California communities’ recovery time between wildfire smoke events is shrinking

(Press-News.org) Californians have long dealt with wildfire smoke as a seasonal fact of life, but those fires have become more intense and frequent, raising the profile of wildfire smoke as a public health issue. Now, a study led by researchers at UC San Diego’s Scripps Institution of Oceanography finds that the time between multi-day smoke events is shrinking — leaving communities with less time to recover before smoke returns. 

The new study, published March 11 in the journal GeoHealth, found that in California the window of cleaner air between smoke waves shrank by more than 60% from 2006 to 2020. The study also finds that the communities hit hardest by shrinking recovery times are also among the state’s most socially and economically vulnerable.

Research on the effects of wildfire smoke on human health has grown substantially in recent years, establishing links between smoke exposure and respiratory illness, cardiovascular disease and premature death. A 2025 study found that short-term exposure to wildfire smoke contributes to some 40,000 deaths each year in the United States. Wildfire smoke fine particulate matter has been shown to be more harmful per unit of pollution than the same particles generated by cars or industrial sources.

But most prior research treats each wildfire smoke event as a standalone episode. What hasn’t been well studied is what happens when those events come in rapid succession. Evidence from wildland firefighters suggests that cumulative exposure across a season can worsen airway inflammation and reduce lung function more than any single event would on its own. Whether similar effects occur in the broader population remains an open question.

At the same time, California has seen increases in the frequency and intensity of wildfires. Smoke from these fires has reversed decades of air quality progress in the state and is now the leading source of fine particle pollution in California. The cumulative toll on communities, particularly those with fewer resources to adapt, is a growing public health concern as places like California grapple with the new normal in a warming world.

“Until recently, wildfire smoke was considered rare for the general population, but in California wildfire smoke is now a chronic problem,” said Tarik Benmarhnia, an environmental epidemiologist at Scripps who co-authored the study. “In California we are leading the way to shift how we study and address the health impacts of wildfire as these events are becoming prevalent, and there is a lot to do.”

Benmarhnia and his co-authors wanted to find out whether Californians are getting less time between smoke events than they used to, and if so, whether some communities are experiencing more change than others. 

The team analyzed daily wildfire-specific fine particulate air pollution for every census tract in California from 2006 through 2020 — a dataset built using machine learning models that separate smoke particles originating from wildfires from those generated by other sources such as cars and industry. They defined a “smoke wave” as two or more consecutive days with detectable wildfire smoke above a specified concentration threshold, then calculated the recovery period by tallying the number of days between the end of one smoke wave and the start of the next.

To assess how patterns changed over time, the researchers divided the study period into three five-year windows and compared smoke wave frequency, duration and recovery periods across those periods. They also looked at whether changes in recovery time were associated with demographic and socioeconomic variables including race and ethnicity, household income and single female-headed households. 

Between the first five-year period (2006–2010) and the last (2016–2020), smoke wave frequency across California census tracts increased by roughly 85%, and the average recovery period shortened by more than 60%. 

The picture varied considerably by region. Northern California was consistently the most smoke-affected part of the state throughout the study period, but did not see shortened recovery periods over time. The most pronounced changes occurred in Southern California and the Central Valley, where communities experienced smoke waves with progressively less time between them across the study’s three five-year periods.

The analysis also found that the census tracts that experienced the largest reductions in recovery time were home to higher proportions of Black, Hispanic, Asian and other racial minority residents, as well as lower-income households and single female-headed households. The main reason for these socioeconomic and demographic patterns was geography: The Central Valley and Southern California, which saw the biggest reductions in recovery time, are home to more people among these populations. 

The precise health consequences of shrinking recovery periods aren’t yet fully understood, but research showing cumulative impacts from repeated exposure suggest that it is a key area for future research. The socioeconomic and demographic patterns identified by the study also highlight additional risk factors that can reduce residents’ ability to adapt to these shorter respites from wildfire smoke such as inadequate housing, limited healthcare access, outdoor occupational exposures and fewer financial resources to purchase air filtration equipment or temporarily relocate when smoke descends. 

“The geographic and socioeconomic dimensions of these risks can highlight places and communities that may need more resources and help hone public health messaging,” said Caitlin Jones-Ngo, the study’s first author who performed the research while a postdoctoral researcher at Scripps. “The idea is to make sure the communities facing the greatest risks get the support they need.” 

If subsequent research finds that repeated wildfire smoke exposure with reduced recovery time between episodes causes cumulative health, social or economic impacts, then public health policies calibrated to single smoke events may be insufficient. For example, California’s air quality alerts, which warn residents to limit their exposure, are triggered by fine particulate air pollution exceeding specific thresholds that remain the same regardless of when the prior smoke wave has ended. 

“The threshold level of exposure that’s considered safe or harmful may need to consider this temporal component that may compound risk,” said Benmarhnia. “This idea of compounding risks could also be extended to other events such as extreme heat alongside wildfire smoke. Thinking about how to deal with compounding impacts is the big next step for climate policy.”

Benmarhnia and others are now conducting follow-up research that seeks to analyze how different amounts of recovery time between bouts of wildfire smoke exposure translate into health impacts. While this study focused on California, the authors said similar analyses could be worth considering for any other region in the U.S. or around the world with frequent wildfires, such as parts of Canada and Australia. 

In addition to Benmarhnia and Jones-Ngo, who is now a scientist at the California Department of Public Health, Chen Chen and Rosana Aguilera of Scripps Oceanography as well as Miriam Marlier of UCLA co-authored the study. The research was supported by the University of California as well as California’s Office of Environmental Health Hazard Assessment.

 

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