Fall is the time when many homeowners hire contractors to seal the asphalt pavement in their driveways. But they may be unwittingly spreading pollution in the process, because most sealants are made from coal tar, and scientists have identified coal tar-based pavement sealers as a major source of toxic pollution to streams and lakes across the U.S.
Since the Clean Water Act stopped rampant discharges of pollution from industrial pipes, the greatest source of pollution to most streams, rivers and lakes comes from rainwater runoff: As rain washes oils, salts and other pollutants off driveways, roads, parking lots and rooftops, the many small sources of pollution add up to big problems that can contaminate fish, damage streams and undermine entire ecosystems.
For the nation's urban lakes, there's no greater source of one major pollutant, polcyclic aromatic hydrocarbons (PAHs) than coal tar-based pavement sealant, according to a U.S. Geological Survey study. The agency has found that runoff from coal tar-sealed pavement can be 65 times as polluted as unsealed pavement. (Asphalt-based sealants also cause pollution levels to spike, but "only" 10 times, compared to unsealed pavement runoff.)
What's more, the dust from sealed driveways can be hundreds or thousands of times more contaminated with PAHs, and homes near sealed pavement have indoor air pollution levels 25 times higher than those near unsealed pavement. Children, because they tend to spend more time crawling on floors where dust settles (and playing on recently sealed pavement), are likely to be exposed to more PAHs than adults.
Exposure to PAHs has been linked to many illnesses in laboratory studies that have been well-documented by U.S. health authorities, though the risk of exposure to the levels found in dust from sealed pavement hasn't specifically been studied. Research is continuing all the time, and scientists continue to identify health risks from exposure that include everything from lung diseases like asthma and cancer to brain development and reproductive health. For the environment, PAHs can be toxic to fish and other aquatic life.
Some cities (Austin, Texas, and Washington, D.C. among them) and one state (Washington) have banned the use of coal tar-based pavement sealants. If you must seal pavement, look for asphalt sealants, but know that even they will increase pollution levels near your home. Other alternatives include different surfaces, like gravel or pavers, both of which have the added benefit of being permeable to rainwater, so they won't contribute to the flow of urban runoff.
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