Organophosphate insecticides, especially chlorpy-rifos, are quite unique among pesticides because three epidemiological studies are ongoing that are designed to investigate developmental neurotoxic-ity of these compounds. Results from these studies suggest adverse effects at levels of current dietary exposure, but these results are in contrast to animal studies which are the basis of the regulatory ap-proval process. Chlorpyrifos is therefore an inter-esting example to illustrate how different types of studies are weighted by the regulatory authorities.
Beginning in the mid-1990s, researchers observed that in animal studies, compounds of a group of
insecticides called organophosphates have a nega-tive effect on the development of the brain and nervous system of the offspring. This effect is called neurodevelopmental toxicity; the rationale is that small damages to the developing brain may have life-long consequences.
Evidence of effects for cognitive skills
In 1998, Guilette and co-workers provided some of the first evidence of the negative effects of pes-ticides on the development of cognitive skills in children. Four to five-year-old Yaqui children in Mexico were exposed or non-exposed to pesti-cides, depending on whether they lived in the Ya-qui valley (with pesticide-intensive agriculture) or the nearby foothills (with low or no pesticide use).
Exposed children had “decreases in stamina, gross and fine eye-hand coordination, 30-minute mem-ory, and the ability to draw a person”88.
Starting around the year 2000, three large cohort studies (observational studies that repeatedly follow up participants) were initiated, measuring the ex-posure of pregnant women and infants to organo-phosphate pesticides, and later measuring various cognitive outcomes in the children at several ages.
One study, the ”Columbia Center for Children’s Environmental Health Study”89, is designed to in-vestigate the effect of prenatal exposure to air pol-lutants, amongst them chlorpyrifos (as a household insecticide), on neurodevelopment in children, in a low-income urban community in New York.
The second study, the CHAMACOS (Center for the Health Assessment of Mothers and Children of Salinas) cohort study, assesses how children are exposed to pesticides and other pollutants during their mothers pregnancy, infancy and childhood.
The study is designed to investigate the effect of these exposures on children’s growth, neurodevel-opment, and other health parameters90.
The third study, the “Children‘s Environmental Health Cohort” of the Mount Sinai Hospital in New York, is designed to investigate the impact of the prenatal exposure to some indoor pesticides on the growth and development of children91.
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All three studies report to have taken into account a range of potential confounding factors (chemical, socio-economic, and others). Although the stud-ies vary in design, methods and populations, some important common trends are apparent. Mental development at age 7–9 was negatively affected in children that had been exposed to chlorpyrifos/or-ganophosphates during pregnancy.
Three studies discovered similar effects In the Columbia study, the full-scale IQ at age sev-en was lower in childrsev-en with a higher exposure during pregnancy. Also, the working memory was negatively affected by chlorpyrifos exposure dur-ing pregnancy, but not perceptual reasondur-ing, verbal comprehension, and processing speed92. Also, in the Columbia study, a high exposure to chlorpyrifos during pregnancy was associated with changes in brain morphology at age 6–1193.
In the CHAMACOS study, children of the 20
per-cent of women with highest exposure to chlorpyri-fos during pregnancy had on average a score seven points lower on the IQ scale at age seven compared to children of women with the 20 percent lowest exposure. Also working memory, processing speed, verbal comprehension and perceptual reasoning at age 7 were negatively affected by high chlorpyri-fos exposure during pregnancy94. The Mount Sinai study, with a smaller number of children in this anal-ysis, found comparable but not statistically signifi-cant effects95. Importantly, the observed associations of exposure during pregnancy and the effect on neurodeve lopment were consistent over three differ-ent study populations, and their magnitude was quite large. It is also important to note that associations found in epidemiological studies are not a proof of causality. However, the Bradford Hill criteria of cau-sation96, a group of originally nine criteria that have recently been further developed, can provide guid-ance on whether there is adequate evidence for a causal relationship in epidemiological studies.
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Based on results from these and other epidemio-logical studies, scientists have recently added chlor-pyrifos to a list of compounds known to exert de-velopmental neurotoxicity97. A recent systematic review summarises all available epidemiological studies on the developmental neurotoxicity of or-ganophosphate pesticides and finds that “most of the studies evaluating prenatal exposure observed a negative effect on mental development and an increase in attention problems in preschool and school children”98.
In contrast to these epidemiological studies, in ani-mal studies carried out as part of the regulatory ap-proval process according to specific guidelines, such neurodevelopmental effects of chlorpyrifos have not been found; adverse effects have only been ob-served at much higher concentrations than in the human studies. It is important to note, however, that knowledge of neurodevelopmental effects has increased greatly in recent years, while the test of neurodevelopmental effects used in the regulatory process dates from 199899. That test is probably not able to detect decreases in the IQ scale of the mag-nitude observed in the epidemiological studies.
The most frequently discussed cause of develop-mental neurotoxicity of chlorpyrifos is the ability of the chlorpyrifos oxon, a metabolite of chlorpy-rifos, to block (inhibit) the enzyme acetylcholines-terase (AChE) which is important in the develop-ment and function of the brain and nervous system.
AChE inhibition is the mode of action of chlorpy-rifos, as well as of other organophosphorus and car-bamate insecticides. Apart from AChE inhibition, a number of other potential modes of action have been proposed84, 100. However the animal studies that authorities base their toxicological assessment on, focus solely on the AChE inhibition under vari-ous exposure scenarios; no other potential modes of action are investigated. The lowest daily dose of chlorpyrifos found in animal studies that causes AChE inhibition in long-term studies in animals is roughly 1 000 times higher than the exposures that are associated with observed adverse effects on chil-dren’s neurodevelopment in the epidemiological studies100.
It is a characteristic of the current regulatory assess-ment practices that in such cases, epidemiological studies have lower weight, although the applica-tion of the Precauapplica-tionary Principle is anchored in
PhOTO & TExT: ISTOCKPhOTO © NANDy NEhRING: Watsonville, California, USA – January 24, 2011: Farm workers spot spraying artichokes with herbicide. Workers are dressed in protective clothing to reduce their expose to chemicals. Use of pesticides in fields near housing developments such as this is an issue in California.
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rele vant EU regulation61. For example, for the most prominent and most widely used organophosphate pesticide, chlorpyrifos, EFSA summarizes the evi-dence from epidemiological studies:
“In summary, the weight of evidence suggest that the results of the three cohort studies in concert with the animal studies indicate that maternal CPF [chlorpyrifos] exposure would be likely as-sociated with adverse neurodevelopmental out-comes in humans. However, the exposure to multiple cholinesterase-inhibiting pesticides or other neuro toxicants might result in additive or interactive effects. The Columbia study was con-sidered the most robust because it measured CPF in maternal and cord blood (rather than non-specific metabolites). The epidemiology hu-man studies should not be considered quantita-tively to establish reference dose.”100
That is, although the association between chlorpyri-fos exposure and negative effect is likely and EFSA acknowledges that the epidemiological studies are of high quality, EFSA does not regard the causality
to be established. Therefore, these epidemiological studies are disregarded when the toxicity of chlor-pyrifos is assessed, and an ADI is established based on AChE inhibition in the red blood cells of rats101. Accordingly, EFSA requires a causality to be es-tablished before epidemiological studies can be taken into account, although the precautionary principle should apply, according to Regulation 1107/200961. It is also remarkable that the poten-tial causality apparently has not been evaluated us-ing e.g. the Bradford-Hill criteria96.