/2020.04.05.20054502

15 Landguth EL, Holden ZA, Graham J, et al. The delayed effect of wildfire season particulate matter on subsequent influenza season in a mountain west region of the USA. Environment International 2020;139:105668. doi:10.1016/j.envint.2020.105668

16 Henderson SB. The COVID-19 Pandemic and Wildfire Smoke: Potentially Concomitant Disasters. Am J Public Health 2020;110:1140–2. doi:10.2105/AJPH.2020.305744