Climate Matters•October 4, 2023
Wildfire Smoke: Nationwide Health Risk
KEY CONCEPTS
The average person in the U.S. has breathed in more harmful wildfire smoke in 2023 than in any year since 2006.
This year is part of a trend of increasing fire weather, which fuels the flames and smoke that put health and safety at risk in our warming climate.
Fine particle pollution from wildfire smoke is a nationwide health concern. Studies suggest that smoke exposure in the U.S. has increased 27-fold over the last decade.
The West has more large fires and local smoke-related pollution, but fires from the western U.S. and Canada affect the entire contiguous U.S.
New research links wildfire smoke with stalled or reversed air quality improvements in 30 U.S. states since 2016, with the worst effects in the West.
Download a .kml version of the Annual Average Wildfire Smoke Days map
Wildfires in 2023: record-breaking, devastating, and smoky
This year’s wildfire season has broken records across the Northern Hemisphere.
In August, dry weather and strong winds from Hurricane Dora fueled the deadliest U.S. fire in over a century, devastating the historic town of Lahaina on the Hawaiian island of Maui.
Extreme heat fueled intense fires in northern and central Greece, resulting in evacuations, loss of life, and the country’s third-smokiest July-August period on record.
Canada’s wildfire season has been relentless and record-shattering, leaving a total burned area roughly equivalent to the size of Oklahoma. That’s about 6.5 times Canada’s 10-year average burned area.
Climate change more than doubled the likelihood of extreme fire weather conditions in Eastern Canada in May and June that began Canada’s record fire season and brought unhealthy smoke to many U.S. cities and even across the Atlantic to Europe.
Record U.S. smoke pollution in 2023
The smoke from Canada’s massive, ongoing fire season has more than doubled the country’s previous annual record for fire-related air pollution.
Smoke pollution has continued to climb through September as fires continue to rage in western Canada, forcing mass evacuations.
Wildfire smoke can travel far and harm health hundreds or even thousands of miles away from where fires burn.
This is what the U.S. has experienced in 2023, largely due to Canada’s record fire season.
The average person in the U.S. has been exposed to far more smoke-related air pollution in 2023 than during any other year since 2006, according to data from the Stanford Environmental Change and Human Outcomes Lab.
In 2023 (through August 31), the average person in the U.S. has experienced about 66% more fine particle pollution from smoke than during the previous record year (2021).
Wildfire smoke harms health.
Wildfire smoke is a complex mix of pollutants, but fine particle pollution accounts for about 90% — and is the main threat to human health.
Fine particle pollution (known as PM2.5) includes particles with a diameter of 2.5 micrometers or smaller. That’s less than 5% of the width of a human hair.
When people breathe in wildfire smoke, these tiny particles can make their way deep into the lungs and even into the bloodstream, causing a range of health effects from minor irritation to serious cardiovascular and respiratory illness.
Emerging research suggests that wildfire smoke can also affect learning outcomes. A recent study found that exposure to fine particle pollution from wildfire smoke corresponds to lower test scores among U.S. students aged eight to 14 years.
Fine particle pollution can affect anyone but some people face higher risks, including the elderly, pregnant people, children, and those with chronic conditions such as asthma, COPD, and heart disease. Some people, including wildland firefighters, first responders, and outdoor workers, also face higher risks of exposure to fire and smoke during wildfire outbreaks.
Wildfire smoke exposure is on the rise.
According to a recent study, the number of people in the U.S. that experience at least one day each year with smoke-related fine particle pollution levels at 3x over the EPA standard has increased 27-fold over the last decade.
In 2020 alone, 25 million people in the U.S. experienced at least one smoke exposure day at this level. And per-capita smoke pollution exposure in 2023 has already been more than double the 2020 levels.
Wildfire smoke: an air quality issue everywhere
The average number of wildfire smoke days each year is highest in the western U.S. Parts of California, Oregon, Arizona, Texas, Louisiana, and Florida experience an average of 90-111 wildfire smoke days each year.
Although wildfire smoke is most acute in parts of the West, it’s a universal air quality issue. Every county in the contiguous U.S. now sees at least 16 wildfire smoke days each year.
Western and Canadian fires affect air quality across the U.S.
Wildfire smoke accounts for up to one-quarter of all fine particle pollution across the U.S., and up to half in parts of the West.
Western fires account for about half of the smoke experienced in the rest of the contiguous U.S.
The Northeast and Midwest are routinely affected by wildfire smoke from Canadian wildfires. About 60% of smoke in the Northeast in recent years came from fires outside the U.S.
Coast to coast health effects from wildfire smoke
Although most large fires occur in the western U.S., about three-quarters of mortality and asthma morbidity linked to smoke fine particle pollution occurs in other U.S. regions, due to long-range smoke transport and higher population density in eastern U.S. regions.
The same study found that asthma morbidity linked to smoke is highest during spring and summer iIn most U.S. regions.
Wildfire smoke has stalled air quality improvements in most U.S. states
Over the last six years, wildfire smoke has influenced trends in fine particle pollution in nearly three-quarters of states in the contiguous U.S.
Increases in wildfire smoke since 2016 have either stalled (in 22 states) or reversed (in eight states) decades-long air quality improvements. The worst effects have been felt in California, Oregon, and Washington, where air quality is affected both by local fires and by fires in western Canada.
If these trends continue, they threaten to undo decades of air quality improvements under the Clean Air Act.
Long-range smoke transport: how it works
Large-scale atmospheric circulation can transport wildfire smoke hundreds and even thousands of miles, affecting local air quality in places far from active fires.
The transport and dispersion of wildfire smoke involves both horizontal and vertical motion processes. The extreme energy and intense convection associated with wildfires can discharge smoke aerosols beyond the planetary boundary layer into the free troposphere — as high as 60,000 feet in the air. Prevailing winds in the upper atmosphere can then transport smoke particles long distances.
This is how smoke emissions from wildfires in northwestern Canada can travel across the Atlantic Ocean to create hazy skies over Europe.
Local air quality is impacted when smoke particles aloft descend to the surface. Turbulence, mixing, and subsidence can all drive the downward transport of smoke pollutants.
How does weather influence smoke pollution?
Local and regional weather conditions can influence the build-up of wildfire smoke in the air we breathe.
Multi-day smoke events are often caused by a weather phenomenon known as a temperature inversion — an atypical vertical temperature profile whereby air gets warmer (instead of cooler) with height.
Since warm air rises, air and smoke under the inversion cannot escape because it’s cooler than the air above it, preventing it from rising, mixing and dispersing. This is how a temperature inversion acts as a cap under which smoke becomes trapped in the air we breathe.
A shift in the atmospheric set-up and/or direction of the surface wind flow is required for the smoke pollution to clear.
More fire weather in a warming climate
In our warming climate, more frequent hot, dry, windy conditions, known as fire weather, makes fires more likely to start and spread — feeding the flames and smoke that put health and safety at risk.
According to Climate Central analysis, U.S. fire weather seasons have lengthened and intensified over the last 50 years (1973-2022), especially in the western U.S.
Southern California, Texas, and New Mexico now see around two more months of fire weather compared to 1973.
More modest increases in fire weather in the densely-populated eastern U.S., with nearly 28 million homes in burn-prone areas, put more people at risk.
Find your local fire weather trend: Burning Hot: 50 Years of Fire Weather Across the United States
Human-caused climate change accounts for at least two-thirds of the rapid increase in fire weather in the western U.S. in recent decades.
And the latest IPCC reports project more frequent fire weather conditions with increased warming.
LOCAL STORY ANGLES
Are wildfires affecting air quality in your area — today, or this month?
Find wildfire smoke tracking maps through the NOAA Office of Satellite and Product Operations and AirNow. View this month’s outlook for wildfire-related health risks in your local area using the Climate and Health Outlook Portal from the federal HHS Office of Climate Change & Health Equity.
CONTACT EXPERTS
Marshall Burke, PhD
Associate Professor
Stanford University Environmental Change and Human Outcomes Lab
Relevant expertise: Wildfires and air pollution
Contact: mburke@stanford.edu
Minghao Qiu, PhD
Postdoctoral Scholar
Stanford University Environmental Change and Human Outcomes Lab
Relevant expertise: Smoke-related air pollution
Contact: mhqiu@stanford.edu
FIND EXPERTS
Submit a request to SciLine from the American Association for the Advancement of Science or to the Climate Data Concierge from Columbia University. These free services rapidly connect journalists to relevant scientific experts.
Browse maps of climate experts and services at regional NOAA, USDA, and Department of the Interior offices.
Explore databases such as 500 Women Scientists, BIPOC Climate and Energy Justice PhDs, and Diverse Sources to find and amplify diverse expert voices.
Reach out to your State Climate Office or the nearest Land-Grant University to connect with scientists, educators, and extension staff in your local area.
METHODOLOGY
Cumulative daily PM2.5 exposure data from 2006-2023 were obtained from Stanford University’s Environmental Change and Human Outcomes Lab and was calculated using station data from the EPA. Daily smoke PM2.5 predictions by county from 2006-2020 come from Childs et al., 2022. Any non-zero day was considered a wildfire smoke day and was used to calculate the average annual predicted days with wildfire-related smoke by county. State-level data related to the contributions of wildfire smoke to PM2.5 trends in the U.S. were obtained from Burke et al., 2023. Special thanks to Marshall Burke and Stanford University’s Environmental Change and Human Outcomes Lab for their help with this release.