The cloak of wildfire smoke that has descended across huge swaths of Northern California is a visible reminder of air pollution’s health threats. For people at high risk for severe COVID-19 symptoms, the bad air presents a new challenge. For firefighters and others exposed to large amounts of smoke, the long-term effects are uncertain.

Kari Nadeau, director of Stanford’s Sean N. Parker Center for Allergy & Asthma Research, and Mary Prunicki, the center’s director of air pollution and health research, study wildfire smoke’s effects on the heart, lungs and immune system. One of their current projects involves analyzing blood from current and retired firefighters, as well as members of the public, to see how smoke alters the immune system over time and how air purifiers and masks might lessen the impact (read more about their research).

Here, Nadeau and Prunicki discuss health threats, preparedness and ongoing research related to wildfire smoke.


What kind of health effects are you seeing from these fires? How are you studying them?

Kari Nadeau (Image credit: Sean N. Parker Center for Allergy Research at Stanford University)

Nadeau: We are doing a comprehensive collection, via searches in the electronic medical record system of Stanford’s emergency department, of health-emergency data from the past few days compared to the time period prior to the wildfires. We are finding that there are increases in asthma, heart attacks and strokes during this week of wildfires compared to the week prior. People who are over 65 years of age have a higher chance of heart attacks and strokes even after two to three days of bad air quality due to wildfire smoke.

The smoke from wildfires goes through the lungs and the airways and can be absorbed into the blood. There are over 400 toxins associated with wildfire smoke, which have a multitude of bad effects on the body. In the blood, these toxins can activate the immune system and platelets and the lining of blood vessels. When this happens, the inflammation can induce clots which lodge in the heart or the brain vessels and cause heart attacks or strokes, respectively. Asthma can get worse with wildfire smoke because the smoke gets into the lungs and causes irritation and muscle spasms, which lead to wheezing and difficulty breathing.


Any implications of wildfire smoke for long-term organ functioning that would make someone more susceptible to COVID-19?

Prunicki: We know air pollution causes immune changes, and increased levels of long-term exposure to small particulates called PM2.5 is associated with decreased life expectancy. Given that wildfire smoke is 80 percent PM2.5, we expect acute immune dysregulation. Our own research has shown increases in inflammatory markers in teenagers exposed to wildfire smoke. Less is known about long-term impacts of wildfire smoke exposure on health, which is why our work with retired firefighters is so important. Given that areas of elevated air pollution are associated with increased COVID-19 rates, exposure probably renders the body less able to fight off viral infections, such as COVID-19.


Mary Prunicki (Image credit: Sean N. Parker Center for Allergy Research at Stanford University)

Some COVID-19 symptoms, such as headache and shortness of breath, are similar to the symptoms people suffer from poor air quality. How will doctors differentiate between the two in a timely manner without rapid COVID tests?

Nadeau: It will be difficult to distinguish these symptoms without seeing a clinician first. Typically, with a virus, a person can have fatigue and muscle aches and fever, which are not associated with wildfire exposure. The smoke and toxins from wildfire exposure can cause headaches, wheezing, shortness of breath, coughing, heart attacks and strokes. In addition, the two can be differentiated because when the wildfire smoke dissipates, symptoms may improve, whereas for viral infections like COVID, symptoms can worsen over a period of one to two weeks and then improve.


How should vulnerable populations, such as older adults, weigh whether to open windows – if they don’t have air conditioning – and risk respiratory issues, or keep windows closed and risk heat issues?

Nadeau: This is a tough question. Vulnerable populations, like young children, infants, pregnant women and older adults, have worse symptoms after wildfire exposure. If you or someone you know needs to evacuate, it is better to go to a place with air conditioning and filtration, such as cooling stations set up by municipalities. People should contact agencies such as the American Red Cross or municipal offices to see where those might be. I also recommend using an indoor and outdoor monitor and filter for your home and staying indoors to decrease exposure to bad air.


Do cloth masks that many people are wearing as a precaution against COVID-19 offer any protection from wildfire smoke? How should the average person weigh the benefits of an N95 mask, in terms of reducing particulate inhalation, and the need to reserve them for public health workers dealing with COVID-19?

Prunicki: Cloth masks offer very limited protection but are better than nothing. One study used the smoke from a candle to measure filtration and found that cloth masks were about 50 percent effective in filtering very small particles. However, the best solution is to stay indoors if you can. Otherwise, standard N95 masks are in limited supply for health care workers, but an N95 mask with an exhalation valve could be used. These help protect you from the particulate matter in wildfire smoke, but can’t be used in the health care setting because the valve could potentially allow the transmission of virus.

Nadeau is also the Naddisy Foundation Professor in Pediatric Food Allergy, Immunology, and Asthma, professor of medicine and pediatrics and, by courtesy, of otolaryngology at the Stanford School of Medicine, and a senior fellow at the Stanford Woods Institute for the Environment.

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Media Contacts

Kari Nadeau, Sean N. Parker Center for Allergy and Asthma Research: (650) 724-6780,

Mary Prunicki, Sean N. Parker Center for Allergy and Asthma Research: (408) 483-6082,

Rob Jordan, Stanford Woods Institute for the Environment: (650) 721-1881,