Particles and Air Pollution
Why tiny particles cause big problems
A few years ago, if you had driven along certain residential streets in Long Beach or Los Angeles, you might have seen Ann Miguel pushing a vacuum cleaner down the center of the road. No, she’s not an obsessive neatnik. A scientist at California Institute of Technology, Miguel is one of a growing number of researchers studying an environmental threat all around us: dust.
Dust is not just the collection of lint that drifts into the corners of your rooms. Tiny particles, often called particulate matter, are swept up by passing car tires, blown into the wind from construction sites and farms and spewed into the air by tail pipes and power plants. And experts say there is growing evidence that all this dust and soot may cause serious health problems—not only for humans but for other species as well.
One alarm sounded last summer, when a comparison of levels of particulate matter and death rates in the nation’s 90 largest metropolitan areas was released. The study, conducted by scientists at the Johns Hopkins University School of Public Health, found that for every cubic meter of air, an increase of 20 micrograms of airborne particulate matter (that’s a scant 70 millionths of an ounce) brought a 1 percent rise in the death rate. Hospital admissions for elderly people exposed to the increased pollution rose between 2 and 4 percent.
These troubling findings are just the latest danger sign. "Once we got rid of most of the big, obvious nasty stuff in the air during the 1970s and 1980s, we thought we had addressed the particulate-matter problem," says John Vandenberg, director of the National Particulate Matter Research Program at the U.S. Environmental Protection Agency (EPA). "Then in the early 1990s studies came along that knocked us back on our heels." In one, the nonprofit Natural Resources Defense Council calculated that as many as 60,000 Americans die each year from lung diseases, heart conditions and other health problems brought on by breathing concentrations of airborne dust. More people may die prematurely as a result of exposure to minute particles in the air than die in accidents on the highway, experts now think.
The particles of greatest concern are 10 microns or less in diameter. (A human hair, by comparison, is about 70 microns thick.) Dust motes that spin in the air when the sun pours in the window measure roughly 10 microns. The much smaller airborne bits that make up cigarette smoke are only about half a micron or less in diameter—so small that instead of seeing individual particles you perceive a cloud of them concentrated together. About 60 percent of particulate matter 10 microns or less in size comes from combustion sources, such as cars and power plants. The rest come from construction, agriculture and particles stirred into the air by passing cars on the nation’s roads and highways.
When Miguel and her team analyzed road dust vacuumed off byways of Southern California using a standard Shop-Vac, they found soil, deposited motor vehicle exhaust, pollens, animal dander, even minute particles from brake linings and tires. Scientists have also identified heavy metals, cancer-causing toxic substances, bacteria and viruses in airborne dust.
The problem of particulate pollution isn’t limited to cities and suburbs. Jay Turner, an engineer at Washington University in St. Louis, set up monitors on both an urban interstate and a rural Illinois road. The average urban vehicle generates between 30 and 40 milligrams of particulate matter per mile traveled, he found. The average rural vehicle sends between 200 to 300 milligrams into the air. The reason: There is more soil dust on rural roads and more diesel-powered vehicles.
Those particles can cause breathing problems. In 1999, British researchers exposed 15 volunteers to air mixed with diesel exhaust particles for the period of one hour—about the level you would inhale on a smoggy day in Los Angeles. Six hours later, the scientists found signs of inflammation in the lungs. In another experiment, researchers from the University of Texas-Houston Health Science Center exposed immune cells called macrophages to ash collected from the Mount St. Helen’s eruption and to airborne dust from St. Louis and Washington, D.C. Volcanic dust had no effect on the cells. The urban dust, on the other hand, caused macrophages that normally keep immune reactions under control to die. The result: overly aggressive immune responses that cause inflammatory damage to the lungs.
Researchers think that inhaled particles can cause asthmatic attacks and may pose a serious threat to elderly patients whose lungs are already weakened by age and illness. Babies and young children are also at increased risk; per pound of body weight, they inhale more particulates than adults.
Lung problems are not the only worry. There is new evidence that tiny particles—from diesel trucks, cars, industrial plants and even windblown dust—also alter normal heart rate and rhythm. A healthy heart is able to vary its beats per minute widely, depending on the demands being placed on the body. But when researchers at Harvard Medical School took electrocardiogram and blood-pressure readings from 21 Boston residents over age 50, heart rate variability decreased dramatically when levels of very fine particulate matter in the air were high. "That’s worrisome, because decreased heart rate variability is known to be a risk factor for sudden heart failure," says the EPA’s Vandenberg.
Why tiny inhaled particles impact heartbeat is anyone’s guess. "One possibility is that when you inhale these very small particles deep into your lungs, some of them make their way into the bloodstream, where they would tend to reach organs that process a lot of blood, such as the heart," says University of Delaware environmental scientist Anthony Wexler, who has studied how very small particulates make their way into the lungs. "The particles could then become lodged in cardiac muscle, reducing blood flow and affecting heart rhythm." According to one estimate, particulates are responsible for 1 percent of all heart disease fatalities in the United States, or about 10,000 deaths a year.
Virtually no one has looked at the effects of airborne particles on wildlife, although many laboratory studies show that inhaled dust can damage both the hearts and lungs of dogs, mice and other animals. In one investigation, dogs exposed to levels of particulates no higher than those found in many U.S. cities experienced significant changes in their heart rhythms. And last year, scientists at Duke University warned that increasing amounts of atmospheric dust from the prolonged drought in the Sahel region of Africa have begun to threaten coral reefs in different parts of the planet. The dust, which is five times as thick as normal due to the dry spell, contains bacteria, viruses and fungi. Included is a soil fungus called Aspergillus that has killed more than 90 percent of the Carribbean’s sea fans, a form of soft coral.
There is little scientists can do to block dust storms whipped up by drought conditions, of course. But as worry over the health effects of airborne particles increases, experts are looking for ways to reduce exposure levels. The EPA is making a major effort to clean up diesel exhaust, which is a large contributor to particulate levels. In California, researchers at the University of California-Davis, have developed a novel way to "fingerprint" dust in order to trace its source. The technique, which identifies unique mixtures of soil bacteria and fungi, may be used eventually to enforce regulations that limit the amount of dust kicked up by agricultural operations.
Indeed, airborne dust may become a rallying cry for those opposed to development. In Phoenix, one of the biggest sources of air pollution is "fugitive dust," whipped into the air by the area’s booming new housing tracts. The dust poses a special danger because it has been shown to contain a fungus called Coccidioides immitis, which normally resides in the soil. Once inhaled, the fungus causes valley fever. Symptoms resemble a bad case of the flu, but if the fungus spreads from the lungs into other parts of the body, the disease can be fatal.
Luckily there are a few simple ways to reduce your exposure to particulates. First, heed warnings to stay indoors during days when air pollution levels are high—especially if you suffer from asthma, respiratory allergies or heart disease. If you exercise outdoors, schedule your workouts for early morning or in the evening, when particulate levels are generally lowest. Avoid jogging on streets with heavy traffic. If you find yourself driving behind a diesel-powered vehicle, leave extra open space between your car and the exhaust belching into the air ahead. At home, filters on heating and air-conditioning systems help remove dust from indoor air, as long as they are cleaned and maintained regularly.
At the national level, officials with the EPA are currently drafting revised clean air standards, due to be released in 2002. Given accumulating evidence of the health threat from fine particulates, the new standards are likely to represent a first big step toward reducing levels of these very tiny particles.
California writer Peter Jaret cleaned the filters on his furnace at home after reporting this article.
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