As wildfires scorched record swaths of California, Oregon, and Washington State, massive plumes of smoke caused air pollution there to soar to unprecedented levels—rivaling that of the world’s most polluted cities and confining residents to their homes. Health officials raised alarms about the threat the pollutants posed, which some researchers believe might have indirectly accounted for hundreds of deaths.

Wildfire smoke is filled with noxious gases and tiny particles that come from construction materials in burned buildings, as well as from trees and plants. According to a study in the Journal of the American Heart Association, wildfire smoke is linked to respiratory and cardiovascular illnesses—and those tiny particles present the biggest problem.

Known as PM2.5 because they are smaller than 2.5 micrometers in diameter, the particles can penetrate deep into the lungs and can’t be broken down by the body’s immune system. There’s also some evidence that they can enter the bloodstream and cause widespread inflammation. (How breathing in wildfire smoke affects the body.)

The U.S. EPA’s Air Quality Index (AQI) tracks five major air pollutants, including PM2.5, converting concentration measurements for each onto a common scale that communicates the level of hazard. The color-coded scale stretches from 0 to 500 and is split across six categories, from good to hazardous. Air officially becomes unhealthy for at-risk groups at 101, and unhealthy for all populations at 151.

When the AQI for PM2.5 rises above 300, it triggers warnings of emergency conditions: The air is considered hazardous for everyone. Even healthy people are warned to avoid all physical activity outside; sensitive groups are instructed to remain indoors and limit activity even there.

As the wildfires intensified during one week in September, air pollution from PM2.5 climbed above 300 in areas across the West Coast. On September 10, the Oregon coast was enveloped by smoky air that ranged from unhealthy to hazardous; in subsequent days, it pushed north into Washington State and Canada, as well as south into Northern California. (See photographs of the polluted orange skies that threatened the health of Californians.)

For days, cities across the region had the worst air quality in the world—breaking their own records for PM2.5 pollution and even topping that of Delhi, India, one of the world’s most heavily polluted cities. On September 13, Portland, Oregon, sat at the top of the list with a daily average of PM2.5 pollution that spiked to 358.2 micrograms per cubic meter—equivalent to an AQI of 406.

Stanford University researchers estimate that the wildfire smoke may have been indirectly responsible for the deaths of hundreds of people 65 and older. Although actual data will not be available for months, the researchers relied on a 2019 study that used Medicare data to determine the relationship between PM2.5 exposure and mortality. By applying the results from that study to California’s recent PM2.5 exposure, they determined that the polluted air may have caused an additional 1,200 deaths and 4,800 visits to the emergency room by elderly who fell ill.

There is some evidence, too, that exposure to high levels of PM2.5 can cause long-term damage to the lungs in the elderly. A study published this year in the journal Toxics found diminished lung capacity among some participants for two years after being exposed to what the AQI describes as “very unhealthy” levels of PM2.5 over 49 days during a 2017 wildfire.

Christopher Migliaccio, an immunologist at the University of Montana and one of the authors of the study, stresses that more research is needed into the potential health effects of wildfire smoke—a question that is taking on greater urgency as climate change fuels more intense wildfire seasons.

Right now, however, Migliaccio is worried about this year’s potential “twindemic” of seasonal flu and COVID-19. Exposure to air pollution can make people more prone to respiratory disease; a recent study published in Environmental International found that exposure to wildfire smoke correlated with three to five times more flu cases. “If you have an increase in influenza cases, that makes people more susceptible to other respiratory infections like pneumonia or coronavirus,” Migliaccio says. (Here’s what happens if you catch flu and COVID-19 at the same time.)

That’s why it’s particularly important for people who have been exposed to wildfire smoke to get their flu shots this year and practice good hand washing, Migliaccio says. He also recommends using a HEPA filter to create a safe space at home.