The Particulars of PM 2.5

Why particulate matter…matters.

November 14, 2014

Jean-Etienne Minh-Duy Poirrier

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

Delhi New Delhi has the most polluted air on earth. In 2019, particulate matter in the city reached levels more than 20 times higher than the worst air pollution measured in Manhattan that same year. Specifically, it got to 900 micrograms per cubic meter (µg/m3); for reference, the U.S. Environmental Protection Agency’s (EPA) scale to measure particulate matter maxes out at 500 µg/m3, at which point the air is considered “hazardous.” 

What is particulate matter pollution?

While we can’t clear the air in New Delhi (that’s the Indian government’s job), we can at least clear up some of this lingo. 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. 

Unlike the other major air pollutants—ground-level ozone, sulfur dioxide, carbon dioxide, 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. Which is why they’re regulated in the United States by the Clean Air Act (and why our air no longer approaches the dirtiness of China's or India's).

What is 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 New Delhi and other polluted cities, you can’t see individual pieces of PM 10 or PM 2.5 with the naked eye.

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 and other crushing operations are common sources of PM 10 pollutants. You can trace PM 2.5, on the other hand, primarily to combustion—fireplaces, car engines, and coal- or fracked (natural) gas–fired power plants are all major PM 2.5 sources. That’s why the five U.S. cities with the highest PM 2.5 levels are all in car-crazy California. Other problem cities have large coal-fired power plants (Pittsburgh), rely heavily on wood stoves for heat (Fairbanks, Alaska), or are prone to wildfires (Medford, Oregon)

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. PM 2.5 is responsible for an estimated 4.2 million deaths around the world every year. In other words, when it comes to air pollution, size matters.

What level of PM 2.5 is unhealthy?

While the EPA marks the upper limit for “good” air quality at 12 µg/m3 over 24 hours, evidence shows that there is no safe level of air pollution exposure. This problem is a global menace and is imposing huge costs on our society in terms of deaths, illnesses, lost wages, and reduced worker productivity.

How can you filter out PM 2.5 particles?

First, check the Air Quality Index of your city here. 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, it has two straps, and it fits over your nose and under your chin, sealing tightly to your face. Air purifiers can be used inside your home to get rid of 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.


onEarth provides reporting and analysis about environmental science, policy, and culture. All opinions expressed are those of the authors and do not necessarily reflect the policies or positions of NRDC. Learn more or follow us on Facebook and Twitter.

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