Making Sense of Wildfire Smoke

Ted Schettler, MD, MPH

Record-setting drought and heat in the West, driven in large part by climate change, set the stage for an early start to the 2021 wild-

fire season. By early July fires had engulfed more than twice the acreage that burned this time last year, the largest wildfire season recorded in California history. Although wildfires are natural features of this landscape, their increased intensity in prolonged seasons with more people living in the wildland-urban interface has deepened and widened their destructive impacts. Deaths and injuries to residents and firefighters, catastrophic property loss, socio-economic upheaval and forced displacement of people from their homes are the most wrenching, long-lasting consequences where the fires burn. But health impacts extend far beyond, primarily due to hazardous smoke that spreads over vast regions, sometimes for many days or weeks.

Wildfire Smoke Characteristics

Wildfire smoke is physically and chemically complex. Its composition, formation, behavior, aging, and dispersion are influenced by the fuel mix, kind of fire, rate of fuel consumption, meteorological conditions, and landscape features. The primary emissions from wildfires are coarse and fine particulate matter (PM), including aerosols; gases such as carbon monoxide, hydrogen cyanide, methane, nitrous oxide, nitrogen oxides, other volatile organic carbon compounds, including benzene, formaldehyde and acrolein; trace metals; polycyclic aromatic hydrocarbons (PAHs) and other toxicants. Some of the gases form secondary pollutants including organic aerosols and ozone when they photo-react in the atmosphere.

Particulate matter is typically divided into sub-types by size. Particles less than 10 microns in diameter (PM10) are inhalable; those between 2.5-10 microns are largely confined to the upper airways. Smaller particles (< PM 2.5) can penetrate more deeply into lungs and ultrafine particles can pass into the general circulation. Wildfire-related PM 2.5 is often used as a metric of exposure but is only a surrogate for the complex mix of particles, gases, and hazardous air pollutants unique to each fire.

Health Effects of Wildfire Smoke

Well-established health effects of exposure to wildfire smoke range from eye and respiratory tract irritation to reduced lung function, bronchitis, pneumonia, exacerbation of asthma and COPD, and premature death—similar to impacts of urban PM. Even children without asthma show a decline in lung function. A causal association between general particulate air pollution and cardiovascular morbidity and mortality is well established. Data linking wildfire smoke exposure to cardiovascular mortality and morbidity are mixed but most studies find increased risks of adverse cardiovascular events, especially among susceptible populations. Young and healthy people can also develop biological responses including systemic inflammation and vascular activation. Inconsistencies in study findings may be due to differences in exposure and outcome assessment methods, considerations of lag times, and variability in smoke composition. Very close to the fire, carbon monoxide concentrations can be high enough to be an acute health threat causing headache, weakness, dizziness, confusion, visual impairment, coma, and death. In addition to impaired lung function, firefighters are at increased risk of several kinds of cancer, plausibly because of repetitive exposures to a variety of carcinogens associated with products of combustion. Cancer risk associated with exposure to wildfire smoke in the general population, however, is uncertain. There is growing interest in learning more about what happens when smoke blankets communities sometimes for weeks at a time, since wildfire smoke waves, events lasting more than two days, have increased sharply in recent years. Birth outcomes, mental health, cognitive impacts, and cancer have not been sufficiently studied although some evidence shows an increased risk of low birth weight or preterm birth with wildfire smoke exposure during pregnancy.

Children, pregnant women, people with pre-existing respiratory disease and the elderly are especially vulnerable to smoke exposure and should take particular care to limit exposures to wildfire smoke.

Personal and Public Health Protection

Officials from the U.S. Environmental Protection Agency, U.S. Forest Service, Centers for Disease Control and Prevention, and California Air Resources Board have prepared Wildfire Smoke: A Guide for Public Health Officials, which contains a wealth of information useful to all stakeholders, including clinicians who will periodically be advising patients on best practices for minimizing smoke exposure and health risks. Another guide from the CDC summarizes recommendations succinctly for the general public. Health care professionals, health care systems, public health officials, city planners, schools and businesses should all be involved in implementing solutions to mitigate adverse impacts of wildfire smoke.

Strategies for personal protection

Stay indoors as much as possible: Reduce smoke exposure by staying indoors with doors and windows closed. Effectiveness in reducing exposure will depend on whether or not air conditioning that recirculates indoor air is available as well as the tightness of building construction. With windows and doors closed, particulate air pollution can be significantly reduced, although not by much in buildings that allow outdoor air to infiltrate easily. In warm weather, however, without air conditioning it may be difficult to stay in a closed-up house, requiring temporarily moving to a cooler location. High-efficiency particulate air (HEPA) filter air cleaners that do not emit ozone can help reduce indoor particle levels dramatically. They should be matched to the size of the space where they are placed, which may require creating a “clean room” within a home closed off from the rest of the house where the air cleaner can work most efficiently. Most particulate air cleaners do not remove gases but some models are designed to accomplish that as well by adding an activated charcoal layer. If necessary, a cheaper alternative to commercial HEPA air cleaning devices can be made by attaching a one-inch thick high efficiency furnace filter, available in hardware stores, to the back of a box fan so that air entering the fan is drawn through the filter. In a “clean room” these can significantly reduce particulate levels.

Reduce other sources of indoor air pollution during a smoke event such as tobacco smoking, using gas, propane and woodburning stoves and furnaces, spraying aerosol products, frying or broiling meat, burning candles and incense, and vacuuming, which can all increase particle levels.

In vehicles, people can reduce smoke levels by keeping the windows and vents closed, and, if available, operating air conditioning in “re-circulate” mode. However, in hot weather a car’s interior can heat up very quickly to dangerous levels with windows closed.

Reduce activity: Since exercise can dramatically increase respiratory minute volume, reducing physical activity will lower exposure to inhaled air pollutants and reduce health risks during a smoke event.

Respiratory protection with facial masks (respirators):

Facial masks (respirators) should only be used after first implementing other more effective methods of smoke exposure reduction as much as possible. Appropriately-designed, properly-fitting facial masks can help to further reduce exposures, particularly when outdoor activity cannot be avoided. But masks should not be used as a reason to justify spending more time outdoors during smoke events by creating a false sense of security.

One-strap paper masks, surgical masks, or covering the mouth and nose with a bandana or handkerchief are not adequate protection from wildfire smoke. The N95, N100 (or P100) particulate filtering masks, sometimes called face piece respirators, are most appropriate for the general public. (N indicates not resistant to oil, P indicates oil proof; either is effective for particulate filtration.) N95 masks are rated to capture at least 95 percent of small particles and N or P100 to capture at least 99.97 percent. However, their performance depends on a snug fit to facial contours. Facial hair reduces their effectiveness. They are also very difficult to fit properly to children and do not provide adequate protection. Facial masks may also make breathing more difficult and resistance increases with respirator efficiency, which can make them uncomfortable and potentially hazardous to people with respiratory or cardiac disease. Tightly-fitting respirators with purple HEPA filters offer a high degree of protection from particulates but may be less comfortable and more expensive than flexible masks. Most readily available masks generally do not filter gases but some models are constructed with an additional carbon layer that absorbs some gases. Tightly-fitting respirators with particulate and cartridge filters can also be effective for protection from particulates and certain gases. All respirators can become clogged as filtered particulates build up and should be regularly replaced, particularly as breathing becomes more difficult with prolonged use.

Strategies for public health protection

Strategic public health interventions that complement and inform protective measures undertaken by individuals and families are critical components of the response to wildfire smoke events. They include public service announcements; sharing of recommendations for people of all ages, health status, and social circumstances; and providing “clean air” shelters. These interventions depend on anticipation, preparation, and building partnerships so that they can be implemented on short notice—perhaps as soon as tomorrow or next week. The 2021 fire season is off to an explosive start and may last longer than ever before.

Additional Resources:

Wildfire Smoke: A Guide for Public Health Officials. EPA. Available at: Wildfire Smoke: A Guide for Public Health Officials | AirNow.gov Protecting Children from Wildfire Smoke and Ash. Fact sheets available at: Public: Protecting Children from Wildfire Smoke and Ash - PEHSU

Cascio W. Wildland fire smoke and human health. Sci Total Environ. 2018; 624:586-595. Reid C, Brauer M, Johnston F, Jerrett M, et al. Critical review of health impacts of wildfire smoke exposure. Environ Health Perspect. 2016; 124(9):1334-1343. National Academy of Sciences. The Chemistry of Urban Wildfires. The Chemistry of Urban Wildfires A Virtual InformationGathering Workshop | National Academies

Ted Schettler, MD, MPH is Science Director, Science and Environmental Health Network and affiliated with Commonweal in Marin. He received his MD from Case-Western Reserve University and a master's degree in public health from the Harvard School of Public Health. He practiced medicine for many years in New England.