Waste are unwanted or unusable materials. Waste is any substance which is discarded after primary use, or is worthless, defective and of no use. A by-product by contrast is a joint product of relatively minor economic value. A waste product may become a by-product, joint product or resource through an invention that raises a waste product's value above zero. Examples include municipal solid waste, hazardous waste, wastewater, radioactive waste, and others. Medical wastes are considered as a special category of waste because they pose potential human health and environmental risks, as they contain sharps, human tissues or body parts, discarded plastic materials contaminated with blood, discarded medical equipment, and other infectious materials.