What is neurotoxicity?
Neurotoxins are substances which alter the function of the nervous system by damaging brain cells or the nerves which carry signals around the body. Some researchers regard substances which cause temporary changes to the function of the nervous system as also being neurotoxic.
Disorders associated with exposure to neurotoxic substances include impaired intelligence, impaired regulation of emotional responses, behavioural problems including attention deficit and hyperactivity disorders, depression, anxiety, memory formation, impaired physical coordination and increased risk of neurodegenerative disorders such as Parkinson’s and Alzheimer’s diseases.
How can chemicals harm the nervous system?
The healthy development and long-term function of the nervous system is governed by a wide range of physiological factors. Chemicals can have neurotoxic effects by directly and indirectly interfering with these processes.
For example, in the developing foetus some chemicals may prevent brain cells forming enough contact points with each other to communicate effectively. Other chemicals can impair IQ and cause behavioural problems by altering levels of hormones which are vital for the correct development of the foetal nervous system. In this respect, endocrine disruptors which target the thyroid system are of particular concern to researchers.
As adults, chemicals and pollutants can accelerate the rate at which adult nerve and brain cells degrade and die by increasing oxidative stress in the body. This is thought to increase the risk of Parkinson’s and Alzheimer’s diseases, bipolar disorders and chronic fatigue syndrome.
Which chemicals are causing problems?
At least 200 chemicals have been identified as potentially neurotoxic in humans and over 1,000 have been shown to be neurotoxic in animals, including:
- PBDE flame retardants, associated with poorer concentration and lower scores in a range of childhood developmental and behavioural tests (Herbstman et al. 2010);
- Heavy metals such as lead and manganese, which have been shown to impair intelligence (e.g. Ciesielski et al. 2012);
- Phthalates, which impair physical coordination and reduce scores in a range of behavioural tests, with boys in particular affected (e.g. Whyatt et al. 2012);
- Exposure to air pollution, which accelerates cognitive decline in adults (Weuve et al. 2012);
- A range of pesticides, which increase risk of Parkinson’s disease and impair early mental development, affecting working memory, intelligence and perceptual reasoning (e.g. Wang et al. 2011);
- BPA, which may affect regulation of behaviour and emotion (particularly in girls) and may feminise the behaviour of male boys (Braun et al. 2011);
- Perfluorinated (non-stick and stain-resistant compounds), which may increase risk of attention disorders and impulsive behaviours (Hoffman et al. 2010)
Developmental neurotoxicity of industrial chemicals. Scientific review in the medical journal The Lancet, summarising the evidence implicating chemicals in a wide range of neurodevelopmental disorders.
Reducing The Risk of Alzheimer’s and Parkinson’s Diseases. Article in the Huffington Post describing research indicating a strong environmental component in diseases associated with ageing, including Alzheimer’s disease.
Do toxins cause autism? A New York Times writer looks at evidence that chemicals in the environment may be a risk factor in autism and other neurodevelopmental disorders.
Thyroid Hormone Understanding Branches Out: Insights into PBDE Impacts on Brain Development. Synopsis of findings that flame retardant exposure may harm mental development by altering thyroid hormone levels in the foetus.
Selection of references
Braun JM, Kalkbrenner AE, Calafat AM, Yolton K, Ye X, Dietrich KN, Lanphear BP. Impact of early-life bisphenol A exposure on behavior and executive function in children. Pediatrics. 2011 Nov;128(5):873-82. Epub 2011 Oct 24. PubMed PMID: 22025598; PubMed Central PMCID: PMC3208956.
Ciesielski T, Weuve J, Bellinger DC, Schwartz J, Lanphear B, Wright RO 2012. Cadmium Exposure and Neurodevelopmental Outcomes in U.S. Children. Environ Health Perspect :-. http://dx.doi.org/10.1289/ehp.1104152
Herbstman JB, Sjödin A, Kurzon M, Lederman SA, Jones RS, Rauh V, Needham LL, Tang D, Niedzwiecki M, Wang RY, Perera F. Prenatal exposure to PBDEs and neurodevelopment. Environ Health Perspect. 2010 May;118(5):712-9. Epub 2010 Jan 4. PubMed PMID: 20056561; PubMed Central PMCID: PMC2866690.
Hoffman K, Webster TF, Weisskopf MG, Weinberg J, Vieira VM 2010. Exposure to Polyfluoroalkyl Chemicals and Attention Deficit/Hyperactivity Disorder in U.S. Children 12-15 Years of Age. Environ Health Perspect 118:1762-1767. http://dx.doi.org/10.1289/ehp.1001898
Wang A, Costello S, Cockburn M, Zhang X, Bronstein J, Ritz B. Parkinson’s disease risk from ambient exposure to pesticides. Eur J Epidemiol. 2011 Jul;26(7):547-55. Epub 2011 Apr 20. PubMed PMID: 21505849.
Weuve J, Puett RC, Schwartz J, Yanosky JD, Laden F, Grodstein F. Exposure to particulate air pollution and cognitive decline in older women. Arch Intern Med. 2012 Feb 13;172(3):219-27. PubMed PMID: 22332151.
Whyatt RM, Liu X, Rauh VA, Calafat AM, Just AC, Hoepner L, Diaz D, Quinn J, Adibi J, Perera FP, Factor-Litvak P. Maternal prenatal urinary phthalate metabolite concentrations and child mental, psychomotor, and behavioural development at 3 years of age. Environ Health Perspect. 2012 Feb;120(2):290-5. Epub 2011 Aug 31. PubMed PMID: 21893441; PubMed Central PMCID: PMC3279439.