EPA establishes long-term (chronic, lifetime) and short term (acute, single day) exposure standards based on laboratory animal testing at corresponding exposure durations (e.g., repeated doses over a lifetime or a single dose).

EPA's Exposure Limits to Chlorpyrifos

(nanograms per kilogram of body weight)

Non-Adverse Effect 1,000,000

No Effect 500,000

Maximum Allowable One-time Exposure from Labeled Use 500

Maximum Allowable Daily Exposure from Labeled Use 30

As part of the scientific reevaluation of chlorpyrifos that concluded in 2006 under reregistration, the EPA, in 2000, set its health-based standard for acceptable chronic (lifetime) exposure to chlorpyrifos at 30 ng/kg/BW (nanograms per kilogram of body weight). Similarly, EPA set the acceptable acute (single day) exposure standard at 500 ng/kg/BW. How did the Agency arrive at these figures? They include a special uncertainty factor for sensitive populations and are based on results of animal testing in which levels shown to have no effect on cholinesterase enzymes were determined. Lifetime feeding studies showed no adverse effects in rodents with daily doses up to 30,000 ng/kg/bw. Moreover, laboratory studies found that single doses of up to 500,000 ng/kg/BW had no measurable effect. Even at single exposures of 1,000,000 ng/kg/BW chlorpyrifos produced only non-toxic changes in the nerve enzymes typically tested for chlorpyrifos sensitivity.

The large difference between EPA exposure standards and levels demonstrated to have no effect in laboratory testing is due to the use of several safety factors. Because animals may respond differently than people, one person differently from another, and children may be different from adults, the EPA builds in various safety factors to account for these potential differences in sensitivity. For chlorpyrifos, the safety factor determined by EPA in its 2000 human health assessment was 1,000 — or 1,000 times below the level at which the product produces no effect in laboratory tests. This included a standard 10-fold factor for animal-to-human variation, a standard 10-fold factor for person-to-person variation, and a special 10-fold factor for child-to-adult variation. This latter safety factor resulted from changes in policy which occurred as part of the Food Quality Protection Act , which required the special safety factor when specific information to compare juvenile and adult sensitivity is incomplete.

Exposure levels to Chlorpyrifos related to the height of the Eiffel Tower

The following graphic helps put the EPA’s safety requirements regarding chlorpyrifos exposure into perspective:

At the very top of the tower (320 meters high) would be the dose of chlorpyrifos above which there may be an effect in an enzyme measurable in laboratory tests following a single dose.

Halfway down the tower (160 meters high) would be the level shown to have no effect following a single dose.

Only one centimeter from the base of the tower would be the EPA’s current maximum allowable daily exposure from labeled use.

From this illustration, it is clear that the margin of protection available for the EPA’s registered uses of chlorpyrifos is very large.

These margins have been established by the EPA based on extensive research and testing and are intended to provide ample protection not only for the population at large but also for potentially sensitive subgroups such as women and children.

Learn more about how the EPA sets its standards and the EPA’s process for registering a pesticide product. Further details on EPA’s Registration Review of chlorpyrifos and proposed revisions in human health standards are available, and have been summarized in comparison to existing standards.

Further Resources

The Insecticide Chlorpyrifos: How Standards are Set for the Protection of Human Health”, Dow AgroSciences.

“Reregistration Eligibility Decision (RED) Document for Chlorpyrifos”, U.S. Environmental Protection Agency, 2006.