In chemistry, ‘organic’ refers to substances with a chemical structure containing carbon−hydrogen bonds. Contaminants are any substances that pollute, spoil or poison something. Organic contaminants, therefore, are substances that contain carbon and hydrogen atoms and can have adverse effects on air, water, soil, or living organisms when they enter the environment. Examples include polycyclic aromatic hydrocarbons (PAHs) which come from fossil fuels, and dichloro-diphenyl-trichloroethane (DDT) which was a commonly used insecticide.