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Pesticide drift is the process by which sprayed pesticides travel through the air, away from the site of usage. This can occur in two different mechanisms: spray drift and volatilization drift. Spray drift occurs when sprayed pesticides drift off due to wind. Volatilization drift, on the other hand, happens when pesticides evaporate to form gases, which then drift away. Overall, around 70 million pounds of pesticide drift off-site, based on EPA estimates.

Historical Background
Pesticide drift has been an issue since the widespread use of pesticides ushered in by World War II. Because the U.S. and the Allies needed to send vast quantities of food overseas to feed troops, food production needed to be increased. The most widely used pesticide used to increase food production during and after the war was DDT, discovered by Paul Muller in 1939. It was popular because it was cheap, effective, and lasted for long periods of time; it did not break down in the environment easily. As a result, it was able to drift easily. For example, high concentrations of DDT were found in the bloodstream of Eskimos in Alaska, which indicates that DDT drifted over long distances. There were two main issues that made DDT so dangerous: it was prone to drift, and it was able to accumulate in organisms up the food chain. It was absorbed by organisms that were lower in the food chain, such as plankton, in low concentrations. In the case of plankton, DDT was found in concentrations less than 1 part per million. However, as these plankton were eaten up by larger animals such as fish, and those fish were in turn consumed by birds and other predators, the DDT concentrations were accumulated. As a result, the DDT concentration observed in ospreys, for example, was 25 parts per million in some cases. As a result, DDT wreaked havoc on the environment, killing off vast quantities of birds and other predators. This prompted Rachel Carson's book Silent Spring, which swayed public opinion against DDT. This resulted in environmental regulations banning DDT in the U.S. However, the U.S. has not banned the export of DDT overseas, so the pesticide is still used around the world. In addition to DDT, a number of other fat-soluble pesticides that are prone to long-distance drift were identified by the Stockholm Convention on Persistent Organic Pollutants. These pesticides, known as Persistent Organic Pollutants, include Aldrin, Chlordane, Toxaphene, and Dioxins. As part of the Convention, 176 countries agreed to ban these pesticides, but the U.S. is not one of them. In addition to Persistent Organic Pollutants, fumigant pesticides are also prone to drift. Once they are applied to the soil, fumigant pesticides turn into a gases and move through air pockets in the soil. The fumigant pesticide Methyl Bromide was developed in the 1940s, and EDB was developed in the 1950s. Although the use of EDB has been outlawed in the U.S. due to its harmful effects, other fumigants are commonly used today.

Contributing Factors
There are a number of factors surrounding the application of pesticides that affect whether or not they drift away: the type of pesticide, temperature, and nozzle height.

Of the various types of pesticides, fumigants are the most likely to contribute to drift. This is because they are especially prone to volatilization drift. Other forms of pesticides, such as 2,4-D amines, propagate by spray drift as droplets, but this is less dangerous than volatilization drift. Fumigants are prone to volatilization drift because they turn into a gaseous state after they are applied to the soil. Although many farmers cover their fields with plastic tarps after fumigant usage, this practice is not sufficient because drift can occur for weeks after usage. Fumigants are used widely across the United States, especially in strawberry fields. In California for example, 20% of all pesticides used are fumigants. The reason for the widespread use of fumigants is their effectiveness and speed. Because they turn into gases once they are applied to the soil, they are able to move through soil pores, or air pockets within the soil, quickly. When these pesticides are in a gaseous state, they can also dissolve in soil water, or the water at the surface of the soil. The ability of these particles to dissolve in the water depends on their vapor pressure and the temperature at the time of application, but most particles remain as gasses, and so they are likely to escape the soil and drift away. For non-fumigant pesticides, droplet size affects susceptibility to drift: larger droplets are less likely to drift. Farmers can control the droplet size of a pesticide by using drift control additives. When mixed with pesticides, drift control additives can increase droplet size, reducing drift. However, the additional costs incurred by drift control additives can make them prohibitive for some farmers. In addition, the weather affects pesticide drift in two ways: by increasing volatility and creating inversion layers. While fumigants are designed to form gases, other pesticides are also likely to evaporate, especially when the temperature is 85 degrees Fahrenheit or higher. Whether or not a pesticide evaporates depends on two main factors: vapor pressure and temperature. Chemicals with high vapor pressure are more likely to evaporate than chemicals with low vapor pressure; however, once the temperature rises to a high enough threshold, most pesticides will evaporate. Both gaseous pesticides and pesticide particles are more likely to drift horizontally when there is a warm inversion layer above the application site. A warm inversion is a layer of warm air, sandwiched by colder air both above and below. When an inversion layer is absent, the pesticide particles simply drift upwards. However, the inversion layer prevents the particles from rising above the layer; therefore, the particles remain closer to the ground and are more likely to be swept away by the wind. As a result, pesticides can drift miles away.

Another factor is the height of the spray nozzle above the ground during pesticide application. Boom sprayers, which are used to distribute pesticides across fields, must be calibrated properly to minimize drift. Ideally, the distance between the nozzles should be the same as the distance as the distance between the boom and the ground. Otherwise, if the distance is too large, then the wind can interfere with the spraying process, leading to drift.

Health Hazards
Pesticide drift negatively impacts both human life and the environment. When pesticides and pesticide vapors travel across farm fields, they poison farmworkers and nearby residents. Farmworkers are particularly affected by pesticide drift because they spent most of their days working in pesticide-treated fields. One of the biggest poisonings as a result of drift occurred in Nevada. In 2007, 121 farmworkers were hospitalized because fumigant pesticide applied a quarter of a mile a way drifted over to their work site. Fumigant pesticides, which are more likely to drift because they turn into a gaseous state, are also some of the most dangerous because they can enter the human bloodstream more easily. In addition to inhalation, fumigants can enter the bloodstream through the skin or eyes. Contact with skin can result in rashes or burns. Over long periods of time, pesticides can be absorbed through clothing, and when they absorbed through the skin, they can affect internal organs. Contact with the eyes can result in loss of vision. Some of the acute symptoms of exposure to pesticide drift are headaches, nausea, coughing, and slurred speech. The health hazards of pesticide drift have been observed across the U.S., both in farmworkers and bystanders. A study based on data from the National Institute for Occupational Safety found that up to 68 percent of pesticide-related illness in farmworkers can be attributed to drift. Furthermore, 33 percent of pesticide-related illnesses in schools nationwide are a result of drift. Although health hazards from pesticide drift occur all over the country, states that are more dependent on agriculture are affected more. Some of the most persistent pesticide drift poisonings have been occurring in California and Hawaii.

Cases Observed in California
California produces almost half of the agricultural produce in the U.S. . As a result, pesticide usage, along with drift, is common. In 2017, over the course of a week, 4 pesticide drift events incapacitated over a 100 farmworkers in Bakersfield, Salinas, and Watsonville. In Bakersfield, chlorpyrifos drifted half a mile from the application site to poison farmworkers. Along with the acute effects, which include fainting, nausea, and vomiting, drifting chlorpyrifos has been linked to asthma, memory loss, and ADHD in children. Furthermore, pesticide drift in the Central Valley results in long-term impacts such as Parkinson's disease. A UCLA study on people living in the Central Valley found that people living within 500 meters of farm fields were 75 percent more likely to contract Parkinson's disease.

Cases Observed in Hawaii
Because the Hawaiian economy is particularly dependent on the agricultural industry, there have been many observed cases of the harmful effects of pesticide drift. The variable weather patterns in Hawaii also enable pesticides to drift further and faster. This can wreak havoc on bystanders in residential areas. For example, after pesticides drifted over to a school in Kaua'i in 2008, 60 children and 2 teachers experienced the acute symptoms of pesticide poisoning: nausea, vomiting, and breathing problems. At least 10 children had to be hospitalized. Physicians in Kaua'i and Waimea have observed that patients near a seed corn field experienced respiratory issues, infertility, and gout. Because the 1000-acre field uses pesticides that latch onto dust, any dust that drifts to nearby communities brings the pesticides with it. As a result, residents in Waimea are ten times as likely to contract cancer than people living elsewhere in Hawaii. Pesticide drift also results in birth defects; infants in Kaua'i are ten times as likely to be born with defective hearts than infants in other parts of the country.

Environmental Impacts
In addition to negatively impacting human health, pesticide drift negatively impacts the atmosphere and non-target plants. Pesticide drift contributes to depleting the ozone layer. For example, the fumigant pesticide Methyl Bromide is a Class 1 Ozone Depleting Substance, classified by the Montreal Protocol. This means that it is a potent factor in degrading ozone in the atmosphere. This is a problem because the ozone layer absorbs ultraviolet radiation. Gaps in the ozone layer enable harmful radiation to reach the surface of the Earth, which can harm plants and animals.

In addition to breaking down ozone in the atmosphere, pesticide drift also harms plants that were not targeted by pesticide application. Some pesticides kill plants that are not genetically modified to be pesticide resistant. One example is the herbicide Dicamba, made by Monsanto and DuPont. It is primarily used on Dicamba-resistant soybean and cotton plants. However, when Dicamba is sprayed, it often drifts away to other fields, where it kills crops that are not genetically modified to be Dicamba resistant. According to Monsanto, around 25 million acres of Dicamba-resistant crops were planted in the U.S. in 2017. Drifting Dicamba damaged plants in at least 2.5 million acres that same year. The costs associated with that damage have been high enough so that Missouri, Arkansas, and Tennessee now regulate the times in the year when Dicamba can be applied.

Regulation and Legislation
Regulation surrounding pesticide drift is mainly handled at the state level. There are very few federal standards for pesticide drift laws. However the FIFRA (Federal Insecticide, Fungicide and Rodenticide Act) enables the EPA enforce labeling of pesticide products. The FIFRA introduced guidelines for minimizing pesticide drift, which are printed on pesticide labels. Manufacturers that fail to print these guidelines, along with farmers that fail to follow the instructions, can be prosecuted by the EPA. Another way the EPA enforces measures to reduce drift is through the Worker Protection Standard. Under this standard, workers that handle pesticides must go through training that includes information on minimizing pesticide drift. Another clause of the standard requires that employers must provide immediate medical assistance to workers that are affected by pesticide drift. The rest of the regulation of pesticide drift varies from state to state. The 3 main factors in pesticide drift regulation are: liability issues, requirements for buffer zones, and restrictions on aerial pesticide application.

State legislation regulating the liability of pesticide drift is mostly uniform in the sense that the party that sprays drifting pesticides must pay for any damage. For example, the Alabama Supreme Court maintains that pesticide drift is only illegal when it results in damages. When damages occur, the landowner that sprays the pesticides must pay for them, even if the actual spraying was carried out by contractors. California also has legal clauses that prohibit damage from pesticide drift. A common cause of pesticide drift is overspray, or the application of pesticides in amounts that are greater than what is allowed by regulatory standards. In California, applicators and contractors that overspray pesticides can face both civil and criminal charges. They can also have their applicator licenses suspended in some cases. Other states, such as Oregon and Oklahoma, lack general regulations preventing pesticide drift, but ensure that landowners are responsible for any damage caused by aerial pesticide spraying.

Some states require buffer zones between farm fields and residential areas in order to ensure that pesticide drift does not affect bystanders. Arizona, for example, has buffer zones around schools, healthcare facilities, and residential areas. Pesticide applicators must remain at a prescribed distance away from these areas at all times. Some states, such as California and Florida, mandate buffer zones around water sources to prevent contamination from drift. In California, designated groundwater sources have buffer zones of about 1 square mile around them. Florida requires 300 foot buffer zones around wells used for human water consumption. Florida also enforces buffer zones between fields that are being sprayed and non-target fields. The size of the buffer zone is dependent on wind speed and temperature. In California, pesticides can be sprayed a quarter of a mile away from schools. Prior to March 2017, pesticide applicators were required to notify schools 48 hours before spraying pesticides near this buffer zone. However, the California Department of Pesticide Regulation repealed this regulation, and prior notification is no longer required.

Another way states restrict pesticide drift is by banning crop dusting, or aerial application. Alabama has made aerial pesticide application illegal when wind speeds are over 10 miles per hour. Arkansas has banned aerial pesticide spraying altogether, while California requires pilots to obtain permits before spraying pesticides. Most states regulate aerial pesticide application, by banning it, or restricting the times when pilots can spray pesticides, or limiting the altitude that pilots fly while crop dusting.

Although most U.S. states have comprehensive regulation designed to reduce pesticide drift, the majority of developing countries lack these kinds of regulations.