Autism has a strong genetic component, and, even in cases that aren't inherited, newly formed mutations appear to be common. But genetics isn't everything, raising questions about exactly what the environmental influences are. A variety of factors have been suggested, ranging from immune disorders to environmental toxins.

One of the factors that has been suggested is pesticide use. These chemicals, which typically target the nervous system of insects, can also be toxic to humans. Exposure has been associated with some disorders of the nervous system, most notably Parkinson's disease. Now, a study is out suggesting that autism is also associated with pesticide exposure. But the study has some pretty significant limitations, so it shouldn't be viewed as anything more than a reason to work on a more thorough study.

The study relies on a group of infants termed the Childhood Autism Risks from Genetics and Environment (CHARGE) study population. These children are recruited when they register with the state social services; matched controls are recruited at the same time.

The ability to track pesticide use comes from California environmental regulations, which require that any commercial pesticide use, along with its location, be registered with the state. That location can then be compared to the residence of the parents of children in the CHARGE study. This provides a very rough indication of potential pesticide exposure.

The authors of the paper, who are all based in California, managed to get data for 970 children, which seems like a reasonable number. But a sizable majority of them (70 percent) had no exposure. The remainder were then split into individual groups based on their proximity to the pesticides being used, with the distances set at 1.25km, 1.5km, and 1.75km. (The reasoning behind using these distances isn't made clear in the text.) Then, the groups were further divided based on which of five classes of pesticides were used in the area.

There are two consequences to all this dividing. One is that the populations get very small. For three of the pesticides, the total number of autistic individuals who might have been exposed at any distance was in the area of 50 to 60 people.

The other problem is that you end up with a lot of tests. The authors look at exposure during the period immediately before conception, the entire pregnancy, and in each of the three trimesters. That's five conditions, multiplied by three distances to give 15 individual tests for each pesticide. They also look at four pesticides for a total of 60 tests. There are actually 63, since a fifth pesticide was only tested with the entire pregnancy, as the number of individuals exposed was too small. Then, they repeat the entire process for developmental delay.

That's 126 different tests. Which means the odds of a spurious positive result are rather high.

In the end, most of these tests come out negative. There are some very weak positive associations with organophosphates late in pregnancy. There's a somewhat stronger result with Chlorpyrifos, but the association actually increases as the distance to the site of use goes up. And then there are a couple of barely significant results scattered through the remainder of the tests. Results with developmental delay appear similar in that the few significant ones are weak and don't display any particular pattern.

The authors conclude that their work "strengthens the evidence linking neurodevelopmental disorders with gestational pesticide exposures." But really, the study mostly indicates that a link can't entirely be ruled out. A larger, more focused study might be able to determine if any of these results are more than a statistical fluctuation.

Environmental Health Perspectives, 2014. DOI: 10.1289/ehp.1307044 (About DOIs).