Herbicides are used in agriculture to eliminate weeds that would otherwise damage the crop of interest or to provide practical advantages in facilitating the harvesting of crops (e.g., glyphosate on cereals). They are also used in areas of public interest (e.g., railways, public and private gardens, or lawns). Their phytotoxic action occurs either on the site of contact or following absorption by the plant and systemic translocation to other organs. Postemergence herbicides are applied on developed undesired weeds, while preemergence herbicides are typically applied on the ground soon after seeding of the desired culture to prevent weed's development. Herbicides have different selectivity for plants. The amount of global herbicide use has been increasing over the years, from approximately of 600,000 tonnes in 1990 to more than 1,200,000 tonnes yearly since 2011. When herbicides are applied downward, human exposure during their application may be less than that following the application of insecticides or fungicides. Exposure can quantitatively change if the application is performed by handheld or by vehicle, with the latter generally reporting a lower exposure. In addition, since herbicides exert their pesticidal mode of action on metabolic processes that are generally unique to plants, they are, in principle, of low toxicity in mammals with the notable exception of paraquat. Some herbicides were reported to have irritating properties. Formulated products may contain adjuvants (stabilizers, penetrants, surfactants, dispersants) that could have, alone or in combination, more severe toxicity or irritating properties than the active herbicide alone.