After studying coal-fired utility wastes in 1993, the EPA decided to permanently exclude large volume coal fired utility wastes, including fly ash, bottom ash, boiler slag and flue gas emission control waste from the definition of hazardous waste. Studies have shown that although trace elements may leach from coal ash in prolonged contact with the water table, they do not migrate far from the ash site and are present in very low concentrations, and therefore do not present a health threat.