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Much of the haze that once blanketed our cities has cleared since the 1970s, thanks to tough environmental laws. But air pollution, including the kind you can't see, still poses health risks to millions of Americans. On any given day, the following five pollutants can be found at dangerous levels in the air somewhere in the nation. Learn how they could be affecting your health, and what you can do to clear the air.

Diesel

Sooty, smelly diesel exhaust is more than just unpleasant -- it's a mix of more than 40 toxic air contaminants and has been linked to cancer, asthma and more than 21,000 premature deaths across the nation every year. Many Americans live and work near diesel hotspots such as truck depots, bus terminals, ports, construction sites and busy roads and highways. Some studies have shown that kids riding inside diesel school buses can be exposed to higher levels of unhealthy diesel soot than people riding in nearby cars.

Since the early 1990s, NRDC's Dump Dirty Diesels campaign has been at the forefront of efforts to clean up the nation's diesel engines. Today, soot-free natural gas buses are replacing diesel ones in some cities, while new hybrid-electric buses and trucks are starting to enter the marketplace. New York City's bus fleet is more than 90 percent cleaner than it was when our campaign launched in 1993, Los Angeles has several thousand natural gas buses and Washington, D.C., has added hundreds of natural gas buses to its fleet. At the same time, upcoming federal diesel fuel and pollution standards will dramatically cut future pollution from trucks, buses and nonroad diesel vehicles like tractors and bulldozers. But because these engines can last for decades, we'll still see dirty diesels on our roads for years to come.

Until then, take care to keep children, as well as yourself, away from anything spewing black, smelly smoke from its tailpipe. You can also urge your local transit authority, school district or municipal government to clean up its fleet. When cities and towns retire their oldest, dirtiest diesels, replace them with the newest models and retrofit their remaining diesels with pollution controls, they can make a huge contribution to the fight for clean air.

Formaldehyde

Formaldehyde is a cancer-causing chemical used for a number of purposes, including the manufacture of insulation, pesticides and disinfectants. But most industrial emissions of formaldehyde -- 42 percent -- are from the lumber industry, which uses adhesives containing formaldehyde to make plywood. Formaldehyde has been linked to lung cancer, and may also cause leukemia and asthma attacks. Diesel emissions also contribute to formaldehyde pollution, and NRDC's ongoing work to clean up diesel is having an important impact.

To minimize formaldehyde exposures at home, make sure any wood furniture or fixtures that use particleboard are laminated or otherwise coated, choose exterior- rather than interior-grade pressed wood products for remodeling and be sure that areas where you are using products containing formaldehyde are well ventilated.

Benzene

Used in motor fuels, solvents, detergents, pesticides and many other substances, benzene is a carcinogen that causes leukemia as well as a number of other illnesses. Virtually the entire U.S. population is exposed to benzene in at least small amounts -- at gas stations (it's in the gasoline), in diesel exhaust or from cigarette smoke, including second-hand smoke. Benzene also a problem in a number of workplaces, including oil refineries, coal-coking operations at steel mills, chemical processing plants, rubber manufacturing plants and laboratories, where it is often used as a solvent for other chemicals. You can avoid one significant source of benzene by stepping away from the gas pump when refueling your car.

Particulate Matter

When you inhale microscopic bits of particulate matter -- otherwise known as soot -- fine particles can become embedded in your lungs and impair their function. As many as 64,000 premature deaths occur each year from cardiopulmonary causes attributable to particulate air pollution, according to NRDC estimates. Most particulate emissions result from burning fossil fuels -- coal, oil, diesel, gasoline -- or wood. Old coal-fired power plants, industrial boilers, diesel and gas-powered vehicles and wood stoves are some of the worst culprits. High-temperature industrial processes such as metal smelting and steel production are also significant sources.

Everyone can help cut down on particulate matter pollution by conserving energy and choosing cleaner, more efficient energy sources for home heating and cooling, transportation, and appliances. See NRDC's guides to find out how to buy clean energy, reduce your energy consumption and save fuel now. Certainly, limiting your use of wood-burning fireplaces and stoves is a good start.

Ground-Level Ozone

High up in the stratosphere, ozone forms a protective layer blocking harmful ultraviolet rays from the sun; but when pollution causes excess ozone to form at the ground level, it can cause severe health problems. Ground-level ozone forms when nitrogen oxides and other pollutants emitted by cars, trucks, buses, coal-fired power plants and other fossil-fuel burners react with sunlight to form the principal ingredient in smog. Nationally, more than 150 million Americans live in counties where they are exposed to unhealthful levels of ozone smog or particulate pollution. Every summer, tens of millions of Americans, especially those with vulnerable lungs or immune systems, are warned to stay indoors on certain high-pollution days because of smog.

To protect yourself and your family, take those warnings seriously. NRDC is pressing the EPA to adopt tougher new national ambient air quality standards that would effectively tighten restrictions on ozone emissions.

last revised 7/12/2005