The study, to be published Thursday, March 2, in the journal Pharmacogenetics and Genomics, reveals far greater variability in susceptibility to pesticides than previously predicted. Current U.S. Environmental Protection Agency (EPA) standards require an extra tenfold safety factor to protect children compared with adults if there are gaps in information about the children's susceptibility. The EPA may select a lower safety factor if it determines that enough information is available, and based on an EPA review, many other pesticides have lower or no additional safety factors.
But the new study "raises the question of whether current standards for safe levels of pesticide exposure are sufficiently protective of a vulnerable population," said Nina Holland, UC Berkeley adjunct professor of environmental health sciences and co-lead author of the paper. "Based on our study, I feel that more research is urgently needed to establish whether the standards need to be re-evaluated."
Since 2001, home use of organophosphate pesticides - diazinon and chlorpyrifos, in particular - has been restricted by the EPA, mainly because of risk to children. However, the researchers said there may still be residual exposure to diazinon and chlorpyrifos from household use before they were banned. Some structural uses for the pesticides are still approved, including treatment of house foundations with chlorpyrifos.
While household use of diazinon and chlorpyrifos is now restricted, these and other organophosphate pesticides are still widely used in agriculture. According to state figures, approximately 143,000 pounds of diazinon and 52,000 pounds of chlorpyrifos are used annually in the region studied - California's Salinas Valley, an agricultural community.
The study, conducted through UC Berkeley's Center for the Health Assessment of Mothers and Children of Salinas (CHAMACOS), included 130 Latina women and their newborns living in the valley. The researchers report that approximately 28 percent of the women in the study had worked in the fields during their pregnancies, and another 14 percent had other jobs in agriculture that included nursery or greenhouse work. Overall, 82 percent of the women had agricultural workers living in their homes during their pregnancies.
Various studies are underway to determine the levels of pesticide exposure to residents in both agricultural and urban/suburban communities in California and elsewhere around the country.
The researchers used levels of paraoxonase 1 (PON1) activity measured in blood samples as a marker for pesticide susceptibility. PON1 is an enzyme that breaks down the toxic metabolites of organophosphate pesticides, including diazinon and chlorpyrifos.
"The significance of PON1 has been shown in prior animal studies," said Clement Furlong, research professor in the departments of medicine and genome sciences at the University of Washington, and co-lead author of the paper. "In these studies, exposure to low levels of organophosphates has killed mice that lacked the PON1 enzyme, while the same dose elicited no symptoms in mice with normal levels of the enzyme. Both the quantity and quality of the PON1 enzyme is important in determining the ability to detoxify pesticides."
Furlong pointed out that there is a greater risk when dealing with exposure to the much more toxic, oxidized forms of chlorpyrifos and diazinon, or chlorpyrifos oxon and diazoxon, respectively.
"Chlorpyrifos and diazinon are manufactured as sulfur compounds," said Furlong. "But these pesticides can transform into chlorpyrifos oxon and diazoxon before or after they are applied and then by specific enzymes when they are inside the body. The enzyme we studied breaks down these oxidized forms. Lower enzyme activity means less protection. Risk assessments should consider exposures to these oxidized forms. We also need more field research to quantify the levels of these oxidized compounds in the environment."
The ability of the PON1 enzyme to protect the body from the toxicity of pesticides is determined by genetics, specifically whether a person has the Q or R form of the PON1 gene at position 192 on the chromosome.
People with the QQ genotype have two copies of the Q variant of the PON1 gene, producing a PON1 enzyme that is significantly less efficient at detoxifying chlorpyrifos and chlorpyrifos oxon. People with the RR genotype have two copies of the R variant of the PON1 gene, producing a PON1 enzyme that is more resistant to chlorpyrifos and chlorpyrifos oxon exposure. Inheriting one type of gene from each parent leads to a QR genotype with intermediate sensitivity to chlorpyrifos and chlorpyrifos oxon.
In addition to the factors affecting the type of PON1 enzyme produced, there are additional genetic variants that affect the levels of enzyme available. A recent study of Latino mothers and children from the CHAMACOS group found that several markers of genetic variability have a stronger effect on enzyme activity in children than in adults.
It is mainly the level of PON1 enzyme that determines resistance to diazinon or diazoxon exposure.
The PON1 enzyme also varies by ethnicity. Approximately 10 to 20 percent of African Americans have the QQ genotype, compared with 50 percent of whites. Approximately 25 to 35 percent of the Latino population has the QQ genotype.
For all groups, infants are at particular risk because the level of PON1 enzyme in newborns averages one-third or less than that of adults. It can take six months to two years for a baby to develop mature levels of PON1.
"People have this remarkable difference in enzymes that defend their health from pesticide exposure," said Holland. "In developing regulatory standards for safe levels of exposure, we need to protect the most sensitive in a population, particularly because children and unborn fetuses are involved."
Researchers were able to estimate sensitivity in mothers and children using data on transgenic mice that carried human Q or R genes and that produced levels of PON1 comparable to that in human newborns and some adults. Combined analysis of the animal model data from Furlong's laboratory and PON1 results from the CHAMACOS cohort allowed a more specific estimation of pesticide sensitivity than was previously possible.
The results of this analysis predict that some newborns may be 26 times more susceptible to diazinon and diazoxon exposure than newborns with the highest PON 1 enzyme levels and up to 65 times more susceptible than adults with the highest enzyme levels.
The differences are even greater when predicting susceptibility to chlorpyrifos and chlorpyrifos oxon. Some of the QQ newborns may be 50 times more susceptible to this pesticide than RR newborns with high PON1 levels, and 130 to 164 times more susceptible than some of the RR adults.
"Several animal studies have shown adverse effects on neurodevelopment from exposure to organophosphates," said Brenda Eskenazi, UC Berkeley professor of epidemiology and director of CHAMACOS and the Center for Children's Environmental Health Research. "Our next step is to look at the relationship between pesticide exposure and neurodevelopment, specifically for young children and genetically susceptible populations."
Other co-authors of the study include Rebecca Richter, research scientist at the University of Washington; Asa Bradman, associate director of CHAMACOS at UC Berkeley; and Alan Ho, research assistant at UC Berkeley.
Funding from the National Institute of Environmental Health Sciences and the EPA helped support this research.
Journal
Pharmacogenetics and Genomics