Hazardous waste is any waste with properties that make it potentially dangerous or harmful to human health or the environment. The EPA has developed four categories of specific wastes (Listed Wastes) and four with defined characteristics (Characteristic Wastes) to identify wastes considered to be hazardous. Waste is classified as "hazardous" when it is discarded or no longer of value such as in the case of expired drugs or thermostats containing mercury. These wastes should always be disposed in an approved hazardous waste container.