Pesticides are a general class of chemicals or biological agents used to control, repel, attract or kill pests. Pests are organisms that include insects, weeds or other plants, birds, mammals, fish and microorganisms that compete with humans for food, destroy property, spread disease or are considered a nuisance. Pesticides are usually, but not always poisons that kill their target organism. They are often classified according to the organism they are used to control. Although the term pesticide is frequently used interchangeably with insecticide, an agent designed specifically to kill an insect, the term has a much broader meaning. In addition to insecticides, pesticides include herbicides that kill plants and most often weeds, fungicides that kill fungi and molds, fumigants that sterilize soil or other materials by killing all organisms present, rodenticides that kill rodents such as rats and mice, nematicides that kill nematodes, algicides that kill algae and molluscicides that kill mollusks. Herbicides can kill outright by defoliation or can be used to regulate plant growth. Pesticides may consist of a single chemical compound, but are more often part of a complex formulation consisting of a carrier medium like a solvent and the active ingredient, the pesticide. The formulation may be as a dust, an emulsifiable concentrate, granules, or a soluble or wettable powder. They can consist of synthetic or natural organic chemicals, inorganic compounds or biological organisms. They may work on contact or have to be ingested. In the case of plants, they make act systemically. They can be toxic to animals and human who come into contact with them as a result of the application process and from touching, breathing or eating materials containing the pesticides. Drinking water contamination and foods are two primary sources for humans to be affected by pesticides. Foods can be directly contaminated through application or can transmit the pesticide and its residues through the food chain. Herbicides are a category of pesticides that are synthetically produced or naturally derived chemicals used to kill or control the growth of undesirable plants like weeds and trees that compete with crops for root space, nutrients and water or to remove brush or foliage so that land may be used for other purposes or to control aquatic weeds. Nearly 70% of all pesticides used by farmers and ranchers are herbicides. They are also widely used on lawns and in gardens and beside roadways. Selective herbicides kill certain target species, e.g. broadleaf (dicot) weeds are selectively killed by 2,4-D in turf areas leaving the desired plants unharmed, while herbicides used to clear ground are designed to be non-selective and kill all plants contacted. Some herbicides interfere with plant growth by acting as hormones. They can be toxic to soil microorganisms, beneficial insects, animals and humans and can contaminate water supplies.