By: Jessica J.
Image from IStockPhoto
What is Autism?
According to the American Psychiatric Association, autism spectrum disorder (ASD) is a lifelong "complex developmental condition that involves persistent challenges in social interaction, speech and nonverbal communication, and restricted/repetitive behaviors." It is a spectrum condition meaning that while people with Autism share certain difficulties, their condition is experienced and projected differently. Each person has a different way of relating and connecting with others and perceiving their surroundings. It is further classified as a brain-based disease because it is essentially a neurodevelopmental difference where symptoms are typically expressed in the first two years of life. There are numerous subtypes under the spectrum. Autism also has different forms, such as Asperger syndrome. People with Asperger syndrome might have average or above-average intelligence, few problems with speech, or difficulty processing emotional situations around them.
Illustration by Wikimedia Commons
For a long time, researchers wondered what caused the brain to develop so differently. Many assumed the answer would be definite. But studies over the past 100 years have concluded the opposite. Television, power lines, and parental attention have all been associated as causes of Autism. None have been confirmed with certitude as to the definitive cause. There was a firmly held belief between the 1950s and 1960s that Autism was caused by distant parents, who were termed "refrigerator mothers." This false information left behind a bequest of shame and guilt in the autism community. Bernard Rimland, fortunately, disproved this claim in his book Infantile Autism: The Syndrome and Its Implications for a Neural Theory of Behavior in 1964. He includes the research and theories of many scientists whose main argument against the theory is that parents who fitted the "refrigerator" stereotype had children who did not have Autism. The book initiated a public debate between two opposing views of Autism's causation: psychological vs. biology and development.
The past years of study into the topic have found very clearly that there is no one cause of Autism. Since then, research has focused on biological factors that may lead to autistic behaviors. A variety of genetic factors is linked to the cause of Autism. These may result in Autism individually or combine with environmental factors to cause the brain to develop differently.
Genetics Role in Autism
Scientists study twins to examine the influences of nature (genetics) and nurture (environment) on a variable human quality. Scientists then look at the frequency of that trait between identical twins and fraternal twins (of a defined population) and if both or just one have the trait. The trait can be deemed genetic if the percentage of people with the traits are higher for identical twins rather than fraternal twins (This is because identical twins share all their DNA). The current best study supporting that autism has a genetic link is a 50-80% frequency for identical twins and a 5-20% frequency for fraternal twins (Sahu & Prasuna, 2016). The data indicates a strong genetic factor influencing Autism.
Environmental Links to Autism
Genetic factors have been identified as either causes or risk factors of Autism. Additionally, exposure to environmental pollutants-such as endocrine disruptors which are hormonally active agents that can interfere with endocrine systems and potentially cause cancerous tumors, birth defects, and other developmental disorders- has been associated with health effects such as neurodevelopmental disorders. Studies suggest an overlap and combination of genetic mutations and various environmental factors could result in Autism. Autism Speaks, the largest Autism research organization in the United States, as of 2021, states that environmental influences do not appear to cause Autism directly; instead, they seem to influence risk in those genetically predisposed to the disorder.
Air Pollution:
According to the National Institute of Health, researchers found that infancy and childhood exposure to air pollution may be a risk factor for Autism. The researchers who were mainly based in Los Angeles took into account the maternal residence at the time of delivery and discovered more autism cases for children of mothers living near a freeway, and concluded that that "living near a freeway was associated with autism" (Volk et al., 2011). Furthermore, residential proximity during the third semester was more associated with Autism. The conclusion is because traffic-related air pollution contains substances (like carbon monoxide, nitrogen dioxide, and lead) found to have deleterious prenatal effects such as various neurological or cardiovascular diseases and disorders. A meta-analysis by the Harvard School of Public Health further supported this conclusion. The analysis correlated exposure to fine particulate air pollution with a significantly increased risk of Autism, mainly if exposure occurs during the third trimester of pregnancy or early childhood. The recent study found that the risk of ASD increased by "64% with exposure to 10 micrograms of PM2.5 per cubic meter of air (mcg/m3) during early childhood and by 31% during prenatal periods. During the prenatal period, the greatest risk [of Autism] was found during the third trimester" (Lin et al., 2021). The results suggest that air pollution exposure to fine particles (PM2. 5) from other sources such as vehicle exhausts, industrial emissions and other sources of outdoor pollution increase the risk of developing Autism.
Studies suggest that air pollution may cause developmental neurotoxicity-an adverse effect on the anatomy and physiology of the nervous system during development or maturity due to chemical or physical influences-and may contribute to the etiology, cause or origin, of neurodevelopmental disorders like autism. The diagram below briefly outlines how air pollution can affect the brain.
Diagram from ResearchGate
Metals, Pesticides, and Other contaminants:
Prenatal and early childhood exposure to heavy metals, altered levels of essential metals like zinc or manganese, pesticides, and other contaminants are linked with Autism.
For the correlation between metallic elements that are toxic ( they may result in health problems or environmental damage and are termed as “heavy metals” they include arsenic, cadmium, chromium, lead, and mercury) and Autism, a study of twins used baby teeth to determine and compare lead, manganese, and zinc levels in children with Autism to their twin without the condition. The study found that the Autistic twins were low on manganese and zinc (essential nutrients for human life) but had higher levels of lead, a harmful metal that can be detrimental to human health, especially during prenatal and early childhood stages of life (Aurora et al., 2017). Additionally, scientists found that altered zinc-copper cycles (processes that regulate metal metabolism, a procedure that delivers the correct metal ion to its destination in the body, because metal regulation is essential to many biochemical processes within the body) are disrupted in certain people with Autism (Curtin et al., 2018). These findings suggest that altered zinc-copper cycles resulting from elemental exposure lead to the emergence of ASD, and the biochemical measures of metal cycles are different Autism cases from people who don't have Autism.
Persistent organic pollutants (POP) are toxic chemicals that adversely affect human health and the environment around the world. They can be referred to as Per- and Polyfluoroalkyl Substances (PFAS),a group of man-made chemicals that have been manufactured and used in a variety of industries around the globe since the 1940s and are often referred to as “forever chemicals” because they do not break down in the environment as they are resistant to environmental degradation through chemical, biological, and photolytic processes. Dichlorodiphenyltrichloroethane (DDT) is a type of POP, and p,p’-dichlorodiphenyldichloroethylene (p,p’-DDE) along with polychlorinated biphenyls (PCBs) are its metabolites. These compounds were widely used in developed countries as insecticides (DDT) and in electrical equipment (PCBs). Although these chemicals were widely banned in many nations over 30 years ago, the chemicals are still present in developed countries like the United States. Due to their easily absorbable nature and chemical half-lives ranging as long as several decades, they persist in the food chain, particularly through fatty food sources resulting in continuing exposure to populations. Maternal exposure (which is the exposure of the female parent to a potentially harmful agent or factor) to these insecticides, substances used to kill insects, during the prenatal stage is also associated with a higher risk of Autism in children. A group of doctors based in New York and Finland conducted a study within the Finnish Prenatal Study of Autism, a national birth cohort study based on a nested case-control design (Brown et al., 2018). The national registry was then used to collect autism cases of people born between 1987 to 2005 and each respective child’s mother’s maternal serum sample from early pregnancy (maternal serum samples are from maternal serum screening in which a blood test is used to identify pregnancies with an increased chance of having a chromosome condition such as Down syndrome) and then analyzed for p,p’-DDE, and total PCBs then compared with pregnancies of children who developed versus children who did not develop autism. Autism among children of mothers with high levels of insecticides significantly increased for maternal p,p’-DDE levels in the highest 75th percentile. These findings are the first biomarker-based evidence- biological molecules (usually found in blood, other body fluids, or tissues) that signify a normal or abnormal process or condition or disease- that maternal exposure to an insecticide is associated with Autism among offspring. This is a perturbing discovery because PFAS international production and importation to the US still continues, and knowing that they will not break down for a long time they are serious hazards to human health.
According to the National Institute of Health, researchers are also studying the possible links of contaminants such as Bisphenol A, phthalates, flame retardants, and polychlorinated biphenyls to Autism.
Conclusion
Autism is still a mysterious disorder and researchers are optimistic that more significant findings will be made to truly understand Autism and its development. Studies seeking to find correlations between environmental links and Autism are actively being conducted with definite results than ever before, and with the aid and development of technology, more solutions will be revealed especially in regards to the environment’s effect on Autism.
Discussion Questions:
Why does residential proximity to certain areas result in a higher correlation for the risk of autism? Why are some areas more polluted than others?
Pollutants in air pollution matter have the potential to lead to what other types of disorders?
Works Cited
Arora, M., Reichenberg, A., Willfors, C., Austin, C., Gennings, C., Berggren, S., Lichtenstein, P., Anckarsäter, H., Tammimies, K., & Bölte, S. (2017). Fetal and postnatal metal dysregulation in autism. Nature communications, 8, 15493. https://doi.org/10.1038/ncomms15493
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Lin, C.-K., Chang, Y.-T., Lee, F.-S., Chen, S.-T., & Christiani, D. (2021). Association between exposure to ambient particulate matters and risks of autism spectrum disorder in children: A systematic review and exposure-response meta-analysis. Environmental Research Letters, 16(6), 063003. https://doi.org/10.1088/1748-9326/abfcf7
Sahu, M., & Prasuna, J. G. (2016). Twin Studies: A Unique Epidemiological Tool. Indian journal of community medicine : official publication of Indian Association of Preventive & Social Medicine, 41(3), 177–182. https://doi.org/10.4103/0970-0218.183593
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