Over 156 million people in the US are exposed to unhealthy levels of air pollutants, according to the recently published 2025 State-of-the Air report from the American Lung Association (ALA). For 25 years the report has helped people across the country understand the quality of the air in their communities. It presents air pollution data at the county-level for two pollutants, deadly fine particulate matter (PM2.5) and ground-level ozone. Internal combustion vehicles are largely to blame, since PM2.5 is emitted directly from the tailpipes of gasoline and diesel vehicles, while ozone and even more PM2.5 are formed indirectly in the atmosphere from other toxic pollutants also emitted from tailpipes.
The report provides renewed evidence of the consequences for our health of using fossil fuels
In 1970 the Clean Air Act and the newly formed Environmental Protection Agency (EPA) were given authority by Congress to regulate pollution from passenger cars and other forms of transportation. Since then, there has been much progress in cleaning up our air, the result of effective environmental policies that control combustion emissions from sources such as power plants, industry, ports, warehouses and our vast network of roads.
However, the new report shows that compared to the 2024 report there are 25 million more people living in areas with dangerous air quality. The greater number of people exposed to polluted air can be partially explained by the now stricter PM2.5 standards for assessing exposure (explained below). But greatly to blame is the global warming-induced increased frequency and intensity of extreme heat and wildfires, which exacerbate both PM2.5 and ozone pollution 1. The findings add to the extensive evidence that global warming is making it harder to protect the hard-fought progress on air quality and public health and make it crystal clear that there is still a lot of work to be done to reduce emissions.
Reducing emissions from transportation is critical for the climate and for air quality
The role of transportation is a significant part of this ongoing work because that sector is the largest emitter of global warming emissions in the country, having surpassed the power sector in 2017. These emissions, which in 2022 were 37.4 percent of US CO2 emissions, are all from fossil fuel combustion in our vehicles, with the vast majority coming from pick-up trucks, SUVs and cars. The number of vehicles on our highways has increased dramatically over the decades. By 1970, with increasing travel demand, there were over 108 million vehicles on the road, more than double the number from twenty years before. This led to large contributions to both CO2 emissions and toxic local pollution and serious impacts on public health and the environment.
PM2.5 is directly emitted from the tailpipes of gasoline and diesel vehicles, and by tire wear, brake wear, and road dust. But beyond this primary formation of PM2.5, even more PM2.5 – referred to as secondary emissions – is formed in the atmosphere from precursor pollutants that also originate from the combustion in fossil-fuel powered engines, such as nitrogen oxides (NOx), sulfur dioxide (SO2) and volatile organic compounds (VOCs). Secondary PM2.5 can be a huge share of overall PM2.5, ranging from 30 to 90 percent of all PM2.5.
Ground-level ozone, or tropospheric ozone, is also produced from precursors that originate in internal combustion engines. It should be distinguished from stratospheric ozone, the good kind of ozone that shields us from harmful ultraviolet rays. Ground-level ozone is formed from the precursors NOx and VOCs and is not always immediately associated with vehicles or industry because it is not directly emitted from tailpipes or smokestacks.
Transportation’s disproportionate contribution to local pollution is further evidenced by the fact that up to 45 percent of NOx and up to one-third of VOCs from all sources comes from on-road vehicles. Medium and large trucks, which make up just 1 in 10 vehicles on our roads, contribute disproportionately to 58 percent of the ozone-forming nitrogen oxides and 52 percent of PM2.5 emissions.
According to multiple studies, transportation emissions alone cause from about 20,000 to over 50,000 PM2.5 -related deaths2 and 5,000 ozone-related deaths per year (Choma, Goodkind, Chao Li). In comparison, PM2.5 emitted solely from burning fossil fuels from all sources leads to about 200,000 premature deaths per year and about 10,000 due to ozone, meaning transportation is responsible for up to about one quarter of premature deaths from PM2.5.
Breathing any amount of PM2.5, for any length of time, is dangerous for your health
You can often smell the noxious fumes from the tailpipe of an idling truck or school bus but most of the time you can’t see or smell the toxic substances that enter your lungs and bloodstream and spread throughout many of your organs, leading to increased risks of all kinds to your health.
PM2.5 is a complex mixture of solid particles and droplets, less than 2.5 millionths of a meter in diameter, tiny enough to enter bloodstream via the lungs and even enter the brain. Strong evidence exists that breathing PM2.5 increases the risk of pulmonary and heart diseases, aggravated asthma, impaired neurological functions, type 2 diabetes mellitus, pre-term births, infant mortality, and even childhood obesity. The developing fetus, children, the elderly, the disadvantaged and people with pre-existing health conditions are especially susceptible to air pollution (CARB on PM, UCS blogs on PM)
The report looks at two kinds of PM2.5 measures.
- Year-round or annual PM2.5 reflects the overall state of pollution and is often highest in places subject to multiple sources of emissions all year long, such as highways and power plants. 85 million people live in 115 counties across 31 states that did not meet the EPA year-round standard, a small improvement over the 90.7 million people in last year’s report. The five metropolitan areas that rank worst in year-round PM2.5 are Bakersfield, Visalia, Fresno-Hanford-Corcoran, all in California, followed by Eugene-Springfield in Oregon, and Los Angeles.
- Short-term PM2.5 levels capture spikes of emission, which can be frequent in high traffic areas. These emissions are not evident from the larger scope year-round averages but can be particularly dangerous, especially for people who have preexisting heart and lung conditions. Here is a striking statement from the report: “Short-term spikes in fine particle pollution that last from a few hours to a few days can kill. Premature deaths from breathing these particles can occur on the very day that particle levels are high, or up to a month or two afterwards”. 77.2 million people live in 154 counties across 27 states that earned failing grades in dangerous spikes of PM2.5, an increase of 12.1 million more people than in last year’s report.
There is growing evidence that there is no ‘safe’ level of PM2.5, as exposure to low levels of PM2.5 is also deadly. A study shows that the elderly faced a higher risk of premature death even when levels of particle pollution remained well below the current short-term national EPA- set standard (to be described below).
For ground-level ozone, the severity of the problem is unprecedented
Ozone is a powerful irritant of the respiratory system, causing inflammation of the airways, a decrease in lung capacity and aggravation of asthma. Both short-term and long-term exposure are associated with increased mortality.
Between 1990 and 2020, national concentrations of ozone were reduced by 25 percent, but this progress is eroding. Ozone is produced from precursors in the presence of ultraviolet radiation from sunlight, and global warming creates conditions favorable to ozone formation by accelerating chemical reactions.
According to the report, 125.2 million people are breathing dangerous ozone levels, an unprecedented increase of 24.6 million compared to the previous report. The report states that ozone levels have increased in many locations, sometimes considerably. The report considers this a distressing reversal of the progress attained so far, with 137 counties losing their A grade, including 10 that went from an A to an F. There are now five times as many counties with ozone levels that have worsened compared to counties where there has been an improvement. The metropolitan area that ranks worst in the country in ozone pollution is Los Angeles, which has occupied this rank for 25 of the 26 years of the report.
The health burden of air pollution is not equitably shared
Key risk factors affect an individual or a community’s pollution burden. Over time disadvantaged communities have been subject to multiple discriminatory decisions such as transportation planning that builds highways through communities, underfunds public transit and prioritizes car dependence. Lack of adequate access to healthcare and psychosocial stress due to poverty and discrimination can all amplify the impact of air pollution on health.
The report finds that people of color are twice as likely as white people to live in a neighborhood with unhealthy levels of PM2.5 and ozone. Although people of color make up slightly over 40 percent of the total population, they make up about half of the people living in a county with at least one failing grade. Latinos are three times more likely than white people to be breathing in dangerous levels of both air pollutants. This evidence adds to a large body of studies showing the inequitable exposure to PM2.5 and other pollutants (see case studies on inequitable exposure for the US, California, Washington DC). A factor that exacerbates inequity is the large number of counties that do not have air pollution monitors. The ALA has a supplemental series, Something in the Air, that explores emerging and other technologies such as the use of satellites to collect data and help understand the pollution exposure in under-monitored regions.
The report analyses the concentration levels of PM2.5 and ozone at multiple locations where monitors are present
As context, it is important to know that the Clean Air Act requires the EPA to set health-based limits for six dangerous pollutants3. These limits, called the National Ambient Air Quality Standards (NAAQS), are levels of the pollutants which are considered ‘safe to breathe’ and are determined by experts at the EPA who base their recommendations on the most recent health and medical science.
The report compares PM2.5 and ozone levels obtained from monitors to the current US Environmental Protection Agency (EPA) standards, determining which locations attain the standards and are therefore assigned passing grades, and which locations are given failing grades4. For instance, the standard level for the annual average PM2.5 concentration is 9 micrograms per cubic meter (µg/m3). This means that any location with higher levels is considered not to have attained the standard. States and local governments are required to take steps to attain the standards5.
Standards have been strengthened by the EPA but still fall short
Under the Clean Air Act, the NAAQS standards must be updated periodically. In February 2024 the EPA finally made a long overdue update to the annual PM2.5 standard from 12 to 9 µg/m3, an update that has yielded large benefits to millions of people. The stricter standard was the one used for the grades and ranking published in the 2024 and 2025 reports, which led to more locations receiving failing grades in the recent report.
However, although this update represents progress, it still falls short of recommendations from a broad front of health organizations, scientists and the public. The Clean Air Scientific Advisory Committee (CASAC) is an independent group of experts that provides science-based advice and guidance to the EPA on the NAAQS66. At the time of the update, a committee majority strongly recommended standards ranging from 8 to 10 µg/m3. Adopting the most stringent PM2.5 annual standard proposed by the EPA (9 µg/m3 ) would save more than 4,200 lives compared to the previous 12 µg/m3, but adopting the strictest standard (8 µg/m3) would save more than twice that number of lives in the year 2032 and would improve racial disparities. It’s worth mentioning that the World Health Organization recommends a much stricter PM2.5 annual standard of 5 µg/m3 (see blogs on UCS hosting of independent review panel, Industry tactics, EPA air pollution science).
Another reason why standards fall short is that when the EPA revised the annual PM2.5 standard it opted not to revise the current short-term standard of 35 µg/m3, in spite of recommendations that lowering the limit to between 25 and 30 µg/m3 would more effectively address the health impacts of PM2.5 spikes. I should note that the World Health Organization recommends a much stricter 15 µg/m3 for the short-term standard.
Since 2014 CASAC has consistently recommended that the ozone primary standard be lowered from the current standard of 70 parts per billion (ppb) to 60 ppb, but the 2024 EPA rule did not make the standard stricter. CASAC said that the current 70ppb “provides little margin of safety for the protection of public health, particularly for sensitive subpopulations”7.
Much of the progress achieved so far is at stake, including vehicle electrification
For over 45 years, thanks to measures enacted under the Clean Air Act, there has been a 78 percent reduction in the combined six criteria air pollutants. From 2000 to 2020, national PM2.5 concentrations decreased by 41 percent while from 1990 to 2000 ozone concentrations decreased by 25 percent, as already mentioned.
This reflects decades of hard work to enact regulations that help clean up not only transportation, but also electricity generation and other sectors. While we need to continue this progress, there are now serious headwinds in our way. The executive order announced in January and the EPA announcement from March 2025 indicate that the current administration is seeking to roll back fuel economy and greenhouse gas standards that have reduced vehicle fuel consumption dramatically over the decades and have led to tremendous savings and benefited the health of people all over the country. Fuel economy and greenhouse gas standards have together saved 2 trillion gallons of gasoline since 1975 and reduced global warming gases by 14 billion tons.
This UCS blogpost talks about the vehicle standards that could be targeted, namely the Multi-Pollutant Passenger Vehicle Emission standards, the Federal Heavy-Duty Truck standards, and the Corporate Average Fuel Economy standards. The waiver that allows states to adopt the stronger California vehicle standards is also under threat. Another regulation that is being challenged is the California-based Heavy-duty Omnibus rule, which requires new heavy-duty engines to achieve a 75 percent reduction in NOx emissions, on average, compared to the current federal standards that have been in effect since 2010. The legality of these rollbacks is uncertain, but if they become reality the number of people impacted by breathing polluted air is going to increase significantly, beyond the 156 million already affected. The progress made so far in electrifying our vehicles will be at risk, as standards mandate the increasing sale of zero-emissions vehicles and set stricter emissions levels for gas and diesel vehicles. Electric vehicles do not have tailpipe emissions, have reduced break wear and are of supreme importance in cleaning up air in cities and near highways, warehouses and ports. And last but not least, the rollbacks will be a major setback in reducing global warming emissions and achieving US climate targets.
The report makes a ‘clarion call’ for action to defend the EPA
For decades the EPA has played a critical role in reducing transportation emissions by setting and enforcing standards for vehicles, promoting fuel efficiency and other important strategies. The report provides recommendations for actions that decision makers and communities can take to improve air quality and calls very strongly for the defense of EPA staff, funding and rules. At the beginning of May, the EPA announced a new phase of ‘organizational improvements’ which include massive layoffs to staffing and a $300 million reduction to the EPA annual budget. The EPA’s core mission is under a severe threat, particularly the dismantling of the Office of Research and Development and other offices, and of the advisory committees CASAC and SAB.
As the report points out, the EPA is first and foremost a public health agency. Access to air quality data is highly dependent on the EPA’s work, some of which the report lists:
- Monitoring air quality and providing access to a wide variety of air quality resources. Making air quality forecasts available to state and local governments and to communities across the country.
- Reviewing health science and making updates of health-based standards such as the NAAQS.
- Keeping up with the latest science through CASAC and the SAB or any other reconvened committees.
- Analyzing vehicle pollution control technologies and testing pollution emissions from vehicles.
- Gathering input from stakeholders and the public in developing standards and on the benefits of policy options.
- Providing grants to governments and communities to help monitor and reduce air pollution and enforcing safeguards.
The recent attacks on the Clean Air Act and the EPA risk jeopardizing more than 50 years of hard-won progress to reduce emissions and improve public health. The EPA’s work, including the State-of-the-Air report, has had an extraordinary impact on improving air quality from transportation and other sectors, preventing thousands of premature deaths and hospitalizations every year. We cannot afford to allow the EPA’s work to be undercut.
- The report finds that the distribution of air pollution is shifting back to East because of the exclusion of data from 2020, which was a disastrous year for California (due to wildfires), and because of the inclusion of data from 2023, when West Coast conditions improved but wildfires in Canada drove up levels of PM2.5 and ozone in central and eastern states. ↩︎
- This is more than the deaths from car accidents in 2024 in the US. ↩︎
- These are the criteria air pollutants, namely PM2.5, ozone, nitrogen dioxide, sulfur dioxide, carbon monoxide and lead. ↩︎
- For PM2.5, the report identifies the number of days in a county when the maximum daily PM2.5 concentration occurred within defined ranges. The underlying data for the 2025 report was collected by the EPA in 2021, 2022 and 2023, the most recent 3 years of quality-assured publicly available data. The data are from federal, state, local and Tribal government monitoring sites and include data associated with exceptional and natural events, such as wildfires. ↩︎
- States must develop State Implementation Plans for areas designated to be in non-attainment of the standard. ↩︎
- Members of CASAC, as well as of EPA’s Science Advisory Board (SAB), were all ousted in January, shortly after the new administration took over, signaling an attack on public health protections. ↩︎
- Letter from Dr. H. Christopher Frey, Chair, Clean Air Scientific Advisory Committee, to Gina
McCarthy, Administrator, EPA, June 26, 2014 ↩︎