Science & Policy


Science Drift

The science is catching up with on-the-ground realities of pesticide drift: inhalation toxicity and volatilization are being seriously studied; long-lasting effects of low-level, tragically timed exposures are beginning to be taken into account; and the dangers of inherently drift-prone pesticides are more clearly understood. Yet if history is any guide, policy will lag a decade or two behind unless pressed to move more quickly by an informed public.

Pesticide drift, or “secondhand” exposure, is unintended, largely invisible and common.

Pesticide drift, or “secondhand” exposure, is unintended, largely invisible and common. Studies show that people who work with pesticides are at greater risk for certain cancers, neurological disorders, respiratory disease, miscarriages, and infertility. Farmers, field workers, and exterminators bear the brunt of these exposure risks, but they are not the only ones facing the consequences of pesticide drift. Children who grow up near fields treated with pesticides face developmental delays. Many pesticides are prone to drift away from where they are applied, ensuring “secondhand” exposure opportunities for people who just happen to be in the area.

The Science of Drift

CropdusterDrift happens when a pesticide applied in one place moves through the air and ends up somewhere else. The offsite movement can be just few feet or up to a few miles, and the degree of contamination can be high enough to sicken neighbors and livestock and ruin non-target crops. Long-range drift can also put wildlife in the surrounding environment at risk. Primary types of drift are spray drift and volatilization.

Spray drift occurs during and immediately after a pesticide application when pesticide droplets, dust, or gases are blown off their intended target. All pesticides have the potential for spray drift; here are a few factors determining likelihood:

  • Particle size. Pesticides are often applied as a mist of fine droplets, or as a powder or dust. The smaller their size, the longer they take to settle, and the higher the potential for drift.
  • Application method. Aerial applications are the most drift-prone. In these applications, the product is released from above the crop and has to travel a long distance before settling on the target. This increases the potential for drift. Ground applications are generally less drift prone, but still the cause of numerous incidents every year. “Airblast” applications, in which a tractor-mounted fan blasts pesticide particles into an orchard canopy, are particularly problematic.
  • Wind. Most pesticide labels prohibit application in high winds, since they can easily blow a pesticide off its target.

Volatilization drift happens after an application is complete, hours and even days later, when the pesticides evaporate (or “volatilize”) into a gas, which can then drift long distances. In contrast to spray drift, it is a pesticide’s inherent physical properties - not how it’s applied - that determine its potential for volatilization drift.

  • Vapor pressure. Pesticides with higher vapor pressures & lower boiling points are most likely to volatilize.
  • Pesticide type. Fumigant pesticides (used to treat homes, storage bins & soil before planting) are especially volatile - some are even gases under ambient conditions - and pose the worst problem of all drifting pesticides. Unlike spray drift, volatilization drift is invisible to the human eye, making it difficult to detect without monitoring equipment.

The plot below for the insecticide diazinon is derived from data collected as part of the State of California’s pesticide air monitoring program. It shows how volatilized pesticides can persist in the air long after the application is complete. The graph demonstrates the phenomenon of volatilization drift and it shows that actual levels of a volatilized pesticide in the air can greatly exceed safe levels. For a detailed explanation of this plot and how the data were collected, see chapter two in Secondhand Pesticides.


Pesticide rules in U.S. simply do not adequately protect people from drift. Volatilization is ignored for all but a few pesticides, and regulations meant to protect people and the environment from spray drift are inadequate for many of the most toxic pesticides. Compounding the situation, the state agencies charged with enforcing these rules lack the resources, and often the will, to do so effectively.

Spray Drift :: EPA attempts to mitigate spray drift on a product-by-product basis by placing restrictions on pesticide labels about what types of applications are permitted. Some examples:

  • Prohibitions on aerial spraying for some particularly toxic pesticides.
  • Requirements to use of special drift-reducing spray nozzles.
  • The use of unsprayed “buffer zones” around sensitive areas like waterways, endangered species habitat, or (rarely) schools, homes, and other occupied structures.

In recent years, some states have put policies in place to protect communities from spray drift. Lawmakers in California, Washington state and Maine have taken action to require drift monitoring, put drift response plans in place, or place restrictions and notification requirements on the most drift-prone types of applications, among other measures.

Volatilization Drift :: EPA ignores volatilization drift for all but the most toxic and volatile pesticides, the fumigants. Since it’s a pesticide’s physical properties that make it so drift-prone, “no spray” buffer zones and limits on application rates are the best ways mitigate this threat.

EPA recently announced increased buffer zones for fumigants, but their own calculations show that the new rules fail to protect people who live, work, or go to school adjacent to treated fields. And despite ample evidence produced by PAN, the State of California, and others showing that many insecticides are also subject to volatilization, the EPA has yet to do anything to address this problem.