It’s the Smoke, Not the Fire

Most Americans are breathing increasingly unhealthy air from wildfire smoke. This is the first conclusion of the study by Vargo et al. in this issue of AJPH (p. 759) quantifying the magnitude of a growing climate change hazard in the United States, namely exposure to the toxic smoke of wildland fires. These increases are happening across both advantaged and disadvantaged communities. This is the second, and more important, conclusion because it counters the dominant media narrative that wildfires are an affliction of the rich. This study highlights the need for public health to do more to protect socially disadvantaged groups and to direct national attention to the smoke and away from the flames.

Climate change is producing longer fire seasons, with more frequent and severe wildfires. The resulting smoke is a complex mixture of corrosive gases and carcinogenic compounds, including particulate matter less than 2.5 microns in diameter (PM_2.5)—a toxic mixture of solid and liquid particles that gets into the bloodstream. PM_2.5 causes a wide range of physical, mental, developmental, and cognitive health harms.

The smoke can travel thousands of kilometers from its source, polluting distant communities for weeks and months. For instance, forest fires in Quebec, Canada, led to air pollution in Baltimore, Maryland, with peak PM_2.5 values 17 times higher than the Environmental Protection Agency’s National Ambient Air Quality standards and to increased cardiorespiratory hospitalization rates across the mid-Atlantic and Northeastern United States.^1,2 Although large fires more often occur in the Western United States, much of the attributable mortality and morbidity may occur in the Eastern United States because of its higher population density.³ As the world enters a time of increasing wildfires, chronic, multiweek fires—as were seen in Australia in 2019 and California in 2020—will become more usual.

With this background in mind, consider the study by Vargo et al. of national-level trends in smoke exposure from 2011 to 2021 and their intersections with social vulnerability.

The authors combined satellite data on wildfire smoke plumes with US Census population data. They assigned daily smoke plume densities (high, medium, light, none) to each census block group and its 2010 population. Person-days of smoke, which is the product of the number of people in a census block group and the number of days that block group experienced smoke, were summed to the census tract to estimate exposure levels.

The results are alarming. Across the United States we are exposed to more wildfire smoke, and few counties are unaffected. The five-year annual average exposure to heavy smoke increased 350%, from 307 million person-days during 2011 to 2015 to 1.381 billion person-days during 2017 to 2021. The authors estimate a total of 2.9 billion person-days of heavy smoke across the US population in 2021 (the US population was 331.9 million people in 2021). More than 87% of the US population experienced increases in the number of days of heavy smoke between these periods. Light smoke days increased too, which is important because there is likely no “safe” level⁴ of exposure to pollutants such as PM_2.5. Eastern states experienced these increases too, albeit starting from a lower baseline. Wildfire smoke has become everyone’s problem.

Protecting oneself from wildfire smoke is difficult when it requires staying indoors in homes with air conditioners and good air filtration, avoiding outdoor work, avoiding traveling (so working from home), and even temporarily leaving your home to stay in places beyond the spreading smoke. As in disasters generally, the level of population harm is a product of social disadvantage and the capacity to protect oneself from the hazard. For instance, infiltration of outdoor pollutants into homes is higher on average for older, smaller homes and for lower-income households, and these differences could lead to disparities in overall individual exposure even if ambient exposures are equivalent.⁵ This is where the second part of the authors’ results fits in.

The authors used the Centers for Disease Control and Prevention’s Social Vulnerability Index to investigate characteristics that might affect the health risks of smoke exposure. This index contains census-derived data on sociodemographic, economic, and cultural characteristics, which the authors used to rank census tracts by their levels of social disadvantage. Census tracts at the greatest social disadvantage experienced a 358% increase in the average annual number of heavy smoke days, from 0.92 days in 2011 to 2015 to 4.21 days in 2017 to 2021.

Census tracts with relative advantage showed similar, if not higher, increases in heavy smoke person-days (annual number of heavy smoke days of 1.13 in 2011–2015 to 4.79 days in 2017–2021). Still, this study highlights inequities of the utmost importance. Although everyone is seeing more exposure, those with the greatest disadvantage often start from a baseline of worse health, have the least access to protections, and have more frequent health harms. Add wildfire smoke to the list of inequities that climate change is worsening.

Vargo et al. note that their results may represent the upper bound of estimates because they used satellite plume data rather than ground-level measurements of air quality—the smoke could be aloft and captured by satellites but not be affecting the ground where people are breathing. Still, even more conservative estimates demand our attention and response.⁶ Because disadvantaged communities are more prone to suffer from disaster impacts, any meaningful discussion of wildfire smoke needs to address this health equity issue.

The media’s penchant for photographing large homes burning in Western mountains depicts wildfires as a problem mainly for wealthy owners of second homes in California and the West. But it is the smoke, not the fire. We in public health must redirect media attention to the problem as we know it really is experienced—households near and far from the fire are suffering harms, hospitalizations, and deaths from the smoke menacing our air.

Additional quantitative and qualitative investigation is necessary to gain a deeper understanding of the relationship between social vulnerability and the health effects of smoke. Studies like that of Vargo et al. analyze vulnerability factors independently when it is likely they are operating synergistically. We need better measures of indoor exposure, as the most common advisory during an event is to stay indoors, but poor housing quality may reduce the benefits. We also need to analyze social vulnerability at smaller scales to better understand local impacts, for instance, combining fine-resolution geospatial data with the rich data available in electronic medical record systems to identify subpopulations most harmed by wildfire smoke.

Focusing our public health interventions on the communities with social disadvantages may maximize our impact, and we need program evaluations to assess this. The accelerating increase in smoke exposure calls for new approaches too. For instance, public health recommendations for protection from wildfire smoke include staying indoors in homes equipped with an air purifier and a HEPA (high-efficiency particulate air) filter. As the authors point out, this is a costly protection beyond the reach of many households. Publicly financed housing modifications, such as retrofits and air purification, may be required in communities with high exposures and high social vulnerability.

During the 2020 California wildfires, the Bay Area Air Quality Management District provided portable air filtration units to low-income individuals on Medi-Cal with severe asthma or other respiratory conditions. More public health departments should identify socially disadvantaged neighborhoods and prioritize those that need increased wildfire smoke preparedness messages, supplies, and access to clean air shelters.

The point is the smoke.