The Particulars of PM 2.5

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NRDC

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Why particulate matter … matters.

PM 2.5 (n.): Air pollution consisting of particles less than 2.5 microns across

A year after New York put its congestion pricing program into place, requiring vehicles to pay a fee to enter the central business district of Manhattan, city dwellers noticed changes. The sounds of car honking had quieted. Traffic was down. And the air was cleaner.

Researchers with Cornell University measured air quality citywide after the program had been in place for six months. Specifically for the central business district, they found a 22 percent decline in particulate matter 2.5 (PM 2.5, also known as soot). “As an asthmatic, I can palpably feel improvements in the air quality,” one resident told the New York Times.

Throughout the United States, millions of people live in areas with unsafe levels of soot air pollution. While the federal government made strides in advancing respiratory health protections by tightening limits on PM 2.5 in 2024, the U.S. Environmental Protection Agency (EPA) has since given up defending those limits. Piling on, reports have emerged that the agency will stop considering the economic damage caused by air pollution (including premature deaths, heart disease, lung cancer, and brain damage) as it loosens limits on such air pollutants. Here’s a primer on what the technical lingo means—and why disregarding this health threat will reverse the gains that people so badly need.

What is particulate matter pollution?

Particulate matter, or PM, is one of the primary contributors to air pollution. Any solid or liquid substance that is suspended in the air counts as particulate matter pollution, including dust, dirt, or soot. Components can include metals, sulfates, nitrates, organic compounds, and pollen.

Unlike the other major air pollutants—ground-level ozone, sulfur dioxide, carbon monoxide, and nitrogen dioxide—particulate air pollution is defined by size rather than the chemicals it contains. Typically reported in micrograms per cubic meter, the smaller these particulates are, the more damage they can wreak on the human body. That’s why they’re regulated in the United States by the Clean Air Act.

What are PM 2.5 and PM 10?

Epidemiologists worry about two categories of particulate matter: PM 10 and PM 2.5. The larger size, PM 10, includes particles less than 10 microns across. Likewise, PM 2.5 is 2.5 microns or less across. Although particulate matter aggregates to form the ghostly haze that hangs over smoggy cities like Los Angeles, you can’t see individual pieces of PM 10 or PM 2.5 with the naked eye.

A micron is one ten-thousandth of a centimeter. For the sake of comparison, most bacteria are at least five microns across. The diameter of a red blood cell is six microns. A strand of hair is around 70 microns wide. You could fit several thousand PM 2.5 particles on a period.

Where does PM 2.5 come from?

Road dust and tiny bits of, well, stuff sent into the air by stone processing, tire and brake wear, and other crushing operations are common sources of PM 10 pollutants. You can trace PM 2.5, on the other hand, primarily to combustion—vehicle engines and coal- or fracked (fossil) gas–fired power plants are major PM 2.5 sources. That’s why the three U.S. cities with the highest PM 2.5 levels are all in California. Not only is it a traditionally car-crazy state, but its roadways are clogged with diesel trucks, a primary culprit behind this pollution. These trucks ferry packages from the ports of Los Angeles and Long Beach to giant warehouses inland, used to ship more than 40 percent of goods to the rest of the country. The heavy tailpipe emissions contribute significantly to poor air quality and pose grave health risks, particularly to frontline communities residing near major freight routes and facilities, around the so-called Inland Empire. The fact that the region is surrounded by mountains exacerbates the issue, as they trap the polluted air.

Wildfire smoke, which is an increasing threat fueled by climate change, is another significant source of PM 2.5. Notably, wildfire smoke-generated PM2.5 is more dangerous than traditional PM 2.5 because it tends to contain more toxic materials like lead, mercury, and benzene.

What are the health effects of PM 2.5 versus PM 10?

PM 10 irritates human airways, especially among asthmatics and the elderly. These particles make your eyes burn and your throat dry. Public health experts, however, are less concerned about these larger forms of particulate matter because your body’s defenses are reasonably effective against them. Tiny hairs along the respiratory tract block a portion of PM 10, you cough and sneeze some of it out, and your throat’s mucus elevator transports a fair amount back out of your mouth or harmlessly into your digestive tract.

Your body, however, isn’t as good at blocking PM 2.5. These particles are small enough to bypass your respiratory system’s defenses, getting into your lungs, where they can even penetrate the bloodstream. That’s when they cause all manner of mayhem. Research has shown that an increase of just 10 µg/m3 of PM 2.5 in the air increases the risk of someone dying from heart disease by 10 percent. Exposure can also lead to higher rates of bronchitis, depressed lung function, asthma attacks, and even premature death. In 2019, the EPA’s nearly 2,000-page Integrated Science Assessment highlighted new harms from the chemicals, including nervous system effects, metabolic dysfunction, and elevated cancer risks.

What level of PM 2.5 is unhealthy?

While the EPA marks the upper limit for “good” air quality at 9 µg/m3 over one year, evidence shows that there is no safe level of air pollution exposure. And that threshold is frequently exceeded: Based on 2020–2022 air-monitoring data collected by state environmental agencies, 119 counties (out of a total of 3,143) experienced annual soot air pollution concentrations above these limits. This problem is imposing huge costs on our society in terms of deaths, illnesses, lost wages, and reduced worker productivity.

Have we made progress in limiting PM 2.5?

Yes: Since 2000, the EPA estimates that the Clean Air Act has helped to reduce PM 2.5 levels by 37 percent. Still, there’s more work to be done. By the agency’s own calculations, the stronger legal Clean Air Act limits on PM 2.5 that were finalized in 2024 would avoid 800,000 cases of asthma symptoms, 2,000 hospital visits, and 4,500 premature deaths each year. These outcomes would add up to $46 billion in health benefits in 2032, once all areas come into compliance. In turn, the cleaner air is expected to go hand-in-hand with economic growth, as it has after past air standard improvements.

More broadly, it is estimated that air pollution regulations under the Clean Air Act help the country avoid 370,000 premature deaths annually, delivering $2 trillion in economic benefits.

But that enormous progress achieved over five decades is now at risk, as the Trump EPA actionsloosen pollution limits, promote more fossil fuel burning, and deny the settled science of climate change.

Who is most at risk from PM 2.5 pollution?

Children and pregnant women face the greatest health threats from air pollution. Black communities and other people of color are exposed to roughly 1.5 times more deadly soot pollution than white communities and face higher rates of asthma, heart disease, and premature death. Anyone with heart or lung disease—such as angina, ischemic heart disease, or asthma—should also take extra care when facing high levels of particulate pollution.

How can you reduce exposure to PM 2.5 particles?

First, check the Air Quality Index of your city. If PM 2.5 concentration is high, it is recommended you avoid the outdoors. If you have to go outside, you can wear a face mask with a PM 2.5 filter to help reduce your exposure. If you’re shopping for one, be sure it has “NIOSH” ( National Institute for Occupational Safety and Health) as well as either “N95” or “P100” printed on it, includes two straps, and fits over your nose and under your chin, sealing tightly to your face. Air purifiers can be used inside your home to reduce pollutants in the air. You can also take advantage of public places with good ventilation systems when you need to get out of the house.

Note that wildfire smoke can travel far downwind and persist for days, and just because the air looks clear doesn’t mean it is. To keep track of air quality in your area when wildfire activity is high, visit the EPA’s AirNow fire and smoke map, which conveys levels of PM2.5 in the air using a six-tiered, color-coded system, from green to maroon. The agency states which people are at risk under each air quality level.

This story was originally published on November 14, 2014 and has been updated with new information and links.

Originally published on NRDC.org

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