A systematic strategy
Researchers are currently developing a systematic strategy to review the toxic substances in the modern environment that are suspected links to neurological disorders. An editorial in Environmental Health Perspectives, A Research Strategy to Discover the Environmental Causes of Autism and Neurodevelopmental Disabilties, and several additional articles in the journal are available for further reading. Environmental Health Perspectives is a peer-reviewed open access journal published by the National Institute of Environmental Health Sciences and is available on the World Wide Web. Toxic substances research sparked a workshop on the subject where ten chemicals and mixtures that are spread widely in the environment were called out as possibly toxic to the developing brain. Other substances will surely be added, but for now this short list is intended to be the focus of research.
The List of Ten
The chemical and compound names are familiar. These short explanations provide basic facts about them and highlight where they can be found. Unless otherwise noted, the U.S. Environmental Protection Agency is the source for the following information.
Lead – A naturally-occurring element, lead is abundant. It was used for many years in paints, but was banned in 1978. There is a good chance that homes and other buildings including day-care centers and schools built before 1978 contain some lead-based paint and lead dust. Other sources of lead include batteries, ammunition, pipes, ceramic products, caulking, and pipe solder. Lead in these sources has been reduced in recent years due to health concerns.
Methylmercury – This organic compound is frequently found in fish and shellfish. The health effects depend upon the amount consumed, the age and health of the person, and other factors. A fetus is exposed to methylmercury in the womb as a result of the mother’s consumption of fish and the developing fetal nervous system is more endangered by exposure to methylmercury that the adult’s.
Polychlorinated biphenyls (PCBs) – This organic chemical is manmade and was manufactured from 1929 until 1979. The substance was popular because it was non-flammable and had a high boiling point; it was frequently used as an insulator. Hundreds of products produced before 1979 used PCBs including electrical equipment (transformers, capacitors, voltage regulators, switches, bushings, electromagnets), oil, fluorescent light ballasts, cable insulation, thermal insulators, tape and adhesives, paints, caulking, plastics, carbonless copy paper, and floor finishes. PCBs do not break down easily and therefore recycle in the environment for a long time; PCBs accumulate in plants and animals and have passed into the food chain.
Organophosphate pesticides – This man-made chemical disrupts the nervous system of insects by interfering with the enzyme that regulates acetylcholine, an important chemical regulator of brain activity. Organophosphates were developed in the early nineteenth century and it wasn’t until 1932 that scientists discovered the chemicals had the same effect on humans that they did on insects. Some were used as nerve agents during World War II. They do not persist in the environment.
Organochlorine pesticides – Man-made pesticides known by names such as DDT and chlordane were used in the past, but have been removed from the market for health and safety reasons. They persist in the environment and bioaccumulate in the food chain. They stimulate the central nervous system and cause excitability of neurons. Endosulfan, used on vegetables, apples, and melons, was banned in 2010. It was one of the last organochlorine pesticides on the market.
Endocrine disruptors – The endocrine system is also known at the hormone system. Hormones
are messengers that initiate all sorts of biological processes related to bodily development and ongoing bodily regulation. Some chemicals have been shown to act like hormones and trick the body into over-responding, responding at the wrong time, and blocking reactions. While birth control pills are an example of intentional endocrine disruptors, there are other environmental chemicals that have been found to disrupt hormones in unintended and unexpected ways.
Automotive exhaust – The by-products of burning fuel in an engine are emissions of volatile organic compounds, oxides of nitrogen, particulate matter and carbon monoxide. While emissions of carbon dioxide contribute to climate change and air pollution is related to respiratory health, scientists are only beginning to understand the link between motor vehicle exhaust and brain cell development. The evidence for a link between exhaust and brain-based disorders is growing.
Polycyclic aromatic hydrocarbons (PAH) – These semi-volatile organic compounds are formed during the incomplete burning of gas, coal, garbage, and other organic substances. They are not produced or used commercially but are abundant in the environment. PAHs are found in motor vehicle exhaust, cigarette smoke, industrial soot, cooked foods, and asphalt processing. PAHs contaminate drinking water by leaching from soil and water storage and distribution systems. Humans consume PAHs by inhaling contaminated air, smoking cigarettes, and consuming contaminated food and water. The compounds are related to reduced fertility and viability of young. Cancer is the primary concern of exposure. PAHs are persistent in the environment and have the potential to accumulate in the food chain, thereby becoming more toxic to humans.
Brominated flame retardants – There are many varieties of flame retardants. The chemicals are major components of petroleum-based products and plastics used in everything from furniture foam to coatings on flame-retardant fabrics. Plastics are found throughout the environment in furniture, appliances, electronic devices, vehicles, toys, and home furnishings. They accumulate in the environment and have been found in increasing concentration in humans. They are suspected of altering behavior and learning and causing cancer.
Perfluorinated compounds (PFCs) – When hydrogen atoms are replaced by fluorine atoms in certain chemical chains, a stain, oil, and water resistant material is formed. Teflon and Scotchgard are two of the most readily recognized brand name PFCs, but the compounds are pervasive in the environment and have accumulated in the food chain for a half century. They have been used in everything from popcorn bags to cosmetics. PFCs bind to tissue including those that make up human blood and organs such as the brain. 3M was the principal producer of these polymers before 2003, but the production of PFCs is being phased out globally today. More research is needed to understand the effect of this chemical on human development.