EXPERT REACTION: Chemicals in everyday products linked to brain development disorders in children

Some industrial chemicals and pesticides can mimic the action of hormones, and evidence is growing that they impact on human development and health. When these substances were invented and registered for use they passed the 'acute toxicity' test. However, increasingly they are suspected of causing chronic affects for which there is no obvious infective agent like a bacterium or a virus.

Are they the cause of conditions like obesity and diabetes; late in life conditions like Parkinson's disease; or impaired development of the brain or the immune system? Or do these conditions come about because of some failure of body development and control systems that are not related to infection or toxicity? It's hard to be sure, but there are many voices urging us to play it safe and avoid the use of these substances.

We all carry body burdens of these chemical substances, albeit at low concentrations. Are they affecting our health and development or can the body tolerate these small concentrations? Maybe population studies show associations between body concentrations and impaired functioning - in the present case, brain development linked to chemicals that mimic thyroid hormones. But cause-and-effect is almost impossible to establish because we can't deliberately dose people to see what happens!

As evidence grows of the associations between chemicals and human health and development, some restrictions are being placed on the use of these chemicals but many remain in use.

Industrial chemicals and pesticides are useful substances. Can we do without them, or can we find alternatives that are safer? It's not always as easy as it sounds. Take DDT for example, long banned because of its impacts on wildlife, it is still permitted for use in some circumstances because it's the cheapest effective insecticide that we have. We would not need flame retardants if stuff didn't catch fire, but there are some suggestions that we have overdone their use and we'd be as well off without them in many applications.

We'll never have the kind of proof-of-harm that has enabled us to phase out acutely toxic substances like arsenic and strychnine, but pressure is growing to phase out substances suspected of chronic effects. They will need to be replaced with safer chemicals in applications where we can't do without them. It's a slow road, along which manufacturers and regulators are harried by scientists producing results like this.

This publication is a well written review of the literature of the adverse effects of endocrine-disruptors on the thyroid gland, particularly the possible consequences of these chemicals causing brain damage to the foetus, infant and young child. They are postulating these toxins may be related to the observed increased incidence of neurodevelopmental disorders such as autism and ADHD. It should be noted that they are using experimental animal data, in most instances, and then extrapolating to humans. So some of their material maybe relevant but not all of it.

What is clear is that maternal thyroid hormone transferred from mother across the placenta to stimulate and regulate normal brain development in humans. A deficiency will result in a spectrum of mild to severe neurodevelopmental disorders (see chapter 21 www.thyroidmanager.org.) It is possible, indeed likely, that the noxious effects of exposure of pregnant women and infants is mediated or compounded by iodine deficiency and already compromised thyroid function in the mother. Of course we should be taking action to limit exposure to endocrine disrupting chemicals and at the same neutralising their adverse effects by ensuring optimal iodine intake and normal thyroid function during pregnancy and in the infant.

This recent review in the journal  Endocrine Connections looks at the possibility that a range of chemicals in the environment may interfere with thyroid hormone function which will, in turn, affect the development of children’s brains (and thus their behaviour). Although there is a lot of evidence that many chemicals can potentially affect how our brains develop, the central problem with this review is summed up in the press release: 'We….propose that the data sets provide a plausible link to the recent increased incidence of neurodevelopmental conditions, including autism spectrum disorders and attention deficit hyperactivity disorders.'

However, it is widely accepted that the 'increase' in autism and to some extent ADHD is due to the combination of better diagnosis, diagnostic substitution (where diseases previous diagnosed as a different category and now diagnosed as the disease in question) and with ADHA, some degree of overdiagnosis. Since the increase in these diseases is largely changed accounting in effect, there is nothing to explain. Indeed while many of these chemicals can interfere with a range of hormones, they do so at much higher concentrations than found in the environment, so a lack of effect is not surprising.

Prof Demeneix and colleagues have written a paper on the impact of man-made chemicals that are now prevalent in the environment and in our bodily fluids on brain development of humans. Specifically, the chemicals that interfere with thyroid hormone signalling. Thyroid hormones are crucial for governing proper development of the brain. In severe cases, inadequate thyroid hormones during pregnancy can result in the baby being born small and mentally retarded (a cretin). Milder insufficiency can result in milder forms of mental retardation. The chemicals that interfere with thyroid hormone signalling can do so because they are very similar in structure to thyroid hormones, so they can ‘trick’ the body into thinking they are thyroid hormones, and consequently disrupt the normal thyroid hormone signalling that directs brain development. Because these chemicals are not actually thyroid hormones, they can block the real thyroid hormones from doing their job i.e. normal brain development.

Some of these chemicals have been identified and are no longer used but other similar chemicals have been used to replace them i.e. perpetuating the problem. Examples of common thyroid hormone disrupting chemicals include flame retardants in carpets, curtains, furniture and non-flammable clothing; plastic containers for heating food and “non-stick” agents. Even though some of these chemicals are no longer made, they are still in the environment because the items containing them are still in use. Some chemicals that interfere with thyroid hormone signalling are now found in amniotic fluid and in breast milk of women.

Rachel Carson wrote “The Silent Spring” half a century ago which first brought the devastating impact of pesticides to our attention. In her recent book “Losing Our Minds: How environmental Pollution Impairs Human Intelligence and mental Health” Demeneix expands on the increase in neurodevelopmental disorders (autism spectrum disorders, attention deficit/hyperactivity disorders), which have increased dramatically in the past decade and which cannot be explained by genetic causes. However, Demeneix describes a positive correlation between increase in thyroid hormone disrupting chemicals and neurodevelopmental defects in children.

The role of low thyroid function in the pregnant mother on impaired brain health in the fetus has been well established, particularly by Australians Basil Hetzel and Cres Eastman.

The increased exposure of populations to so-called environmental hormone disrupting chemicals due to modern industrial processes is also well known.

This review paper seeks to link the two by indicating EDCs can impair thyroid function and thereby contribute to brain and behavioural dysfunction in children.

The links are still tenuous, although animal experiments indicate the hypothesis deserves more study in humans.

Regardless, we do need to reduce the exposure of pregnant women to any of these chemicals and early testing of thyroid function in all women thinking of getting pregnant is warranted.

Practically all of the contaminants discussed in this paper are known drinking water and wastewater contaminants.

These include perchlorate, Bisphenol A and other phenols, brominated flame retardants, triclosan, the various pesticides, phthalates, perfluorinated chemicals, and polychlorinated biphenyls (PCBs).

While drinking water is unlikely to be a major exposure source for most people, it's very important that Australian water quality managers pay close attention to the emerging science and keep our drinking water quality guidelines up-to-date with that science.

This paper highlights the fact that there are many chemical contaminants for which new information is still emerging, and we still have a long way to go before we fully understand the risks.

Due to their chemical nature, many of these chemicals, including triclosan, brominated flame retardants and PCBs are known to accumulate in the sludge produced by sewage treatment plants. Due to their persistence in the environment, we need to be very careful about how we dispose of, or reuse, that sludge. In many cases, it is reused for agriculture, so understanding the risks associated with potential accumulation in food crops is an essential aspect of managing the risks of exposure to these chemicals by people.