Do Environmental Toxins Cause Autism? The Evidence Reviewed

Introduction: Kristof's Argument on Toxins and Autism

Nicholas Kristof's op-ed addresses the issue of environmental toxins and autism, specifically the link between exposure to these toxins and the rise in autism spectrum disorders. Autism comprises a clinically heterogeneous group of disorders — collectively referred to as "autism spectrum disorders" (ASD) — that share common features of impaired social relationships, impaired language and communication, and repetitive behaviors or a narrow range of interests. Autism has a current prevalence of approximately 1% in children.

Kristof explores the relationship between toxins and autism by referring to expert opinion — specifically, public health officials who have found epidemiological links between exposure to harmful substances such as mercury, lead, and thalidomide and neurodevelopmental disorders. Kristof cites studies in which researchers "have found that disproportionate shares of children develop autism after they are exposed in the womb to medications such as thalidomide (a sedative), misoprostol (ulcer medicine) and valproic acid (anticonvulsant)."

Kristof's Rhetorical Strategy and Claims

Kristof calls for more research, increased regulation, and consumer protection. Noting that the EPA only regulates 200 of the 80,000 chemicals registered in the United States, and that the use of dangerous plastic containers is widespread, Kristof argues that more attention needs to be directed toward environmental toxins and their health outcomes for diseases such as cancer and autism. While lamenting the decline in vaccinations resulting from national attention focused on the now-discredited link between vaccines and autism, Kristof declares the precautionary principle applicable in the case of toxins and autism, and suggests that heeding this issue would serve Americans well.

Kristof convincingly argues that toxins and autism may be linked in ways that deserve more attention from the press, from public health officials and scientists, and from the government. His rhetorical strategies are evident in the way he shifts from expert pronouncements to political messaging to editorializing. His readers — presumably environmental health officials, politicians, and the general public — would likely respond to his warnings and possibly take action to reduce environmental toxin exposure.

However, a review of the scientific literature reveals a more complicated picture. The jury is still out regarding the issue of heavy metal toxin exposure and the prevalence of autism. Most articles published since Kristof's editorial have been considerably more cautious about establishing an alleged link between toxins and autism. While many researchers do see at least a correlative association, others maintain that the scientific evidence is inconclusive and that much more research is needed.

Scientific Literature on Heavy Metals and Autism

The question of autism occurrence and toxin exposure was raised in early studies including articles by Soden et al. (2007), Barbaresi et al. (2009), and Thompson et al. (2007). In "Sorting out the spinning of autism: heavy metals and the question of incidence," Mary Catherine DeSoto and Robert T. Hitlan of the Department of Psychology at the University of Northern Iowa state that the reasons for the rise in autism prevalence are a subject of heated professional debate. Featuring a critical appraisal of research used to question whether there is a genuine rise in cases — and whether rising levels of autism are related to environmental toxin exposure — they aimed to evaluate the actual state of scientific knowledge. They also surveyed the empirical research on autism and heavy metal toxins and concluded that the causes of increased autism diagnosis are likely multi-faceted, making this one of the most important health topics today.

DeSoto and Hitlan argue that scientific research does not support rejecting the link between autism and toxic exposures. They analyzed data from articles that have been, or might be, taken to support the view that there is no real increase in autism or no environmental connection.

Of the empirical, case-control studies comparing individuals with and without ASD on measures of actual mercury levels in the body that purportedly found no link, only three existed at the time of their review. Two of those data sets actually show that those with an ASD appear to have higher levels of metals — contrary to the original authors' stated conclusions. According to DeSoto and Hitlan, a very recent study reported in 2010 offered evidence against an association without any major methodological flaws.

On the other hand, numerous scientists have investigated individuals with autism, compared measured levels of heavy metals, and concluded that a meaningful difference does exist. Other researchers have used indices of heavy metal exposure such as the zinc/copper ratio, urinary porphyrins, or measured toxins combined with genetic expression data. To summarize: of 58 empirical reports on autism and heavy metal toxins, 43 suggest some link may be present, while 13 found no link. Even accounting for the tendency of null results to go unreported, it cannot be said that there is no evidence for a link between heavy metal toxins and autism. Although the question remains open, the overall evidence favors a link (DeSoto and Hitlan, 2010).

Conclusion: The Jury Is Still Out

The research is ambiguous, making Kristof's opinions on the causative link between toxins and autism appear more controversial than certain. No one would take issue with Kristof that environmental toxins are harmful, but the question is whether exposure to them in developing children and pregnant women is contributing to the increased prevalence of autism. Kristof seems to take the claims of the studies he cites at face value, without rigorously scrutinizing their results or methodology. He does not account for improvements in diagnostic procedures, the role of genetics, or other factors — such as reduced breastfeeding rates and viral infections — that may contribute to the increased prevalence of autism.

Even the Current Opinion in Pediatrics piece raises caveats concerning its explorations of the causes of autism, with Landrigan characterizing the evidence as "indirect." Landrigan called for "expanded toxicological screening, neurobiological research and prospective epidemiological studies." These are all laudable goals, but whether they will yield conclusive answers remains uncertain.

Works Cited

Barbaresi, W.J., Colligan, R.C., Weaver, A.L., and S.K. Katusic. "The incidence of clinically diagnosed vs. research identified autism in Olmsted county, Minnesota, 1976–1997: results from a retrospective, population-based study." J. Autism Dev Disord 39 (2009): 464–470.

DeSoto, Mary Catherine and Robert T. Hitlan. "Sorting out the spinning of autism: heavy metals and the question of incidence." Acta Neurobiologiae Experimentalis 70 (2010): 165–176.

Heron, J., Golding, J., and ALSPAC Study Team. "Thimerosal exposure in infants and developmental disorders: a prospective cohort study in the United Kingdom does not support a causal association." Pediatrics 114 (2004): 577–583.

Kristof, Nicholas D. "Do Toxins Cause Autism?" New York Times 24 February 2010.

Landrigan, Philip J. "What causes autism? Exploring the environmental contribution." Current Opinion in Pediatrics 22 (2010): 219–225.

Schultz, Stephen T. "Does thimerosal or other mercury exposure increase the risk for autism? A review of current literature." Acta Neurobiologiae Experimentalis 70 (2010): 187–195.

Soden, S., Lowrey, J., Garrison, C., and G. Wasserman. "24-hour provoked urine excretion test for heavy metals in children with autism and typically developing controls, a pilot study." Clinical Toxicology 45 (2007): 476–481.

Thompson, W.W., Price, C., Goodson, B., Shay, D.K., Benson, P., Hinrichsen, V.L., Lewis, E., Eriksen, E., Ray, P., Marcy, S.M., Dunn, J., Jackson, L.A., Lieu, T.A., Black, S., Stewart, G., Weintraub, E.S., Davis, R.L., DeStefano, F.; Vaccine Safety Datalink Team. "Early thimerosal exposure and neuropsychological outcomes at 7 to 10 years." New England Journal of Medicine 357 (2007): 1281–1292.