This post is the first in a series of blog posts about wildfire smoke.

Wildfire smoke is no longer a regional concern.

This summer, wildfire—a frequent phenomenon in the West—drew nationwide attention as the smoke from a series of wildland fires in Canada blew across hundreds of miles to reach the East Coast of the United States. The smoke turned the skies of New York City and Washington D.C. hazy in the daytime and deep orange in the evening. Poor air quality led residents to stay indoors or return to wearing N95 masks when traveling outside. Senators from the Western U.S. attending a hearing on wildfire found that the visual aids they’d brought to illustrate the effects of smoke on their home states were no longer necessary, because the same effects could be viewed from the steps of Congress. It’s clear that wildfire and the smoke it creates is no longer the concern of Western states alone, but a national and international issue whose effects extend greatly beyond the areas where wildfires themselves occur.

Given the newly widespread recognition of wildfire smoke and the danger it poses to public health, action to address the ongoing wildfire crisis is more crucial than ever. However, some important questions remain. More research is needed to determine the full health effects of wildfire smoke exposure, the best methods to minimize exposure to smoke at a community level, and the most effective approaches to mitigate the impacts of future wildfires. While the serious harms of smoke—especially for vulnerable individuals and communities—call for an immediate response, they also call for a targeted response that minimizes harm in coordination with other health and ecological goals. In this post, the first part of this series, we will explain the harmful effects of wildfire smoke and describe how communities can act to limit their exposure to smoke. In the second part, we will go on to examine the causes of the wildfire crisis and discuss how the government can act to reduce the harms posed by wildfire.

Why is wildfire smoke harmful and how can we avoid exposure to it?

While the composition of wildfire smoke varies depending on the material being burned, one key pollutant that remains constant across different types of fires is PM_2.5. Unlike most other pollutants, PM_2.5 is designated not by its chemical composition but by its size. What defines PM_2.5—and what makes it dangerous—is its small size of 2.5 micrometers or less, tiny enough to be inhaled by humans and actually pass through the lungs and into the bloodstream. When inhaled, PM_2.5 can accumulate within the body, causing inflammation that contributes to heart and lung disease. Research indicates that the harm caused by this buildup of PM_2.5 within the body is cumulative; in principle, no level of PM_2.5 exposure is safe.¹ Therefore, when levels of atmospheric PM_2.5 are high, everyone should limit their exposure to the greatest extent possible. However, sensitive populations with pre-existing conditions, seniors, children, and pregnant women are at even higher risk. Targeted public health approaches that successfully limit exposure are especially important for these groups.

To take one group as an example, the effects of PM_2.5 exposure are understood to be more serious for infants and small children. For that reason, many school districts limit outdoor activities or tell students to remain at home when outdoor levels of PM_2.5 are elevated. The logic behind this policy choice is clear. However, while outdoor levels of PM_2.5 are relatively well-understood and consistent, variations in indoor levels of PM_2.5 from building to building can be significant, meaning that, in some cases, telling students to stay home when air quality is poor is counterproductive. If the indoor air quality is better in their school than their home, a child may be better off going to school when outdoor air quality is poor. This is especially likely if the school is equipped with an air filtration system that their home lacks.

This issue intersects with other environmental justice concerns. Households in socio-economically disadvantaged communities may be at greater risk of PM_2.5 exposure, especially if they’re located in areas with other sources of pollution or if their home isn’t sealed adequately to prevent pollution from entering.² To address these concerns, research on relative indoor exposure to PM_2.5 in schools and homes should be conducted to make sure that each school district’s policy calls for children to stay where they’ll be exposed to as little PM_2.5 as possible, whether that’s at home or in school. To support these programs, increased monitoring of indoor air quality and greater access to air filtration systems are also needed.³

These concerns about students and PM_2.5 apply to air pollution policies more generally. Indoor air quality filtration and monitoring have received more attention in recent years due to both wildfire and the COVID-19 pandemic. However, U.S. environmental and air pollution laws generally only apply to outdoor ambient air, not indoor air. As our understanding of indoor air quality increases, our approach to pollution like PM_2.5 needs to take indoor air quality into account, not just outdoor air quality, in order to be sure that our policies don’t lead to greater exposure.

Eric Macomber joined the Climate and Energy Policy Program and Stanford Law School as a Wildfire Legal Fellow in September 2022. His work focuses on law and policy issues relating to wildfire and the wildland-urban interface.

This post was originally shared on the Climate and Energy Policy Program‘s Substack.