Decades of progress in cleaning the air in America’s most populated urban centers are being systematically undone by the growing threats of wildfire smoke and extreme heat. What were once considered separate environmental challenges are now frequently combining into hazardous “compound events,” creating more intense and toxic air quality problems that are spreading across the continent and affecting the health of millions.
A new wave of research reveals that this is not a simple addition of pollutants but a complex chemical interaction. Long-distance smoke plumes, carrying fine particulate matter from distant blazes, are mixing with existing urban pollution from traffic and industry. This mixture is then intensified by rising temperatures, which accelerate chemical reactions to form dangerous secondary pollutants like ozone and other toxic compounds. This dynamic is transforming the nature of air pollution, posing new risks that legacy air quality regulations were not designed to address and putting nearly half the U.S. population in areas with unhealthy air.
The Chemistry of a New Smog
Scientists are increasingly focused on the chemical reactions that occur when wildfire emissions reach a dense urban environment during a heat wave. Research led by Colorado State University, using New York City as a case study during the 2023 Canadian wildfires, provides a detailed view of this process. The study found that particles in wildfire smoke do not simply pass through the city; they interact with a dense soup of existing emissions. These include volatile organic compounds (VOCs) from vehicle exhaust, commercial cooking, and even consumer products like cleaning solutions.
Heat acts as a powerful catalyst in this process. Higher temperatures increase the rate at which VOCs are released from both man-made sources and natural ones, such as the gases emitted by trees and other vegetation. These gases then react on the surface of smoke particles, which absorb and concentrate the pollutants, leading to the formation of secondary organic aerosols and ground-level ozone, often referred to as smog. Professor Delphine Farmer, who led the research, noted that while West Coast cities have long dealt with these issues, the emergence of this phenomenon on the East Coast presents new variables, including denser populations and different local vegetation, which contribute to these novel pollution events.
A National Reversal of Progress
The consequences of these atmospheric changes are being documented on a national scale. The American Lung Association’s 2025 “State of the Air” report concluded that approximately 156 million Americans—nearly half the population—now reside in areas with unhealthy levels of ozone or particle pollution. This represents a significant increase of about 25 million people exposed to poor air quality compared to the previous year’s report. The findings, based on data from 2021 to 2023, directly attribute this decline to the increasing frequency of extreme heat events and widespread wildfire smoke.
This trend marks a troubling reversal of air quality improvements achieved under the Clean Air Act. For years, regulations successfully reduced industrial and vehicular emissions, leading to cleaner skies. Now, however, the sheer scale of wildfire smoke is overwhelming that progress. A study by Climate Central found that Americans are now breathing four times more harmful smoke than they did just a decade ago. The geographic scope of the problem has also expanded dramatically. Previously concentrated in Western states, high pollution readings are now common in the Midwest and the Northeast, as smoke from massive Canadian and Western fires travels thousands of miles.
Amplified Dangers of Compound Events
The convergence of extreme heat and air pollution creates a health threat greater than the sum of its parts. Researchers identify these occurrences as compound events, where the combined stress on the human body is magnified. Studies covering more than two decades of data show that U.S. cities are not only hotter but also more polluted during these heat spells. Urban heat waves last longer and are more intense than in surrounding rural areas, and when combined with high pollution, cities bear a significantly higher health burden. Nearly 89% of U.S. cities recorded more intense combined heat and pollution events than nearby rural regions.
This is partly due to the urban heat island effect, where concrete and asphalt absorb and retain more heat than natural landscapes. This elevated temperature in cities creates a feedback loop, accelerating the chemical reactions that form smog while also increasing the demand for air conditioning, which can strain power grids and lead to higher emissions. The result is that millions of urban residents are exposed to a more dangerous environment, with the most vulnerable populations, including children, the elderly, and those with pre-existing respiratory conditions, facing the highest risk.
Severe and Long-Term Health Impacts
The primary pollutant of concern in wildfire smoke is fine particulate matter, known as PM2.5, which consists of particles so small they can penetrate deep into the lungs and enter the bloodstream. Exposure to PM2.5 is linked to a host of serious health issues, including asthma attacks, heart disease, cancer, and even dementia. Unlike PM2.5 from other sources, wildfire smoke contains a complex mixture of chemicals from burning vegetation and, increasingly, man-made materials from homes and businesses, making it uniquely toxic.
Recent research indicates the health consequences of wildfire smoke are more severe and longer-lasting than previously understood. A study published in Nature by Stanford University researchers revealed that the mortality impacts of smoke exposure can manifest for up to three years after an event. This long-term effect is often missed in traditional health assessments, which may underestimate the true burden of wildfires. The Environmental Protection Agency’s Exceptional Events Rule, for example, sometimes allows for the exclusion of wildfire-related PM2.5 from air quality assessments, potentially masking the full public health crisis.
A Future of Worsening Air Quality
Projections for the coming decades paint a grim picture if greenhouse gas emissions continue on their current trajectory. The Stanford study forecasts that by 2050, climate-driven increases in wildfire activity could cause approximately 30,000 additional deaths each year in the United States due to smoke pollution alone. This represents a more than 70% increase from the estimated 40,000 annual smoke-related deaths between 2011 and 2020.
The economic toll is projected to be equally staggering, potentially reaching $608 billion in annual damages by mid-century under a business-as-usual scenario where global temperatures rise about 2 degrees Celsius. According to senior study author Marshall Burke, this figure surpasses current estimates for all other climate-related damages in the U.S. combined, including those from storms and agricultural losses. The states projected to see the largest increases in annual smoke-related deaths include not only Western states like California and Washington but also populous states farther east, such as Texas, New York, and Pennsylvania, underscoring that no community is immune from the far-reaching impacts of wildfire smoke.
