Pesticide drift

Pesticide drift refers to the unintentional diffusion of pesticides and the potential negative effects of pesticide application, including off-target contamination due to spray drift as well as runoff from plants or soil. This can lead to damage in human health, environmental contamination, and property damage.
Types.
With placement (localised) spraying of broad spectrum pesticides, wind drift must be minimised, and considerable efforts have been made to quantify and control spray drift from hydraulic nozzles. Conversely, wind drift is also an efficient mechanism for moving droplets of an appropriate size range to their targets over a wide area with ultra-low volume (ULV) spraying.
Himel (1974) made a distinction between exo-drift (the transfer of spray out of the target area) and endo-drift, where the active ingredient (AI) in droplets falls into the target area, but does not reach the biological target. Endo-drift is volumetrically more significant and may therefore cause greater ecological contamination (e.g. where chemical pesticides pollute ground water).
Bystander exposure describes the event when individuals unintentionally come in contact with airborne pesticides. Bystanders include workers working in an area separate to the pesticide application area, individuals living in the surrounding areas of an application area, or individuals passing by fields as they are being treated with a pesticide.
Herbicide volatilisation.
Herbicide volatilisation refers to evaporation or sublimation of a volatile herbicide. The effect of a gaseous chemical is lost at its intended place of application and may move downwind and affect other plants not intended to be affected causing crop damage. Herbicides vary in their susceptibility to volatilisation. Prompt incorporation of the herbicide into the soil may reduce or prevent volatilisation. Wind, temperature, and humidity also affect the rate of volatilisation, with humidity reducing it. 2,4-D and dicamba are commonly used chemicals that are known to be subject to volatilisation, but there are many others. Application of herbicides later in the season to protect herbicide-resistant genetically modified plants increases the risk of volatilisation as the temperature is higher and incorporation into the soil impractical.
Public concern.
Although there has been much public concern and research into spray drift, point source pollution (e.g. pesticides entering bodies of water following spillage of concentrate or "rinsate") can also cause great environmental harm. Public concern for pesticide drift is not met with adequate regulatory response. Environmental justice advocates in California, for instance, consider moving up the scale of the discourse on pesticide drift by categorizing it as air pollution in order to receive attention from state environmental protection. 
Farm workers and communities surrounding large farms are at a high risk of coming in contact with pesticides. The San Joaquin valley in California has seen numerous cases of illnesses resulting from exposure to pesticides through pesticide drift. Organisations such as the United Farm Workers Union have fought to implement legislation that would reduce and hold farmers accountable for pesticide drift. The California Department of Pesticide Regulation estimates that between 37–68% of pesticide illness among United States agricultural workers come as a result of pesticide drift.
Insecticides sprayed on crop fields can also have detrimental effects on non-human lifeforms that are important to the surrounding ecosystems like bees and other insects.
From 1998 to 2006, Environmental Health Perspectives found nearly 3,000 cases of pesticide drift; nearly half were workers on the fields treated with pesticides and 14% of cases were children under the age of 15. 
Regulations.
In 2001, the United States Environmental Protection Agency published a guidance to "manufacturers, formulators, and registrants of pesticide products" (EPA 2001) that stated the EPA’s stance against pesticide drift as well as suggested product labelling practices.