Wildfires that keep us inside might drive the spread of infectious disease, per study of the U.S. West Coast wildfires of 2020
image:
Air Quality level during the West Coast wildfire season in 2020. (a) Air Quality Index extracted for PM2.5 particle pollution for all counties in Oregon (OR), Washington (WA), and California (CA). Horizontal line at 150 indicates the Air Quality Index (AQI) threshold for health concerns; (b) The map highlights in dark red the most affected counties in Oregon (OR) and Washington (WA) that were included in our study. The orange counties represent unaffected areas used as a baseline. Both affected and unaffected counties are part of the Northern Indoor Seasonality Cluster (in dark grey). Base layer: https://www2.census.gov/geo/tiger/GENZ2020/shp/cb_2020_us_county_20m.zip.
Shift in human behaviors due to wildfire smoke.
The figure shows the indoor activity seasonality index between July 1, 2020, and November, 01, 2020 in the 10 selected affected counties: Multnomah County (Portland City), Washington County, Clackamas County, Lane County, Marion County in Oregon state, and King County (Seattle city), Spokane County, Yakima County, Clark County, Thurston County in Washington state. In each subplot, we also show in yellow the median and 95% CI of the indoor activity seasonality index for the unaffected counties that we used as a baseline. The violet curves represent the AQI of the affected counties during the studied period.
view moreCredit: Image Credit: Arregui-García et al., 2025, PLOS Climate, CC-BY 4.0
Wildfires that keep us inside might drive the spread of infectious disease, per study of the U.S. West Coast wildfires of 2020, highlighting indirect health impacts of extreme weather events.
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Article URL: https://plos.io/4mXg1FC
Article Title: Disruption of outdoor activities caused by wildfire smoke shapes circulation of respiratory pathogens
Author Countries: Denmark, France, Italy, Spain, United States
Funding: Research reported in this publication was supported by the Fritz-Family fellowship program to SB and GP. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. GP received her salary from the Fritz-Family fellowship program.
Journal
PLOS Climate
Article Title
Disruption of outdoor activities caused by wildfire smoke shapes circulation of respiratory pathogens
COI Statement
Competing Interests: The authors have declared that no competing interests exist.
New USC study links wildfire smoke exposure and heat stress to adverse birth outcomes
Keck School of Medicine of USC
Exposure to wildfire smoke and heat stress can negatively affect birth outcomes for women, especially in climate-vulnerable neighborhoods, according to a recent study led by USC postdoctoral researcher Roxana Khalili, PhD. The investigation, just published in Environmental Science & Technology, is one of the first to show that living in areas more susceptible to the harmful effects of climate-related exposures can significantly alter the effects of heat stress on adverse birth outcomes, even among women exposed to these conditions in the month before becoming pregnant.
“We already know that poor air quality is associated with adverse health outcomes and that pregnant women and fetuses are especially vulnerable,” said Khalili, a researcher in the department of population and public health sciences at the Keck School of Medicine of USC and lead author of the study. “Our knowledge about the specific effects of wildfire smoke during pregnancy has been limited. We know even less about the impact of these hazards right around or before conception.”
Khalili worked alongside Rima Habre, director of USC’s CLIMA Center to explore the associations between exposure to wildfire smoke and heat stress on infant health. They used data from the MADRES cohort (Maternal and Developmental Risks from Environmental and Social Stressors), an ongoing study of pregnant women in Los Angeles.
“Where you live makes a difference in your health,” said Khalili. “So does the timing of your exposure during or immediately before pregnancy. We wanted to better understand how these risks might differ for women who lived in neighborhoods that experience more climate related stressors and have fewer resources to deal with them.”
Measuring exposure
The research team examined 713 births among MADRES participants between 2016 and 2020. They used data from CalFIRE (California Department of Forestry and Fire Protection) to identify the location, size, and duration of every wildland fire in southern California during the study period. They used the NOAA hazard mapping system to calculate the smoke density from each fire and applied sophisticated modeling methods to calculate ground-level smoke concentrations, estimating how much particle pollution—tiny droplets of black carbon, soot, and burned vegetation—the women in the cohort were exposed to during these events based on their daily residential location histories.
Heat stress is another risk factor for pregnant women and fetuses, partly because pregnancy impedes a woman’s ability to regulate body temperature. To measure heat stress exposures in study participants, Khalili and her colleagues used meteorological data documenting the daily temperature, relative humidity, and wind speed. They also factored in a separate measurement that takes into account sun angle and cloud cover to gauge heat stress in direct sunlight, providing a better measure of how hot a given day feels when outdoors.
Next, the team pinpointed those LA neighborhoods that are most vulnerable to climate risks with mapping data from the California Urban Heat Island Index and the US Climate Vulnerability Index, two geospatial tools that analyze and map layers of data. These tools allow researchers to conduct deeper, more nuanced analyses that better assess the cumulative impacts of factors ranging from excessive heat and wildfire smoke to socioeconomic conditions.
Where there’s smoke…
The researchers found that greater exposure to wildfire smoke and excessive heat during the month before conception and the first trimester of pregnancy was associated with greater odds of having a small-for-gestational-age (SGA) baby. An infant is considered SGA if their birth weight is below the 10th percentile of their expected weight based on their gestational age. SGA places an infant at greater risk of health problems, from immediate complications at birth such as low oxygen levels to long-term challenges, including potential metabolic, cognitive and neurodevelopmental impairments. The team also found an association between pregnant women exposed to moderate smoke-density days in the first trimester and having a low-birth-weight baby, or an infant weighing less than five pounds, eight ounces. Like SGA, low birth weight is linked to an array of potential health problems, both short and long term.
“Overall, we found consistent associations between increased exposure to wildfire smoke and heat stress during preconception and the first trimester with the odds of having an SGA infant,” Khalili said.
The study also suggested that living in a climate-vulnerable neighborhood could increase the odds of an SGA birth, particularly for women exposed to heat stress during preconception. For women living in the most climate-vulnerable neighborhoods, the study showed the effect of heat stress during preconception on the likelihood of an SGA birth almost doubled.
Khalili and her colleagues also found that Fenton Z-scores, which measure how much an infant’s size deviates from the average for their age, significantly decreased with more days of wildfires experienced during pregnancy. These findings are particularly relevant given the longer-lasting fire events that Los Angeles experienced in January 2025 and that many cities across the US are currently facing.
“Understanding what’s happening to women during wildfires and excessively hot days could help us identify protective measures, develop guidance, and plan interventions,” said Khalili.
Given how frequent and long California’s wildfire seasons are expected to become, such interventions are urgently needed. But these need to go beyond simple measures an individual can take – like staying indoors, running air filters or air conditioners – to truly understanding and addressing what makes a community more resilient to these impacts.
Habre, who developed and oversaw the project, agrees that the study breaks new ground—and also that more holistic, actionable research is needed.
“As the recent Los Angeles fires have demonstrated, infrastructure, socioeconomic, and health vulnerabilities can combine with excessive climate and environmental factors to magnify health risks,” said Habre. “It is only by looking at the cumulative impacts of burdens communities are facing, now and into the future, that we can start to truly quantify health risks of climate hazards and target interventions to strengthen community resilience.”
About the study
The study’s additional authors include Yisi Liu, Yan Xu, Karl O’Sharkey, Tingyu Yang, Xinci Chen, Mario Vigil, Jill Johnston, Theresa M. Bastain, Carrie V. Breton, and Shohreh F. Farzan from the Department of Population and Public Health Sciences, Keck School of Medicine of USC; Nathan Pavlovic, Crystal McClure, and Fred Lurmann from Sonoma Technology, Inc., Petaluma, CA; Brendan Grubbs and Layla Al Marayati from the Department of Obstetrics and Gynecology, Keck School of Medicine of USC; Deborah Lerner andNathana Lurvey from Eisner Health; and Carmen J. Marsit from the Rollins School of Public Health, Emory University, Atlanta, GA.
This study was supported by the National Institute of Environmental Health Sciences, National Institutes of Health (NIH) [P50MD015705, P50ES026086, R01ES027409, P30ES007048], the Environmental Protection Agency [83615801], the National Heart Lung and Blood Institute (NIH) [P20HL176204] and the USC Provost Fund.
Journal
Environmental Science & Technology
Method of Research
Meta-analysis
Article Title
Adverse Birth Outcomes Associated with Heat Stress and Wildfire Smoke Exposure During Preconception and Pregnancy
Article Publication Date
18-Jun-2025
Study finds hidden effects of wildfires on water systems
Highlights:
- Wildfires disrupt microbial communities on land, but also alter aquatic systems when dissolved organic matter on burnt lands is carried into rivers, lakes or drinking water supplies.
- In a new study, researchers found that the impact of wildfires on water systems could lead to nutrient imbalances, depleting oxygen levels or harmful algal blooms.
- More complex treatment methods may be needed to ensure impacted water is safe for consumption or recreation.
Los Angeles, Calif.—Wildfires profoundly influence the unseen microbial world within our waters, directly impacting water quality and ecosystem health, according to a new study presented at ASM Microbe 2025, the annual meeting of the American Society for Microbiology.
“Understanding these critical, subtle changes is key to developing effective strategies for protecting our water resources in an era of increasing wildfire activity,” said presenting study author Courtney Gardner, Ph.D., assistant professor in the Maseeh Department of Civil, Architectural and Environmental Engineering at The University of Texas at Austin.
The researchers conducted the new study because the increasing frequency and intensity of wildfires pose a growing threat, not only to land, but also to vital water resources. “When rain falls on burnt landscapes, it carries dissolved organic matter, often referred to as ‘pyrogenic organic matter,’ into rivers, lakes and even drinking water supplies,” Gardner said. “A critical gap in our understanding was how this fire-derived material specifically impacts the aquatic microbial communities that are essential for maintaining water quality and cycling nutrients like nitrogen. Unbalanced microbial ecosystems can lead to compromised water health, affecting everything from aquatic life to the safety and treatability of our drinking water. Our goal was to uncover these subtle yet significant consequences.”
The new study involved setting up controlled experiments in the laboratory that mimicked what happens in natural water bodies. The scientists prepared water samples and then introduced different forms of dissolved organic matter: some from unburnt plant material, and some from plant material that had been combusted at low (250°C) or moderate (450°C) temperatures, reflecting different wildfire conditions. For 42 days, the researchers meticulously observed how the existing microbial communities in these water samples responded. This involved analyzing changes in their composition—which types of microbes were present and in what numbers—and examining the abundance of specific genes involved in the nitrogen cycle. By comparing these responses across the different water treatments, they could discern the direct effects of wildfire-derived organic matter.
They found the extent of burning dramatically influences the impact on aquatic microbes. Specifically, dissolved organic matter originating from fires at around 250°C led to a decreased diversity of microbial life and fostered less growth of microbial communities compared to organic matter from hotter burns. This suggests that the moderate heat processing creates more bioavailable forms of carbon that these microbes can readily utilize. Furthermore, the scientists observed significant decreases in the capacity for microbial nitrification, a crucial part of the nitrogen cycle that converts ammonia (a form of nitrogen that can be harmful in high concentrations) into nitrate (a form of nitrogen that plants can use).
“These findings carry important implications for how we manage water resources in wildfire-prone regions,” Gardner said. “The observed alterations in microbial communities and nitrogen-cycling processes can disrupt the natural balance of aquatic ecosystems. Such disruptions might lead to imbalances in nutrient availability, potentially fueling harmful algal blooms, depleting oxygen levels essential for aquatic life, or increasing the presence of undesirable compounds.” For communities, this means that post-wildfire water bodies may require more complex and potentially more expensive treatment methods to ensure water remains safe for consumption and recreational use. “Our research provides foundational knowledge to anticipate and address these challenges.”
The researchers said it's crucial to underscore that the new research highlights the interconnectedness of terrestrial and aquatic ecosystems. Wildfires have far-reaching effects on water that are often underestimated. “This study provides scientific evidence that reinforces the need for integrated land and water management strategies in regions vulnerable to wildfires,” Gardner said. “By focusing on fundamental scientific understanding, we can inform the development of more resilient communities and healthier environments in the face of escalating climate challenges.”
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The American Society for Microbiology is one of the largest professional societies dedicated to the life sciences and is composed of over 37,000 scientists and health practitioners. ASM's mission is to promote and advance the microbial sciences.
ASM advances the microbial sciences through conferences, publications, certifications, educational opportunities and advocacy efforts. It enhances laboratory capacity around the globe through training and resources. It provides a network for scientists in academia, industry and clinical settings. Additionally, ASM promotes a deeper understanding of the microbial sciences to all audiences.
Climate change will likely make prescribed fires rarer – but increase their air quality impacts
Prescribed burns are important for land management and preventing wildfires, but a new study finds these managed fires are also significant contributors to air pollution in the southeastern United States – particularly in areas with large minority and low-income populations. The study also finds these air quality impacts could become more pronounced in the decades ahead as the effects of climate change become more pronounced.
The term “prescribed burns” refers to activities where sections of land are intentionally burned to clear undergrowth, reduce the risk of wildfire, manage land for wildlife, and so on.
“However, smoke from prescribed burns can also pose challenges to air quality,” says Fernando Garcia-Menendez, corresponding author of a paper on the work and an associate professor of civil, construction and environmental engineering at North Carolina State University. “And climate change may reduce the number of days when it is possible to conduct a prescribed burn, due to meteorological conditions such as high temperatures, drought or high winds.”
“Given these concerns and given how important prescribed burns are to land management in the southeastern United States, we wanted to get a better understanding of how these burns affect air quality in the Southeast now – and how they may affect air quality in the future,” says Megan Johnson, first author of the paper and a Ph.D. graduate from NC State.
For this study, researchers drew on air quality and emissions data from federal databases for the year 2018, including the number and location of prescribed burns in southeastern states, as well as data from previously published climate change projections. In addition, the researchers conducted a survey of 223 land managers in the Southeast to learn more about how meteorological conditions affect their decisions about prescribed burns and how they think climate change may affect their ability to conduct prescribed burns in the future.
The researchers then fed these data into a complex computational model that allowed them to both capture how prescribed burns affected air quality in 2018 and to project how the burns would likely affect air quality between 2055 and 2059.
“In order to understand how burns and climate may affect air quality in the future, first you need to establish how burns are affecting air quality now,” says Garcia-Menendez. “And we found that, across the Southeast, prescribed burns account for 5-10% of particulate matter (PM2.5) air pollution annually – which is significant.
“However, prescribed burns account for 15-25% of PM2.5 pollution in areas with the most prescribed burns – which are clustered in the region where Alabama, Florida and Georgia meet,” Garcia-Menendez says.
“The areas with the highest levels of smoke are home to around two million people, and these populations have higher fractions of people of color and people with low incomes relative to both the regional average and to the populations experiencing the least smoke,” says Johnson.
“Right now, these regions are not violating air quality standards for PM2.5. But we found that climate change may make it difficult for these areas to comply with those standards in the future.”
The researchers found that climate change may affect smoke (and air quality impacts) from prescribed burns in multiple ways. One major factor is that if there are fewer days when land managers can conduct prescribed burns, it is likely that more burns will be conducted on the viable days. Similarly, land managers are more likely to burn large areas of land at once, rather than burning several small areas over time.
“Whether it’s more burns, larger burns, or both, you’re putting more smoke into the air on a given day, which means a lot of PM2.5 being put into the air at once,” says Johnson.
What’s more, the viable days for conducting a prescribed burn are more likely to occur on days in spring and winter when the pollution effects of the burn can be more pronounced.
“To be clear, these would be days when the conditions for conducting a safe prescribed burn are met, but other atmospheric variables effectively increase air pollution stemming from the fire,” says Johnson.
“It’s important to note that, based on our survey, land managers in the Southeast already feel smoke management is a limiting factor for prescribed burns,” says Garcia-Menendez. “In other words, they’re not able to conduct these burns as often as they would like due to air quality concerns. Our study suggests that this challenge may only get worse.
“It’s also important to note that the calculations in our study were made using an assumption that the area of land treated with prescribed burns would be approximately the same in 2055-2059 as it was in 2018,” Garcia-Menendez says. “But we’re now seeing a big push to increase the number of prescribed burns, largely in an attempt to reduce the underbrush that contributes to uncontrolled wildfires. If we do see an increase in prescribed burns, that will increase the air quality impacts.
“That doesn’t mean those burns aren’t necessary, but it is something we would have to address in terms of both public health and public policy.”
The paper, “Impacts of Climate Change on Land Management and Wildland Fire Smoke in the Southeastern United States,” is published open access in the journal Environmental Research Letters.
This research drew on work supported by the National Science Foundation under grant 1751601 and the Joint Fire Science Program under Project 21-1-01-18. The research was also supported by a U.S. Geological Survey Science to Action Fellowship.
Journal
Environmental Research Letters
Method of Research
Computational simulation/modeling
Subject of Research
Not applicable
Article Title
Impacts of Climate Change on Land Management and Wildland Fire Smoke in the Southeastern United States
Article Publication Date
16-Jun-2025
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