The Greater Los Angeles area is awash in smoke and ash as multiple fires burn in and around the city. It’s too soon to assess the overall pollution impacts from this rare January event, but we know the smoke is filled with tiny particles known as PM2.5, one of the most pernicious public health villains, associated with increased risk of respiratory and heart disease and premature death.
Last year, the Environmental Protection Agency tightened the National Ambient Air Quality Standard for PM2.5 for the first time since 2012. The South Coast Air Quality District, which contains Los Angeles, is known for having some of the worst air quality in the country. State officials have already deemed it to be out of compliance — and that’s without even counting pollution from major wildfires. But new research raises questions about whether complying with the new standard will even be possible in many places due to the increasing frequency and severity of wildfires.
Marshall Burke, who published the not-yet-peer-reviewed findings in December, is a Stanford University researcher who has spent the past several years investigating how wildfires have affected PM2.5 exposure in the U.S. In a 2023 paper published in Nature, he and his co-authors found that over just six years, wildfire smoke eroded decades of air quality improvements throughout the country. The trend was particularly bad in Western states, of course — some of which saw more than half of their gains erased. The pre-print of the new paper updates those findings to include data from 2023. But it also goes deeper on what this means in light of the new air quality standards. The authors find that 34% of air monitoring stations registered PM2.5 above the regulatory limit because of smoke in at least one of the last five years.
Technically, wildfire smoke is completely unregulated. Jurisdictions can request to exclude “exceptional events,” such as days when PM2.5 spiked due to wildfire, from their calculations. But as the “smoke season” has grown longer and more places experience more days with degraded air quality due to smoke, local officials have not been requesting more exemptions. The researchers analyzed applications for exemptions since 2019, and found that they were more common on days with higher levels of wildfire smoke, but were still infrequent overall.
One reason might be that local pollution control officers don’t always recognize when smoke has pushed pollution over the limit on a particular day versus other factors. There is also a “substantial resource burden involved” in demonstrating the influence of wildfire smoke on ambient air quality, the paper says. Also, as smoke becomes more commonplace, it may be more difficult for officials to make the case that a given smoke event is “exceptional.”
In any case, if this low rate of applications for exemptions continues, many more regions may find themselves to be out of compliance with the new PM2.5 standard.
In the paper’s discussion section, the researchers posit that as wildfire smoke continues to get worse, either of two possible scenarios could play out. In the first, air quality districts affected by smoke get better at applying for exemptions and therefore achieve compliance with the Clean Air Act, even as local air quality and public health deteriorate. In the second, they find other ways to stay in compliance with the standards, such as by tightening pollution caps on power plants and factories. “Such mitigation could be cost effective in many regions where abatement costs remain low relative to the benefits of further air quality improvements,” the authors write, “but could become onerous if wildfire smoke concentrations continue to grow, as is expected under a warming climate.”
The first scenario is bleak, and the second comes with a pretty big caveat. But those aren’t the only options — we can also reduce the risk of wildfires with better land-use planning and management. Unfortunately, promising strategies like controlled burns can push PM2.5 levels over the standard, and those are not exempt from reporting the way that wildfires are — creating a perverse incentive not to do them.
