If you plan to seal your blacktop driveway, avoid coal tar-based sealants. They contain polycyclic aromatic hydrocarbons, or PAHs, toxic compounds shown to cause cancer or other genetic mutations. When rainwater and other precipitation hit your driveway, the toxic chemicals run off into your yard and into your local drinking water supply. In fact, this situation has been compared to dumping quarts of motor oil right down a storm drain. The dust is often tracked into homes, too.
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Gravel and other porous materials are best for driveways because they allow rainwater to sink into the ground, where it gets filtered and doesn't inundate water treatment plants. But if you do seal blacktop, pick asphalt sealant and stay away from any product that has coal tar in its name (or products simply called "driveway sealant"). Lowe's and Home Depot have already banned the bad stuff, but smaller hardware stores may still carry it.