One out of every six children suffers from some form of neurodevelopmental abnormality, mostly of unknown causes. Some diagnoses, such as autism spectrum disorder and ADHD have apparently become more common, but we do not know why. These problems are not likely to be of genetic origin, and some environmental factors clearly play a role.
Several environmental chemicals, such as lead, mercury and certain pesticides, are known to cause damage to the brain, especially during the vulnerable early stages of development. After decades of research, we are finally realizing that these chemicals can damage the brain functions of the next generation. Chemical brain drain is a fact.
There are thousands of chemicals in the environment and in consumer products that have not even been tested for possible damage to brain development. Yet, we ignore this problem in risk assessment, where lack of evidence is assumed to mean no risk.
Why we need to act
The brain is the most complex organ of the human body and therefore highly vulnerable to adverse effects. You get only one chance to develop a brain. Developmental processes must happen in a particular sequence and at a particular time. If development is stalled or derailed at some stage, there is little chance of repair or catch-up. Compensation is possible, but optimal brain functioning presumes that the right cells are available at the right locations and with the right connections. Our intelligence depends on the integrity of the complete organ. Thus, the brain differs substantially from other important organs, such as the kidney or the liver, where we can lead successful lives without maximal function.
Even very small changes – which do not necessarily amount to a neurological diagnosis – can easily impact one’s future brain functions, academic achievements, risk of delinquency, and quality of life. Our brain functions are valuable to ourselves and to society. Data on exposures to lead, mercury, and pesticides suggest that the losses to society amount to hundreds of billions of dollars every year. This calculation is based solely on losses of income and does not take into account less tangible damages associated with chemical brain drain.
Brain development is uniquely sensitive
Brain development consists of a series of complex and intricately coordinated stages that need to happen at specific times, and in a particular sequence at specific locations. Damage during early development is likely to be irreversible, and the consequences of derailing the time schedule or the developmental processes are therefore long-term. We get only one chance to develop a brain. Yet no textbook discusses brain development from the point of view of vulnerability to toxic damage. One serious misconception was that the blood-brain barrier (which envelopes the brain) would prevent toxic chemicals from entering. However, unfortunate experiences proved it wrong. Likewise, we originally thought that congenital brain damage would be heritable or would be due to some physical factors, like trauma during birth. We now know that toxic chemicals can interfere with the intricate and complex biochemical processes that generate this exceptional organ. So, exactly because the brain is so complex, and because optimal function depends on the integrity of the brain as a whole, brain development is highly vulnerable to toxic chemicals.
Errors of the past
In 1940, nobody believed that a maternal infection with German measles could be detrimental to the child she was carrying. But an epidemic of eye and brain damage among infants in Australia convinced a doctor that the micro-organism had penetrated the placenta and damaged the brain development in the child. The daring conclusion was met with skepticism and arrogance. It was not until the German measles epidemic in the US in 1968 that experts agreed that the newly discovered German measles virus could make its way from the mother’s circulation to the fetal brain. At that time, thousands of children had already suffered eye and brain damage that could have been prevented.
In parallel, a discovery was made in the 1960s that maternal alcoholism could cause brain damage in the offspring. Again, the findings were at first ignored or disbelieved. Decades later, we have realized that even occasional alcohol intake during pregnancy may be harmful to the fetal brain. We now know that hundreds of industrial chemicals circulate in the fetal blood. These chemicals were absorbed by the mother from her diet, from drinking water, from exposures at work, and from consumer products. Yet, without her knowing, the chemicals pass the placenta and invade the brain of her child. This chemical invasion continues, as we continue to rely on optimistic assumptions that the fetus is protected in the womb, although this is clearly naïve and wrong.
Inertia in science
Science provides an incomplete basis for protection of the brain against chemical toxicity. First of all, science has covered only a handful of chemicals, and we know the dose-response relationships only for substances, such as lead, mercury, arsenic, and polychlorinated biphenyls (PCBs). However, we know from clinical poisonings that more than 200 chemicals can cause neurological symptoms in adults. Thus, these chemicals can obviously gain access to the brain and exert toxic effects on brain cells. If this can occur in adults, the chemicals very likely can also damage the developing brain, with much more serious and permanent consequences likely to occur, even at doses much lower than those that can harm mature brains.
Second, research on chemical brain drain takes time and involves many uncertainties. The best studies have measured chemical exposures of the mother during pregnancy, perhaps analyzed cord blood samples taken at childbirth, then later also assessed exposures during childhood. The detailed brain functions are difficult to test in small children and usually have to wait until the child has reached school age. Thus, such studies take many, many years to complete. Animal experiments may be helpful, but the human brain is very different from the nervous system that usual laboratory animals must be content with. Also, standard toxicology protocols just require that the weight of the brain is measured, and only a decrease of more than 5% is considered an adverse effect. Because of the uncertainties, the conclusions from existing scientific documentation must be cautious.
The third problem is that scientists usually express themselves in ‘soft’ language, using ‘hedged’ expressions with maybe, perhaps and other terms like that, while caveats are highlighted. This tradition in academic science has served to negotiate guarded conclusions, while balancing between skepticism and gullibility. However, vested interests have taken advantage of this tradition and raised doubt about the conclusions that can be drawn from the evidence. This has led to a belief that chemical safety is not needed unless proven by science, that is, ‘sound’ science. So a chemical is considered safe, unless otherwise proven.
The combination of these three problems – a triple whammy – puts the developing brain at extreme risk. We are waiting for science to provide the documentation, but that will take decades, and in the meantime, we will expose the next generation to toxic chemicals that can damage their brains.