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, HealthyStuff.org 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.

Yet Gearhart still finds well-known neurotoxins like lead in the consumer products he tests. He notes that while we still had a significant problem with lead in toys only a decade ago, it took public outrage and federal regulations like the Consumer Product Safety Improvement Act to greatly reduce the number of toys with lead in them. Today, says Gearhart, only 2-3% of products they test have lead in them, a significant reduction.

But the products Gearhart still finds lead in may surprise you. His organization recently found lead in a pair of infant shoes, and found dangerous levels of lead, other toxic metals and toxic flame retardants in Mardis Gras beads.

In these enlightened times, how is it that we're still finding lead in common consumer products? Gearhart says chunks of waste electronics, which contain hazardous compounds, are being recycled, shredded and finding their way back into consumer products. In Mardis Gras beads, this toxic recycled material is used as filler.

When lead and other chemicals pop up in seemingly random ways like this, danger could lurk around any corner—or in any product. So what's the fix for this problem? Gearhart calls for a systemic approach to fix our problems.

“Our public policy on how we regulate chemicals in our material economy is not proactive,” says Gearhart, so we should come to expect lead to show up in seemingly random places. “We don't have a comprehensive way to have public policy around chemistries going into commerce. As a result, why should you be surprised about things popping up where they shouldn't be?”

“We have the right tools in place, we can actually change things. But the key thing is that we have to bring that systems approach to that," says Gearhart.

The current status quo is to react to chemicals in products when they're identified to be hazardous. For example, BPA (bisphenol A), a compound found in plastic bottles was found toxic, many manufacturers switched to a different "safer" plastic, Tritan, now suspected of posing health risks. Gearhart says were "jumping from one hazard into another."

Gearhart is proposing a systemic approach to thoroughly test chemicals before using them in consumer products. He believes that this would give the most protection to consumers.

Even pesticides, compounds created solely for their toxic properties, are not thoroughly tested before commercial use as they should. There are legal loopholes in place that allow for them to be tested after they're put on the market. 

Most other chemicals are regulated under the Toxic Substances Control Act. But the TSCA, which is four decades old, grandfathers in the chemicals on the market before its implementation, and gives the government very little leeway to stop new chemicals from entering the market or removing them after they're already there.

Gearhart works directly with retailers and manufacturers to put safer products on the market and has found success using this approach.

“It's leap-frogging where we are in terms of policy,” Gearhart says. Companies are evolving from a chemical-by-chemical approach to product safety and are now using a systems-oriented approach. For example, a company is using flammable polyurethane foam in a product that requires them to use a chemical flame retardant, they might consider using a less flammable alternative to polyurethane to avoid the need for flame retardants. 

But Gearhart admits that appealing to retailers and manufacturers has its limits. “We've got to have the policy piece in place to make the whole market change, particularly the international component,” he says.