It’s now become a regular topic in our newsfeeds: wildfires are increasing in number, size, and damage every year. Some of the consequences of these fires are obvious but others are less well understood. Our new PNAS paper focuses on the increasingly important contribution of wildfire smoke to overall pollution levels in the US. These far-reaching pollution effects from wildfire smoke mean that consequences of fire extend far beyond property loss; other ongoing research in our lab shows links between wildfire pollution and birth outcomes, test scores, mental health and life expectancy. The growing wildfire risk has a lot of causes including climate change, the fire policy of years prior, US housing policy that causes neighborhoods to expand to areas more likely to be at risk of fire, and the infrastructure we build and how we build it.
Wildfires account for a lot of PM2.5
To understand the contribution of growing wildfire risk to air pollution, we built a model to estimate how much PM2.5 comes from wildfire smoke. Our model used information on the locations of fire and smoke in conjunction with data on other factors that influence overall particulate matter (PM2.5) (e.g., energy use, geography, population, agriculture) to predict observed pollution concentrations at EPA monitors across the nation. Once the model was optimized for predicting PM2.5 we set the fire and smoke inputs to 0 to estimate what PM2.5 would have been in a hypothetical counterfactual scenario absent fire and smoke.
Our estimates suggest that in 2018 wildfires accounted for up to 25% of all PM2.5 nationally and up to 50% of PM2.5 in the western US. Those numbers are two to five times what they were 14 years ago; our estimates for 2006 were 5% nationally and up to 15% in the western US (Fig 1).
What does that mean for us now?
We know that PM2.5 exposure is bad for our health in many ways, so next we looked at what kind of harm pollution from wildfire might be causing. To do this we used existing estimates of PM2.5 impacts on mortality in older adults to translate our estimates of PM2.5 from wildfire smoke into counts of excess deaths from pollution. This exercise pointed to 15,000 deaths per year attributable to PM2.5 from wildfires, in older adults alone! These estimates are approximate and rely on the assumption that PM2.5 from wildfire smoke affects health the same way as PM2.5 from other sources. Nonetheless, the estimate illustrates the substantial threat that wildfire smoke poses. While we know that exposure to overall PM2.5 is bad for our health, we are only beginning to understand how wildfire PM2.5 specifically affects us, this is an important avenue of much ongoing and future research.
What does that mean for the future?
While mortality damages from temperature are typically thought to be the largest cost of climate change in the United States, we find impacts from smoke are of a similar magnitude. In other words, wildfire smoke is likely to be one of the primary health impacts of climate change in the United States.
As the climate continues to change, increases in temperature and the dryness of burnable plant material are expected to create conditions even more suitable for fires to ignite and spread. In parallel, there are already 50 million homes in the wildland-urban interface (WUI) and they are being joined by around 300,000 additional new homes per year. Not only are these areas closer to potential wildfire locations, they are also the most difficult and expensive places to fight fire. In 2018, the US spent 3.15 billion on fire management and suppression, a number that’s been rising by about 66 million per year (Fig 2). The combination of more homes in these areas and worse fire conditions mean that we can only expect the costs of fire seasons to continue to rise.
The smoke experienced by residents of California and Oregon in 2020 demonstrates conditions the future may hold if changes aren’t made — massive plumes, dark skies, and levels of PM2.5 exceeding 300 micrograms per cubic meter. The September peak of the 2020 fire season was estimated to cause hundreds of premature deaths along the West Coast alone (Fig 3). However, this could soon become the norm. An annual fire season with school closures, people unable to go outside across much of the West, and thousands of pollution related deaths from wildfire smoke each year is not the future we’d like to imagine.
What can we do?
The changes we’re currently seeing in wildfire risk are driven by excessive global greenhouse gas emissions and a century of fire policy focusing on suppressing any and all wildland fire. However, “good fire”, or the purposeful application or allowance of lower-intensity fire on the landscape, can prevent the accumulation of dry, burnable material and reduce the risk of larger fires (like the gigafire in California in 2020, which burned more than a million acres of land.) Available evidence suggests that expanding fuel management strategies is the most effective way to reduce fire frequency and intensity. It has long been known among land managers that wildfire management is air quality management, but air quality regulation has created barriers to investment in these types of management strategies.
Wildfire is not currently regulated as an air quality issue at the state level, and is not covered by the Clean Air Act. Instead, wildfire smoke is considered an “exceptional” emissions source, an act of God not subject to human control and thus not considered in assessment of air quality standard attainment. At the same time the use of prescribed burns, one of the main interventions for limiting future fire risk, is subject to regulations because it is under human control.
Though the EPA is beginning to change this stance, these policies create a catch 22. If you don’t use prescribed burning, catastrophic fire events can cause massive loss of human life and property; if you do utilize prescribed burning it may take you out of Clean Air Act attainment despite the potential risk reduction. Because of this conflicting incentive structure, prescribed burning is largely underused as a risk reduction strategy, and is limited primarily to the Southeast US (Fig 4).
Obviously wildfire cannot be regulated the same way as vehicle emissions, but policies that disincentivize investment in a critical method of fire control are unlikely to effectively reduce the frequency and intensity of wildfires and their resulting smoke. Those charged with managing wildfire face the challenge of ever escalating fire suppression costs, regulatory barriers to greater use of fuel management tools, and institutional incentives that promote fire suppression and disaster response over prescribed burns and risk reduction. Revising regulatory frameworks to incentivize risk reduction is an important first step to mitigating fire risk.
What research do we still need?
Many gaps in our knowledge of wildfire remain: Does PM2.5 from wildfire smoke have different health impacts than PM2.5 from other sources? How much does prescribed burning reduce the total wildfire burden? Is smoke from prescribed burning different from that of wildfires? How much does wildfire smoke infiltrate homes (where people spend much of their time) and what factors does this infiltration depend on?
Our team is currently zeroing in on that last question. Environmental issues disproportionately affect lower-income communities, so we were surprised to find in our paper that wealthier communities are more exposed to ambient wildfire PM2.5 on average. Outdoor PM2.5 doesn’t capture the whole story though. Home sealing and extra filtration (which are more common in wealthier areas) can lead to much better indoor air quality, meaning even two households exposed to the same levels of outdoor PM2.5 may have very different indoor exposures. Similarly, pollutants tend to infiltrate more easily into older and smaller homes, which could exacerbate exposure inequalities.
As wildfire risks continue to grow, understanding the effectiveness of potential policy options and the factors that matter for mitigating health impacts will become increasingly important. And while this will require a substantial investment of resources, the alternative of doing nothing is likely to be even more costly.
For more on this topic see our recent work:
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