A review of studies carried out to assess the safety of industrial chemical substances has concluded that 11 substances—including certain metals, organic solvents, pesticides, and flame retardants—can now reliably be classified as developmental neurotoxicants. Such substances have the potential to cause permanent brain damage in developing fetuses and young children.
In the review, environmental health scientists Philippe Grandjean and Philip J. Landrigan of Harvard School of Public Health say that exposure to the identified substances is contributing to “a global pandemic of developmental neurotoxicity” that mirrors smoking cigarettes, alcohol abuse, and processed foods as a public health problem. As a result of their findings, the Harvard scientists call for mandated worldwide assessment of the neurotoxicity of chemicals used in commerce.
The study comes at a time when Congress is looking to update laws that regulate chemical testing and risk assessment. Current regulations require that, once a chemical is on the market, there must be proof it is toxic before its use is restricted or it is removed from commerce. Toxicities of many chemicals used in industry and in consumer products have not been adequately tested, often because they were on the market prior to current regulations. So scientists and regulatory agencies have little understanding of how the compounds might work alone or in combination to cause harm—or whether they are harmless.
Most people agree society can’t afford to pretend that the 11 cited chemical substances pose no risk to human health—some of them have been known to be toxic for decades. But many people would also agree that it is shortsighted to summarily restrict useful chemical products that benefit consumers and the economy or to replace a chemical with an alternative that may be more problematic. The chemical industry is a staunch defender of the current U.S. system, but environmental advocates argue that the industry’s insistence on absolute proof of toxicity is unreachable.
Grandjean and Landrigan advocate taking a precautionary approach to risk assessment that emphasizes preventing early-life exposures to the suspect chemicals—even in the absence of proof of their toxicity. In 2006, they published an initial report identifying lead, methylmercury, polychlorinated biphenyls, arsenic, and toluene as neurotoxicants that could interfere with early-childhood brain development. The researchers have now updated their list by adding manganese, fluoride, chlorpyrifos, DDT, tetrachloroethylene, and polybrominated diphenyl ethers (Lancet Neurol. 2014, DOI: 10.1016/s1474-4422(13)70278-3).
The researchers conducted a literature search for developmental neurotoxicity and reviewed the clinical and epidemiological studies they discovered. The Harvard team concluded that 214 industrial chemicals could be labeled human neurotoxicants. Many of these substances have been detected in umbilical cord blood and breast milk.
Grandjean and Landrigan point out that low-level exposure to these chemicals might have little or no effect on adults. But the Harvard researchers say the chemicals can cause subtle disruptions in critical brain development during pregnancy or in young children and lead to learning and behavioral disabilities. They add that these effects could hamper academic achievement and economic welfare later in life, as well as lead to criminal behavior.
“The Grandjean and Landrigan review provides a powerful summary of the mounting scientific evidence documenting the individual and societal tolls—from IQ deficits to impacts on national GDP—of early-life exposure to key neurodevelopmental toxicants,” says biochemist Jennifer McPartland, a health scientist at the Environmental Defense Fund, a nonprofit advocacy group. “The significance of this situation is further elevated by the real-world cumulative exposures to these chemicals. These scientists rightfully argue for a call to action.”
But Grandjean and Landrigan have been criticized by other scientists for their “strength of evidence” approach. Their analysis relies on making generalizations based on studying research results that link a cause, such as exposure to a ubiquitous chemical, with an effect, such as autism. Critics say that a more rigorous assessment of the amount and duration of exposure and factoring in the potency of a substance are needed.
“No one can really argue with identifying lead or methylmercury as developmental neurotoxicants,” Vanderbilt University toxicologist F. Peter Guengerich says. “But I don’t think we are having an ‘industrial epidemic’ as Grandjean and Landrigan suggest. We have less exposure, at least in the U.S., to these substances today than we did a generation ago.”
When it comes to passing judgment on individual substances, Guengerich adds, all chemical substances, including the medicines we take, are toxic at some dose and safe at some dose. “But it seems a jump to group what we know to be useful chemicals, such as fluoride, in with them.”
Listing fluoride as a developmental neurotoxicant in the study was based primarily on research studying exposure to fluoride in areas of the world where it is at a naturally high concentration in drinking water, Guengerich notes. That is different from an acute work-related exposure or consumption of the low levels added to drinking water and toothpaste to prevent cavities.
“So to say that 214 chemicals may be neurotoxic is less of a beneficial analysis than a possible scare tactic,” Guengerich says.
Several chemical company toxicologists cited those deficits among other reasons in declining to comment for the record for this story. However, their sentiments were summed up in a statement released by the American Chemistry Council, a trade association of chemical manufacturers.
“What is most concerning is that the authors focus largely on chemicals and heavy metals that are well understood to be inappropriate for children’s exposure, are highly regulated, and/or are restricted or being phased out,” the ACC statement reads. “They then extrapolate that similar conclusions should be applied to chemicals that are more widely used in consumer products without evidence to support their claims. Such assertions do nothing to advance true scientific understanding and only create confusion and alarm.”