Why Tiny Particles Pose Big Problems - airborne dust causes health problems

National Wildlife, Feb-March, 2001 by Peter Jaret

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.