How do the increased incidences of autism in America track with exposure levels to suspected environmental toxins?

The question yielded surprising answers in a study recently published by respected open access publisher, BioMed Central, entitled "A Comparison of Temporal Trends in United States Autism Prevalence to Trends in Suspected Environmental Factors." The conclusion of the study, conducted by Cynthia D. Nevison, an atmospheric and environmental research scientist at the University of Colorado, Boulder, is that, while most of the suspected environmental toxins examined have flat or decreaing temporal trends that correlated poorly to the rise in autism, cumulative aluminum adjuvants and cumulative total immunizations are among the few factors that reveal the opposite.

Nevison notes that a recent comprehensive study of autism spectrum disorder rates among twins supports the dominant influence of environmental factors and the importance of genetic susceptibility.  Since genes aloe do not mutate rapidly, she claims, trends in environmental toxins are relevant and can provide valuable clues into the causes of autism. 

In her study, Nevison compared data on autism rates for birth years 1970–2005 derived from the California Department of Developmental Services (CDDS) and the United States Individuals with Disabilities Education Act (IDEA) with data on trends in suspected toxins derived from extensive literature survey. 

I’ve compiled this chart to illustrate the findings of how the rise in autism correlates to the specific toxins/factors:

The reasons for the downward trends are varied and fascinating, ranging from those toxins that have been banned (e.g., lead) to those that have been replaced by newer technology (e.g., organochlorine pesticide reduction due to the advent of genetically modified crops).  The following are excerpts from the study:

Austism and Mercury

Postnatal Thimerosal therefore seems unlikely to be driving the ongoing increase in autism in the 2000s, although a recent reported decrease in the severity of ASD among younger birth cohorts may coincide with the Thimerosal phaseout.

A possible confounding factor in the postnatal Thimerosal analysis is the administration of flu shots to pregnant women, which increased in the late 1990s/early 2000s around the same time that Thimerosal was being phased out of children’s vaccines. Many flu shots still contain 25 μg Hg and thus may be leading to increased prenatal exposure.  

Autism and Aluminum

Other vaccine indices, including cumulative aluminum adjuvants and cumulative total number of immunizations, continue to correlate strongly with autism trends. Aluminum is a demonstrated neurotoxin that can induce neuroimmune disorders and cellular oxidative stress. Several recent studies have described biological mechanisms by which aluminum could contribute to autism and have emphasized the need to consider the interaction of aluminum and vaccines with other pharmaceuticals, including antibiotics and the antipyretic acetaminophen. The upward trend in aluminum adjuvant exposure is also notable in that very young infants have experienced the largest relative increases from the early 1980s to 2005. Newborns have seen essentially an infinite increase due to the hepatitis B birth dose, the receipt of which has been linked epidemiologically to increased autism risk, while 2 month-olds have seen about a 3-fold increase in aluminum adjuvant exposure.

Other Autoimmune Disorders

It is also possible that the drivers of the temporal trend in autism are tied into the factors responsible for the rise in other autoimmune or hyperimmune system diseases such as asthma, Crohn’s disease, lupus, and type 1 diabetes. All of these diseases have increased in recent decades in the U.S as well as in many other countries.

I agree strongly with Nevison that the correspondence between trends in autism and environmental factors is a useful method for identifying possible triggers of autism to help focus future research. And while she emphasizes that the correlation between autism and PBDEs, cumulative aluminum adjuvants, and glyphosate is not proof of causation (given the ecological nature of this study, in which the exposure data were aggregated at the group level), I feel the results of her survey necessitate an urgent need for America’s public and private research communities to dedicate more resources to studying these triggers to halt the alarming rise in autism worldwide.