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Largest-ever Study on Autism & Pollution Shows Strong Link During Pregnancy

Nurses’ Study shows autism rates double with exposure to high levels of fine-particulate air pollution during pregnancy; strongest link in third trimester
December 18, 2014

A new nationwide study found a doubled autism risk among children of women exposed to high levels of particulate air pollution during pregnancy.

The association was strongest when the exposure occurred during the third trimester. The greater the exposure, the greater the risk.

The researchers saw no increased autism risk if the pollution exposure occurred after birth or before conception.

The study, led by researchers at the Harvard School of Public Health, was funded in part by Autism Speaks. It appears online today in Environmental Health Perspectives.

“It’s important to remember that not all mothers exposed to air pollution during pregnancy will have a child with autism and not all children with autism were necessarily exposed to air pollution in utero,” comments epidemiologist Michael Rosanoff, Autism Speaks associate director for public health. (Rosanoff was not directly involved in the study.) “We know autism is a complex disorder and underlying genetic and biological factors interact to influence susceptibility,” he says. “The next step is to identify the biological mechanisms that connect air pollution to autism and identify ways to treat if not prevent the harm to brain development.”

Meanwhile, Rosanoff says, the findings suggest a need to revisit public health policies on pollution limits with an eye to reducing exposures, especially among pregnant women.

Bolstering earlier studies
Smaller studies have suggested that exposure to air pollution during pregnancy increases autism risk. However, these earlier studies were limited in scope – tracking pregnant women and their children in just a few communities.

The new investigation spanned all 50 states by tapping into the national Nurses’ Health Study II, which has 116,000 participants. The analysis looked at pollution exposures before, during and after the women’s pregnancies.

Why look at fine particulate matter?
Particulate matter is a mixture of airborne particles and liquid droplets. (See image above.) It comes in a range of sizes and can be composed of many materials and chemicals. The most worrisome are particles small enough to be inhaled. Fine particulate matter (smaller than 2.5 microns in diameter) is of special concern because it can penetrate deep into the lungs. Vehicle exhaust and other combustion byproducts are high in fine particulate matter. So the greatest exposures tend to occur near busy roadways.

The researchers explored the association between autism and exposure to particulate matter before, during and after pregnancy. They also calculated exposure during each pregnancy trimester. In all, the researchers were able to collect this type of complete exposure information for 160 women whose children developed autism. For comparison, they also looked at 1,000 participants whose children who did not develop autism. The two groups were similar in age, socioeconomic status and other factors - aside from pollution exposure - known to influence health risks.

The analysis found that children born to mothers exposed to the highest levels of fine particulate pollution during pregnancy (above 16.7 µg/m3) were twice as likely to develop autism than were children born to mothers exposed to the lowest levels (below 12.3 µg/m3). However, autism rates increased with exposure levels across the range.

The researchers found the most significant association with autism when the exposure occurred during the third trimester.

By contrast, they saw no association when exposure occurred after birth (early infancy) or before the woman conceived.

They also found little association with exposures to large particulate pollution (dust, mold, etc.).

“This not only gives us important insight as we continue to pursue the origins of autism spectrum disorders, but as a modifiable exposure, opens the door to thinking about possible preventative measures,” says senior author Marc Weisskopf.

Developmental pediatrician Paul Wang, Autism Speaks’ head of medical research, concludes:

“These results powerfully add to the accumulating evidence that air pollution is a significant risk factor for autism. In particular, they suggest that exposure during pregnancy, as opposed to early life, is most critical.”

Science hasn’t identified how pollution exerts its effects on the developing brain, Dr. Wang notes. "It may affect brain cells directly, or through pathways associated with inflammation."

Also see:

* More Research Links Autism to Prenatal Exposure to Air Pollution

* Air Pollution and Autism Risk

* Gene Change Plus Prenatal Exposure to Pollution Ups Autism Risk

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