Waste is considered hazardous if:

• it is ignitable (burns readily);
• it is corrosive or reactive (explosive); or
• it contains certain amounts of toxic chemicals.

The EPA has developed a list of some 500 specific hazardous wastes. Hazardous wastes may be solids, sludges, or liquids. Businesses that produce hazardous waste include dry cleaners, auto repair shops, hospitals, exterminators, and photo processing centers. Larger hazardous waste generators include chemical manufacturers, electroplating companies, and petroleum refineries.

Hazardous waste traditionally was disposed of on the land of the person or facility that generated the waste. Occasionally, the generator would transport the waste to an off-site disposal area. In either case, there often was little or no record keeping, so subsequent landowners were unaware of risks on their property, and waste was stored improperly, which caused land and water contamination and human exposure. Before federal regulation of hazardous waste disposal began, the EPA estimated that 290 million tons of hazardous waste were produced in the United States annually, ninety percent of which was disposed of improperly. Some old hazardous waste sites, like Love Canal, have been the subject of toxic tort lawsuits by people who were injured by exposure to the toxic substances.