Any chemical compound used to kill pests that detract from agricultural production or are in some way harmful to humans. Pesticides include herbicides (such as glyphosate), which kill unwanted plants or weeds; insecticides (such as pyrethrum), which kill insect pests; fungicides, which kill fungi; and rodenticides (such as warfarin), which kill rodents. The main drawback with pesticides is that often they are nonspecific and may therefore be toxic to organisms that are not pests; they may also be nonbiodegradable, so that they persist in the environment and may accumulate in living organisms (see bioaccumulation). Organophosphorus insecticides, such as malathion and parathion, are biodegradable but can also damage the respiratory and nervous systems in humans as well as killing useful insects, such as bees. They act as anticholinesterases by inhibiting the action of the enzyme cholinesterase. Organochlorine insecticides, such as dieldrin, aldrin, and DDT, are very persistent and not easily biodegradable. See also neonicotinoids.

http://www.pesticides.gov.uk/guidance/industries/pesticides/topics/About-The-Chemicals-Regulation-Directorate

http://www.hse.gov.uk/pesticides/ Pesticides section of the UK’s Health and Safety Executive website