The Centers for Disease Control and Prevention reported in March that autism was, as of 2010, diagnosed in one out of every 68 children. The figures represent a steep rise in diagnosis: just 10 years earlier, the prevalence of autism was one in 150 children. Experts are divided as to the cause of the rise; at least one clinical psychologist, Enrico Gnaulati, argues that some slow-to-mature toddlers may be misdiagnosed as autistic.
But environmental factors are thought to be playing an important role in the development of the condition. Two recent studies now link the development of autism to air pollution or to prenatal and early-childhood exposure to agricultural pesticides.
In the air pollution study, scientists at the University of Rochester in New York exposed mice to ultrafine particulate matter air pollution—microscopic particles less than 100 nanometers in diameter, about 40 times as wide as a strand of human DNA—and discovered that the animals’ brains suffered damage analogous to that found in humans with autism and schizophrenia.
Lead researcher Deborah A. Cory-Slechta and colleagues exposed mice within the first two weeks of life—a period of rapid brain growth—to ultrafine particles of air pollution at “levels typically found in American cities during rush hour,” according to a press release from the university. The animals breathed the polluted air for four hours each day during two four-day periods; 24 hours after the last exposure, the scientists killed the mice and dissected the brains. Extensive inflammation was found, and two fluid-filled chambers on each side of the brains “were enlarged two to three times their normal size.” Inflammation, they believe, had injured brain cells surrounding the chambers, which then expanded to fill the space. Similar changes—which occurred mainly in male mice, as with autism in humans—were detected in two additional groups of mice whose brains were examined 40 days and 270 days after exposure, indicating that the damage was permanent.
The mouse experiments, published in June in Environmental Health Perspectives, build on earlier research in humans linking air pollution to autism, and also posit a mechanism for how such pollution may affect the developing brain. Last year, a JAMA Psychiatry study of more than 500 children in California found that those with autism were more likely to live in areas with the greatest exposure to traffic-related air pollution, both in utero and in the first year of life. It isn’t the only such study. “Over the past several years,” Cory-Slechta writes to me via email, “there have now been some six epidemiological studies that have reported associations between air pollution and autism, and two studies that have reported associations between air pollution and schizophrenia.”
Unfortunately, while the U.S. Environmental Protection Agency does regulate “coarse” and “fine” particles such as soot, dust and heavy metals—leading to reductions in air pollution—the same is not true for ultrafine particulate air pollution, even though, as Cory-Slechta explains, such particles are believed to be “the most dangerous [because] they reach the deepest recesses of the lung and from there can access the bloodstream.”
That is a major cause for concern. “It has become increasingly clear over time that autism/autism spectrum disorder is an example of a complex disorder, i.e., one that likely represents the interaction of multiple risk factors that converge,” Cory-Slechta says. “We know for example, that genetic heritability can be high, and that many gene mutations can be shown to play a role in autism, but importantly, genetics do not fully explain the disease and certainly not the increase in diagnosis that has occurred over recent years.
“This has raised the spectrum of environmental risk factors, including chemical exposures as potential influences. Air pollution may be one such influence.”
Next week: autism and pesticides.