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All sprays drift, but pesticide users can reduce the impact of drift by using lower-risk pesticides.

Dr. Alan Felsot, environmental toxicologist with Washington State University, said sprays are made up of particles of pesticides, and how far they move depends on several factors, including the type of spray, the size of the particles, and air movement.

“What can we do to prevent drift?” he asked. “It’s striving to minimize the effect of the inevitable. We can’t stop it, but we can minimize the potential exposure level to a level of reasonable certainty of no harm.”

About 25 percent of complaints received by the Pesticide Incident Reporting and Tracking (PIRT) Review Panel concern drift, Felsott said. People who live near orchards often panic when they see spray plumes, but drift doesn’t necessarily mean risk.

“We have this mistaken notion that toxicity is the same as hazard, and that means risk. They are all different concepts of toxicology. What we want to protect people from is risk.”

“Exposure can occur, but it’s not necessarily the same thing as there’s a hazard,” Felsot explained during the North Central Washington Pear Day.

The U.S. Environmental Protection Agency calculates the level at which a pesticide causes no observable effect in lab tests on rats. The safe level of the pesticide— calledthe acute reference dose—is set 100 or 1,000 times higher than the no observable adverse effect level.

For example, the acute reference dose for azinphos-methyl, the active ingredient in Guthion, is 0.003 milligrams per kilogram per day, which is 100 times higher than the no observable adverse effect level.

No harm

Felsot has done tests to find out how far away a bystander can be from the point of application to meet the standard of reasonable certainty of no harm and what practices an applicator can use to achieve that standard.

In studies, Felsot found that a person standing between 120 feet and 180 feet from the edge of the orchard sprayed with Guthion would receive a dose equivalent to a reasonable certainty of no harm. But if the grower switched pesticides and used a product like Assail (acetamiprid), which is applied at lower rates and has a higher reference dose, the person could be within 20 feet of the orchard with reasonable certainty of no harm level. Because Assail has less mass, there is less drift.

“You can control the situation by choosing a particular pesticide,” Felsot said. “We have control over the situation. These new products are going to guarantee extremely low risk, even nearby the orchard.”

The Guthion label requires that the outward nozzles of the sprayer be turned off when turning to the next row and while spraying the outer two rows, but Felsot’s studies showed what he called an “air backwash” effect. Even when the outer nozzles are turned off, the spray moves around to the other side of the sprayer and can be deposited up to 100 feet outside the orchard. Felsot said some kind of a shield on the tractor might help prevent that effect.


He also tested a drift retardant. When mixed with pesticides, the retardant resulted in a definite reduction in drift, particularly 200 feet away from the target where the particles are smaller. Whether the wind was blowing 3 miles per hour or 16, there was still a big reduction in drift with the retardant. Felsot said when the spray solution is coming out of an airblast sprayer at 200 miles per hour, the wind speed is irrelevant, and the drift pattern is the same.


Bystanders are unlikely to be placed at risk by inhaling airborne residues, Felsot said. In tests, he placed air samplers outside an orchard to sample vapor and particulates. He found that most of the spray is composed of particles, and very little is in the vapor phase during spraying. People are far less likely to be exposed to pesticides through inhalation than through dermal exposure.

Felsot was asked why the smell of a pesticide can be noticed from a long distance if it is not in a vaporized form.

“You’re not smelling the active ingredient,” Felsot replied. “It’s the impurities you can smell, and they have very little toxicological activity.”