(Beyond Pesticides, July 5, 2007) A new study showing that the order of exposure to multiple pesticides may be just as important as the dose, timing and length of exposure adds another dimension to the complex task of risk assessments. Using carbaryl and chlorpyrifos, University of York scientists have observed significant differences in mortality rates of freshwater invertebrates depending on the order of exposure to these frequently used agricultural chemicals.
The study, Modeling Combined Effects of Pulsed Exposure to Carbaryl and Chlorpyrifos on Gammarus Pulex, suggests the sequence of pesticide exposure may be just as important of a variable as the dose, the timing of the dose and the length of exposure when factoring environmental and health endpoints.
The researchers conducted the study by exposing the freshwater invertebrate Gammarus pulex â€“ a tiny shrimp â€“ to pulses of the two insecticides (both of which affect the nervous system through acetylcholinesterase inhibition) mimicking exposure to chemical mixtures in the environment â€“ for example, farmers may apply several different pesticides over the growing season that run off into the aquatic environment. After receiving a pulse of one pesticide, the shrimp were given 14 days, a time period chosen based on previous experiments, to recover and expel the chemical from their systems before exposure to the second pesticide.
When first exposed to carbaryl and then chlorpyrifos two weeks later, mortality rates were observed to be 31% for carbaryl and 21% for chlorpyrifos. When reversed, with chlorpyrifos exposure occurring first, the mortality rates were 12% for chlorpyrifos and 55% for carbaryl. The significant difference in the mortality rates from the carbaryl pulses have led the authors to hypothesize that the shrimp were not able to recover completely from the chlorpyrifos exposure and therefore require greater â€śdamage recovery” times.
As reported by Environmental Science & Technology, Jim Lazorchak, an EPA ecotoxicologist, calls the experiment â€śgroundbreaking.” The team is â€śtrying to explore modeling to predict realistic exposures,” he says, particularly for exposures to nonpoint sources of pesticides. Typical assessment methods donâ€™t incorporate timing and order, which are critical in assessing real-world situations, where even more stressors occur, he emphasizes, from changes in water availability and climate to lack of food and habitat loss. â€śAs far as assessing different exposure regimes, few people are getting involved” in such complex scenarios, he says, â€śbut thatâ€™s the direction [eco]toxicology needs to go.” He continues, â€śThe order in which you are exposed is just as important as the concentration and duration you were exposed.” The question now becomes â€śwhy is the order important?”
The study renews a central discussion over real world scenarios where mixtures and synergistic effects are common. This is a situation that not only impacts our environment but also our health. For example, several studies conducted by a team of Duke University researchers lead by pharmacologist Mohammed Abou-Donia suggest that DEET in conjunction with permethrin-impregnated clothing may be linked to Gulf War Syndrome. Exposing animals to the same doses of DEET and permethrin have been shown to result in similar effects.
Synergistic effects between multiple pesticides and/or other chemicals represent one of the greatest gaps in EPAâ€™s ability to protect the public from the adverse health effects associated with pesticide use and exposure. This current study sheds further light on just how little is understood about exposure to pesticide mixtures and the many variables that can occur.