22 August 2013. At a international meeting of scientists this week in Basel, Switzerland, important new findings were presented on chemical brain drain. While no definite proof emerged, the results add to a complex jigsaw-puzzle. All such studies are observational – they record the links of brain deficits to toxic chemicals in blood samples or other measures of exposure. While such associations alone do not prove a causal relationship, the conference illustrated what the best of science can realistically produce, given that the task is to observe and analyze the possible effects of chemicals that have been released to the environment without prior testing for their effects on the brain. Here are three examples of the types of evidence that we must rely on.
Ecology studies: Some plastics additives are suspected of causing adverse effects on the brain, in particular the phthalates. Swedish researchers from Karlstad University recorded the use of PVC flooring in the homes of over 14,000 children aged 1-5 years in 2000. Five years later, they showed that PVC flooring in the children’s own bedroom was related to phthalate uptake in the infants. At the 10-year follow-up, more than half of the families filled in a questionnaire. Learning difficulties, especially in the girls, and to a lesser degree difficulties in concentration and language, were linked to PVC flooring in the parents’ bedroom – not the child’s bedroom. The researchers assume that exposures from the floor material in the parents’ bedroom during the prenatal period contributed to the cognitive problems recorded in the child later on. As the specific exposures were not measured, but were assumed to be linked to a PVC flooring material, I call this study ecological – it provides evidence of an environmental risk factor, but does not point to specific toxic chemicals and dose levels.
Cocktail studies: A wealth of industrial chemicals may cause endocrine disruption and thereby lead to an increased risk of autism spectrum disorders or other adverse effects in brain development. Researchers from Brown University measured the concentrations of 52 such compounds in blood or urine samples from 175 pregnant women. When children were 4 and 5 years old, the mothers filled in a standardized questionnaire (Social Responsiveness Scale) to assess possible abnormalities in the children. The researchers then applied advanced statistical methods to link the scores to the exposure data. As very little is known on the effects of these chemicals, the analysis assumed that all of them could contribute, depending on their concentrations in the blood and urine samples. Despite the detailed assessment of many chemicals – or one could perhaps say because of the wealth of exposure data – the study provided only limited evidence that single compounds were associated with a risk of autism-like behaviors. Thus, under some circumstances, the cocktail may be too complicated and too difficult to characterize in detail, thus making it virtually impossible to draw conclusions on the effects of specific chemicals.
Domino studies: In some cases, one brain drainer may pave the way for another. This possibility was assessed by California researchers, who examined children belonging to the Chamacos cohort from Salinas, CA. Suspected brain drainers DDT, DDE and brominated flame retardants had been measured in serum obtained from pregnant women, and findings in their 210 7-year-old children have now been analyzed. The researchers had previously reported that the mother’s exposures to flame retardants was associated with impaired brain functions in the child. While the insecticide DDT (and its breakdown product, DDE) was not, by itself, linked to clear adverse effects, children with increased pesticide exposure showed more severe effects of the flame retardants. This domino effect worked both ways, so high flame retardant exposure also led to greater effects of DDT. These effects were primarily seen in regard to prenatal exposures, much less the child’s own exposures after birth. So a toxicant that has no clear effect by itself may nonetheless increase the effects of another.
Overall, studies such as these, presented by some of the best researchers in the field demonstrate that we are only slowly gaining understanding of the causes of chemical brain drain. While growing evidence should leave room for some optimism, we also have to accept that definite proof may never be obtained, and we have to draw responsible conclusions from less than that.