Farm Neighbor Safety

As urban sprawl moves many households closer to working farms, there is concern that these farm neighbors, specifically pregnant women and children, are at risk from breathing air after applications of chlorpyrifos and other pesticides.

This is a natural concern, but based on the extensive scientific research that has been done in this area, neighbors can be assured that authorized uses of chlorpyrifos have only been allowed after EPA determined that they pose a “reasonable certainty of no harm.” 

Inhalation Exposure Studies

Animal studies of chlorpyrifos inhalation have been unable to produce an adverse effect from chlorpyrifos exposure¹ on the most sensitive biological measure: cholinesterase inhibition.² For many of these studies, rodents — rats specifically — are used because they have been found to be one of the species most sensitive to chlorpyrifos exposure.

One study² exposed laboratory rats to the highest concentration of chlorpyrifos physically achievable: 287,000 ng/m3 (or nanograms per cubic meter). In order to simulate outdoor airborne conditions, the chlorpyrifos was subjected to warmed, fan-driven air to increase its vaporization. The resulting concentration was far greater than expected under real-world conditions involving authorized uses. This chlorpyrifos-saturated air was then exposed to rats through nose masks for six hours a day, five days a week, over a period of 13 weeks. At the end of 13 weeks, the rats were thoroughly evaluated, and no effects (adverse or otherwise) were found.¹

EPA Standards and Inhalation Exposures

So how does all of this affect establishing allowable exposures?

Simply this: Because inhalation studies have been unable to produce adverse effects in animals, the EPA is unable to establish true inhalation safety standards. Instead, in setting inhalation standards the Agency uses the high dose in the inhalation study (which did not produce an effect).

California regulators used a similar approach to EPA’s in establishing a short and intermediate term Reference Exposure Level (REL). (A REL is the exposure at which not even a nontoxic effect — i.e. cholinesterase inhibition — is expected.) California regulators also used the chronic oral No Observed Effect Level to establish a chronic REL for airborne chlorpyrifos, essentially maintaining that one should not breathe any more chlorpyrifos chronically than one could eat without having any biological effect.

Regulators recognized that setting the standards in this way is imprecise and would be potentially misleading for those who do not understand the basis for this decision. And the standards have, indeed, misled people. Chlorpyrifos’ critics fail to realize that the standards set for inhalation are not based on safe vs. unsafe levels of chlorpyrifos in the air but, rather, as a benchmark to determine when exposures reach high enough levels to require further regulatory evaluation.

The EPA is currently developing techniques for more refined assessments of exposure to pesticides by inhalation, including chlorpyrifos. In the interim, if regulators require further interpretation of the health significance of chlorpyrifos in the air following farm use, one approach would be to pursue biomonitoring of potentially exposed farm neighbors, both before and after the product is used, to document what exposure — if any — actually occurs as a result of airborne concentrations.

“Drift-Catcher” Studies

Air monitoring devices, many sponsored by chlorpyrifos’ critics, are sometimes used to document levels of chlorpyrifos “drifting” in the air after the product is applied to nearby fields.

The “drift-catcher” studies, as they are collectively known, occasionally measure airborne chlorpyrifos levels higher than the EPA’s Reference Exposure Level (REL).³

These studies may warrant further evaluations of pesticide use practices by individual farmers. However, for a number of reasons they do not indicate a health hazard to farm neighbors, including:

• U.S. regulators do not view the REL as a bright line between risk and safety. An airborne concentration exceeding the REL may require further regulatory scrutiny but does not in itself mean that the air is unsafe.
• Outdoor detections do not measure indoor exposure. To be exposed to the level of chlorpyrifos reported in these studies, one would have to be outside in a fixed position for 24 hours a day, just like the monitoring device is.⁴

¹ The standards currently set for oral exposure to chlorpyrifos are based on preventing a biological change in blood chemistry that occurs without signs or symptoms of toxicity. This change is called “cholinesterase inhibition.” In a nutshell, cholinesterases are enzymes that help regulate our nervous system.

In the brain, substantial inhibition of these enzymes can produce adverse effects. However, prior to inhibiting brain cholinesterase, chlorpyrifos first inhibits cholinesterases found in blood and plasma cells, and this inhibition appears to be the most sensitive effect of exposure to chlorpyrifos. These blood and plasma cholinesterases have no established function in the body, this inhibition is not considered by the regulatory community to be an adverse effect. and the change does not produce signs or symptoms of toxicity.

The EPA bases its standards on exposures at which no observable effect can be found. For more detailed information about cholinesterase inhibition, please see the PDF document: The Insecticide Chlorpyrifos: How Standards are Set for the Protection of Human Health.

²Corley et al., “Chlorpyrifos: A 13-Week Nose-Only Vapor Inhalation Study in Fischer 344 Rats,” Fundamental and Applied Toxicology, 1989, 13, pp. 616-618.

³“Poisons on the Wind: Community Air Monitoring for Chlorpyrifos in the Yakima Valley,” The Pesticide Action Network, December 2006.

⁴Shaw, Mike, “Comments on Representations Relative to Chlorpyrifos in ‘Poisons on the Wind,’ a report by the Farm Worker Pesticide Project and Pesticide Action Network of North America,” Dow AgroSciences, 2006.