Evidence Mounting For Environmental Links to Soaring Autism Rates

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There is no clear explanation for nor understanding of the apparent increase in the incidence of autism. A Harvard study suggests that air pollutants such as mercury and diesel fumes play a major role.

It’s no secret that incidence rates for autism have increased dramatically over the past several decades. And the reasons for this increase have been the subject of considerable scientific debate for some time now. Some researchers argue that much of the seemingly recent explosion of occurrence rates is actually the result of greater professional awareness of the broad spectrum of developmental delays, including the various and less dramatic forms of autism, and the enhanced ability of physicians and various mental health professionals to detect and diagnose these developmental delays in early childhood. What we’re seeing, these researchers suggest, is not so much an actual increase in occurrence of autistic spectrum disorders for some unknown reasons, but rather, a more frequent and accurate diagnosis of conditions that, for too long, either went undetected or were improperly labeled as some other condition. But others in the scientific community argue that even increased awareness and better detection ability can’t explain the phenomenal rise in rates witnessed in recent years, and suspicions have long abounded that environmental factors as yet undetermined might be responsible for the increased incidence of autism. And for some time, one of the prime environmental suspects has been mercury. Furthermore, because many childhood vaccines, especially the multiple vaccines, were for a long time based in solutions containing mercury, an outcry eventually erupted against the use of vaccines. But many rigorous studies have failed to find a direct connection between childhood immunizations and autism, even when they were packed in solutions containing mercury. Moreover, most vaccinations are no longer in solutions containing mercury. Still, many studies continue to point the finger at environmental factors as likely responsible — at least in part — for the increase in autism occurrence, and some of the research evidence is again pointing toward one particular environmental pollutant: mercury.

For some time now there have been indications that baby boys who were conceived, carried, and born in regions of the US with higher than average air pollution are at increased risk for autism. And just recently, a research team from the Harvard University School of Public health published results of a broad-based study that built upon the findings of smaller regional studies (see Scientific American article, US Kids Born in Polluted Areas More Likely to Have Autism, and NIH Environmental Health Perspectives article, Perinatal Air Pollutant Exposures and Autism Spectrum Disorder in the Children of Nurses’ Health Study II Participants). They found that women who live in locales with high concentrations of air pollutants such as mercury, diesel exhaust, manganese, nickel and lead were twice as likely to give birth to autistic babies as women who live in less polluted areas. Unlike the results of the other, smaller regional studies, the nation-wide survey conducted by the Harvard team found that babies of either sex born in polluted areas were at higher risk for autistic spectrum disorder (ASD), although baby boys were at even higher risk than baby girls. Of all the pollutants thought to be factors, the two prime candidates appeared to be mercury and diesel exhaust.

As encompassing as the study is, the recent Harvard study is not without its weaknesses. The methods by which the researchers estimated the mothers’ degree of exposure to the various air pollutants is a weakness cited not only by critics of the study, but also by the authors themselves. Still, for all its weaknesses, the study provides one more bit of damning evidence about the role environmental factors are likely playing in the dramatic rise we’ve been witnessing in autism rates.

Autism is a hot-button topic, to be sure, and will likely continue to be for some time to come. To a great extent, that’s because the still largely unexplained explosion in incidence rates (now estimated to be as high as 1 child in 50, in many areas) has placed tremendous demands on the country’s already strained educational and health resources. Couples hoping or planning for children increasingly live in fear they will face the prospect not only of caring for a special needs child, but also the truly daunting task of securing adequate support resources to adequately deal with those special needs. So, interest in autism and the possible reasons for its apparent increase in occurrence could not be greater. Fortunately, many organizations have risen to the occasion (e.g., Autism Speaks), providing much needed support and information, and energizing the drive to find some answers. And there is a separate section of the Mental Health Reference site that contains some of the latest, best information on the topic. (I’ve also written some articles on the topic — see, for example, “Autism Rates Still on the Rise”.) But despite all the heightened interest in the topic and all the research currently being done, it is proving to be a more challenging endeavor than anyone thought to pinpoint the reasons why so many children are now falling somewhere on the autistic spectrum. The Harvard study is once again pointing us in the direction of the environment, especially to certain chemicals in the environment. But there is much research that still needs to be done. Perhaps studies that take a closer and well-controlled look at how these chemical agents can impact normal human neurobiological development will help us unravel the mystery.

All clinical material on this site is peer reviewed by one or more clinical psychologists or other qualified mental health professionals. This specific article was originally published by on and was last reviewed or updated by Dr Greg Mulhauser, Managing Editor on .

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