Marina Sarris

A version of this article first appeared in iancommunity.org.

Since the autism rate began rising in the 1990s, parents and scientists have wondered about the role of environmental factors in triggering autism spectrum disorder (ASD). In the medical world, the environment can mean anything from a pregnant woman’s age to infections she’s had, the chemicals she uses at work, or the air she and her newborn breathe.

Most autism cases — about 90 percent, according to some estimates — have no known medical cause.^1,2,3 Is a mix of genetic and environmental factors at work? If so, what are those factors and how much does each contribute?

These questions have taken on more urgency in recent years since studies involving twins suggested that unidentified environmental factors play a more important role in autism than was previously believed.^4,5 Researchers discovered that fraternal twins, who shared the same environment before birth but have different DNA, were more likely to both have ASD than siblings who weren’t twins.

INVESTIGATING POLLUTANTS AND AUTISM

Some researchers have zeroed in on exposure to air pollution at home and hazardous chemicals on the job. Several studies found “associations” between pollution or chemical exposure and autism. In other words, children exposed to higher levels of some pollutants or chemicals before and/or after birth were more likely to develop autism.

Such research is in its infancy, and scientists caution that it’s too early to say that pollution or chemicals cause autism just because they occur together. Some recent environmental studies are exploratory in nature: scientists are looking for clues that warrant a more intensive look in future studies.

Certain pollutants have captured researchers’ attention because they are known to affect the human nervous system and development and to affect a fetus.

“Looking at the association between air pollution and neurodevelopmental disorders is a relatively new field,” explains researcher Heather E. Volk, assistant professor in the Department of Mental Health at the Johns Hopkins Bloomberg School of Public Health.

In a study published in 2013, while an assistant professor at the University of Southern California, Volk and her team examined the air pollution exposure of 279 children with autism and 245 children with typical development, called controls, in California. The scientists looked at the locations of homes their mothers occupied during pregnancy and the first year of their children’s lives. They estimated the amount of air pollution at those addresses, factoring in the homes’ proximity to freeways, traffic volume and weather conditions. They also considered the region’s air quality as measured by the U.S. Environmental Protection Agency (EPA).⁶

The results? The children with autism were three times more likely than the typical children to have been exposed to relatively high levels of traffic and regional air pollution in their first year of life. Researchers found a moderate relationship between autism and air pollution even when they took into account social, income and other differences between the families they studied.

Volk explains that diesel exhaust particles and other air pollutants can cause inflammation and “oxidative stress,” two conditions that may also be involved in the development of autism. Like most scientific studies, this one acknowledges that other factors could have contributed to autism, such as indoor air pollution, lifestyle or nutrition.⁶

Another study released in 2013 also found that exposure to air pollutants around the time of birth “may increase risk for ASD.”⁷ This research team, led by Andrea L. Roberts of the Harvard School of Public Health, looked at 325 nurses who had a child with ASD and 22,101 nurses who had a child without ASD. The nurses lived in different parts of the United States. The women who lived in areas with the highest levels of air pollution — diesel particulates or metals — had twice the risk of having a child with ASD as women who lived in areas with the least air pollution.

ANOTHER LOOK AT AIR QUALITY

Those 2013 studies appear to support a 2006 study that found a “potential association” between autism and hazardous air pollution outside the homes where mothers lived at the time of their children’s births.

That study looked at 284 children with ASD and 657 typically developing children born in 1994 in the San Francisco Bay area. Researchers estimated their exposure to hazardous air pollutants using a 1996 EPA database. Cars, trucks, planes, trains, dry cleaners, gas stations and manufacturing plants can all pump hazardous chemicals into the air.⁸

Researchers found a “moderately” higher risk of autism among children born in areas with higher estimated air levels of diesel exhaust particles, metals such as mercury, cadmium and nickel, and chlorinated solvents.⁸

Even rural areas far from superhighways may pose risks. A 2007 study in California’s Central Valley, and a 2014 study in northern California, found that pregnant women living closer to farms treated with pesticides had a higher risk of delivering a child who would be diagnosed with ASD.^9,12

Other studies in North Carolina and West Virginia found associations between autism and air pollution in those states.^6,10

CHEMICALS AT WORK

Whereas those studies looked at pollution where pregnant women lived, a 2013 study examined possible chemical exposures where mothers-to-be worked. “Typically, occupational exposures tend to be higher than [general] environmental exposures,” explains Gayle C. Windham, a reproductive epidemiologist with the California Department of Public Health who worked on the study.

Windham says she became interested in parents’ occupations when she conducted earlier research on the theory that fathers of children with ASD are relatively likely to be engineers. (They aren’t, according to her study.)

In her latest study, her team used California birth certificate information to determine the parents’ occupations at the time of a child’s birth. The team wondered if prenatal exposure to chemicals could affect a child’s development in a way that might trigger autism.¹¹

The researchers discovered that the mothers of children with autism were twice as likely to have worked in jobs that exposed them to chemicals as the mothers of children without autism. That was particularly true of jobs that brought workers into contact with exhaust and disinfectants.

The exploratory study, which involved 284 children with autism and 659 controls, did not determine the mothers’ actual occupational exposures. Instead, the researchers looked at whether they had jobs that were likely to involve certain chemicals, as determined by a doctor certified in occupational medicine.

For example, bus drivers, flight attendants, mechanics and firefighters were presumed to be exposed to exhaust and combustion fumes. Medical and dental workers were presumed to be exposed to disinfectants. The job exposures of fathers did not seem to affect the likelihood of having a child with ASD.¹¹

The researchers said other factors could have contributed to the results. They called for more studies to delve deeper into the subject. In fact, Windham says she is already working on a new study that will look at both genetic and environmental factors in the development of autism.

Volk says she would like to see a different kind of environmental study: one that is “prospective,” or forward-looking.

Many studies begin with children who are already diagnosed with autism and then look back to determine or estimate their exposures, she explains. In a prospective pollution study, however, researchers would monitor air quality at the homes of women while they are pregnant and later check to see if their babies have developed autism. That might shed more light on the issue, she says.

REFERENCES: 

1. Fombonne, E. (2003). Epidemiological surveys of autism and other pervasive developmental disorders: an update. Journal of autism and developmental disorders, 33(4), 365-382. View abstract.
2. Rosenberg, R.E., Law, J.K., et al. (2009). Characteristics and concordance of autism spectrum disorders among 277 twin pairs. Archives of pediatrics and adolescent medicine, 163(10), 907-914. View abstract.
3. Newschaffer, C.J, Croen, L.A., et al. (2007). The epidemiology of autism spectrum disorders.  Annual review of public health, 28, 235-258. View abstract.
4. Hallmayer, J., Cleveland, S., et al. (2011). Genetic heritability and shared environmental factors among twin pairs with autism. Archives of general psychiatry, 68(11), 1095-1102. View
5. Taniai, H., Nishiyama, T., et al. (2008). Genetic influences on the broad spectrum of autism: study of proband-ascertained twins. American journal of medical genetics part B 147B, 844–849. View abstract.
6. Volk, H.E., Lurmann, F., et al. (2013). Traffic-related air pollution, particulate matter, and autism. JAMA Psychiatry, 70(1), 71-77. View abstract.
7. Roberts, A.L, Lyall, K., et al. (2013). Perinatal air pollutant exposures and autism spectrum disorder in the children of Nurses’ Health Study II participants. Environmental health perspectives, DOI:10.1289/ehp.1206187. View article.
8. Windham, G.C., Zhang, L., et al. (2006). Autism spectrum disorders in relation to distribution of hazardous air pollutants in the San Francisco Bay area. Environmental health perspectives, 114(9), 1438-1444. View abstract.
9. Roberts, E.M., English, P.B., et al. (2007). Maternal residence near agricultural pesticide applications and autism spectrum disorders among children in the California Central Valley. Environmental health perspectives. 115(10), 1482-1489. View abstract.
10. Kalkbrenner, A.E., Daniels, J.L., et al. (2010). Perinatal exposure to hazardous air pollutants and autism spectrum disorders at age 8. Epidemiology, 21(5), 631-641. View abstract.
11. Windham, G.C., Sumner, A., et al. (2013). Use of birth certificates to examine maternal occupational exposures and autism spectrum disorders in offspring. Autism research, 6(1), 57-63. View abstract.
12. Shelton, J.F., Geraghty, E.M., et al. (2014). Neurodevelopmental disorders and prenatal residential proximity to agricultural pesticides: the CHARGE study. Environmental health perspectives, DOI:10.1289/ehp.1307044. View article.