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Why It's Impossible to Create a Top 10 List of the Most Neurotoxic Man-Made Chemicals

Toxins are hard to classify, harder to regulate, and even harder to remove from the products we consume.

Classifying human neurotoxins can be tricky. While laboratory research has identified more than 1,000 chemicals to be animal neurotoxins, the known list for humans is small by comparison. Only 214 chemicals have been classified as human neurotoxins, and only 12 have been identified as impacting fetal and child development. 

So why is it that only a fifth of the known animal neurotoxins are identified as posing risks to humans? Even more worrisome, why are only 1% identified as hazardous to infants and fetuses? The reason lies in the ethical and practical considerations of scientific research: You can't study toxicity on humans by feeding people arsenic and comparing their levels of brain damage with a control group. So no authoritative top 10 list of the deadliest neurotoxins is possible. We're in the dark. 

Still, a new study out of Denmark argues that while human neurotoxins can be clearly identified when people who are exposed to them later become ill, it's not so easy to show whether or not small amounts of chemical exposure may have impacts on developing fetuses and infants, and what those affects may be. Or as David P. Rall, the former director of the National Institute of Environmental Health Sciences, who is quoted in the study, says: “If thalidomide had caused a ten-point loss of intelligence quotient (IQ) instead of obvious birth defects of the limbs, it would probably still be on the market."

So, the study points out, we often do not know which chemicals can cause lost IQ points or other developmental problems in children. However, research has identified more developmental neurotoxicants in recent years — doubling the list since 2006 — as scientists do what are called “epidemiological birth cohort studies.”

These studies examine exposure levels of chemicals to fetuses and infants, while comparing growth and development of the children over time. This helps show the chemicals that may cause problems, but the method is not perfect. As individuals, we're all exposed to our own unique cocktail of potentially toxic chemicals over time, and how they may impact us may vary based on many factors.

Still, it can also be assumed the more comprehensive lists of known animal and human neurotoxins will likely have impacts to fetuses. And prenatal exposure to many of these chemicals is common, at least at low levels; more than 200 foreign chemicals have been identified in umbilical cord blood.

Moreover, the study notes that known human neurotoxins are not at all rare: About half are widely used and disseminated around the world, and they include pesticides, organic solvents, metals, and other compounds.

Even if developmental toxicity can be measured in lost IQ points during childhood, the researchers also question impacts of chemicals later in life. Could in-utero exposure increase the chance of developing neurodegenerative diseases like dementia? And if the effects do not show up until a person’s sixth or seventh decade, will we continue to expose future generations to chemicals until we can confirm their toxicity decades from now? 

Questions like these motivate Jeff Gearhart, research director at the Ecology Center. His organization's research website, tests common consumer products and then reports the results to consumers—but also works with manufacturers to make products safer in the future.

Gearhart says the chemicals listed in the Danish study are the ones commonly identified as neurotoxins, but says there is a larger universe of chemicals used in consumer products that might pose health risks. So while we all have the common understanding that lead should not be used in consumer products, there is a considerable amount of concern about many of the chemicals used in common products, particularly in plastics.

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