Herbicides are designed to control or kill weeds. There are over 250 chemicals known to have herbicidal activity, coming from a wide range of chemical types and biological properties. Some herbicides have modes of action specific to weeds and present a low risk to mammals, others are less specific and present a high risk to mammals. The use of herbicides is very widespread but there is some evidence of a decline over recent years. Residues of herbicides in food are generally very low and human intakes are normally well below acceptable exposure levels.