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Introduction
Drift is the offsite movement of airborne pesticides and includes spray droplets, dust particles, vapors from volatile liquid pesticides and gas-phase chemicals such as fumigants. The propensity of a chemical to drift depends on its physical properties, especially vapor pressure and boiling point. Pesticides with high vapor pressures and/or those pesticides that are gases at room temperature are difficult to keep on the application site and will always drift. Higher application rates (pounds per acre) also contribute to higher levels of drift.
Types of Pesticide Drift
There are two types of drift: spray drift and post-application drift. Spray drift occurs during and immediately after a pesticide application. Post-application drift occurs after the application is complete, hours and even days later.
| Spray drift occurs during and immediately after a pesticide application when wind or application equipment blow spray droplets, dust or gases off the intended site. The pesticide droplets sprayed out of nozzles are the most problematic and can drift long distances before settling. These pesticides may be applied by a crop duster, airplane or by a tractor. Applicator error can cause drift; however, even when pesticides are applied correctly, drift can still occur. |  |
Post-application drift occurs after application is complete, hours and even days later. There are two types of post-application drift:
- Volatilization drift: Some pesticides are applied
as liquids or oils but then evaporate (or “volatilize”) into a gas after
they are applied to crops or fields. Once in gas form, volatile pesticides
can drift long distances. Fumigant pesticides
(used to treat homes, storage bins, and soil before planting) are especially
volatile, and pose the worst problem of all drifting pesticides. Unlike
spray drift or drift of dust particles, volatilization drift is invisible,
making it difficult to detect without monitoring equipment. The plot
below (data collected by the California
Air Resources Board) shows how post-application drift persists long
after the application is complete for a semi-volatile insecticide diazinon
at a location 75 feet west of the field boundary. For a detailed explanation
of this plot and how the data were collected, see Chapter 2 in Secondhand
Pesticides: Airborne Pesticide Drift in California.
- Drift of pesticide-coated dust particles: Sometimes high winds in agricultural areas create clouds of dust from pesticide-treated fields. This dust may end up in yards and parks, as well as in homes and cars, where it can be inhaled or ingested. Children are particularly vulnerable to this type of exposure because they play on the floor and often put their hands or other objects into their mouths. Most pesticides can become adsorbed to dust and be transported in this way.
How Much is Too Much?
To assess the potential impacts of drift on human health requires two types of information:
- Air monitoring data that show how much pesticide is in the air during and after applications, and
- Accurate, up-to-date information on pesticide concentrations in air that are likely to harm people.
It is only recently that both types of information have become available for a subset of pesticides. Air monitoring data collected by the California Air Resources Board and the California Department of Pesticide Regulation since 1984 is now available from the California Toxic Air Contaminant website for about 45 pesticides (out of about 900 registered active ingredients). With the passage of the U.S. Food Quality Protection Act in 1996, U.S. EPA was required to re-evaluate the toxicity of all older pesticides and determine Reference Doses equivalent to "acceptable" levels of exposure. As of summer 2005, this process is complete for about half of the older pesticides.
U.S. EPA uses mainly animal studies to estimate how much of a particular pesticide is “acceptable” for the average person to be exposed to, taking into account the special vulnerabilities of infants and children. Unfortunately, U.S. EPA does not evaluate "bystander" (that's you, if you live next door to a pesticide application site) inhalation exposure to drift for most pesticides. Only in early 2005 did EPA announce its intent to evaluate bystander inhalation exposure for the fumigant pesticides.
The "acceptable" levels determined from animal studies can be translated into “acceptable” air concentrations called Reference Exposure Levels (RELs) by taking into accout the breathing rates and body weights of the population being evaluated. The calculation is explained in detail in Appendix 2 of Secondhand Pesticides: Airborne Pesticide Drift in California.The U.S. EPA typically uses these estimates to determine what control measures are necessary to keep exposures below these levels of concern.
There are limitations to this method of determining an "acceptable" level of exposure.
- The effects of exposure to mixtures of pesticides is not currently evaluated by the EPA, even though it is a common occurrence, especially in agricultural areas.
- The toxicity studies are conducted and/or paid for by the pesticide manufacturers.
- U.S. EPA's guidelines for toxicity testing do not include all possible toxicity endpoints. Of particular concern are developmental neurotoxicity and endocrine disrupting effects due to low-dose exposures, which are not currently evaluated.
A more detailed discussion of the limitations of EPA's risk assessment methods can be found at PANNA's PesticideInfo.org site.
When both air monitoring data and a Reference Exposure Level are available, it is possible to compare concentrations in air to U.S. EPA's "acceptable" levels. When we did this comparison for several high-use pesticides, we found that routine (and currently legal) applications often result in problematic levels of pesticides in air. See summary plots below and a detailed discussion of these results in Chapter 2 of Secondhand Pesticides: Airborne Pesticide Drift in California.
Resources
See the complete list of resources about pesticide drift.