The Particulars of PM 2.5

Why particulate matter…matters.

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

Delhi has the most polluted air on earth. The World Health Organization reports that the Indian capital records an average PM 2.5 reading of 153 micrograms per cubic meter. That’s nearly triple the level in Beijing—no slouch when it comes to smog—and more than ten times the average in New York City.

Even those depressing statistics, however, understate the problem in Delhi, where polluters lie about their emissions, and poorly calibrated air monitors systematically underreport PM 2.5 levels by approximately one-half. That said, earlier this week one Delhi neighborhood measured a PM 2.5 reading of 580 micrograms per cubic meter.

While I can’t clear the air in Delhi (that’s the Indian government’s job), I 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. (If you want to know the most common chemical constituents of particulate matter, see page 2 of this WHO document.)

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).

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 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.

Road dust and tiny bits of, well, stuff sent into the air by stone processing and other crushing operations are common PM 10 pollutants. PM 2.5 comes primarily from combustion. Fireplaces, car engines, and coal- or 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 central and southern California. Other problem cities have large coal-fired power plants (Pittsburgh) or rely heavily on wood stoves for heat (Fairbanks, Alaska).

PM 10 irritates human airways, especially among asthmatics and the elderly. These particles make your eyes burn and 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. People who live in high PM 2.5 cities have more heart attacks, depressed lung function, worse asthma, and overall die younger than people who breathe clean air. Indices that score air pollution in cities typically give more weight to PM 2.5 than PM 10 levels.

In other words, when it comes to air pollution, size matters.

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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