A class of compounds called organochlorines were banned from production in the United States in the 1970s, after they emerged as harmful to human health. A new study, however, reveals that exposure to these chemicals could still be high, and it could be putting children at significantly greater risk for autism.
The research was recently published in the journal Environmental Health Perspectives.
Organochlorines are chemicals - including dichlorodiphenyltrichloroethane (DDT) and polychlorinated biphenyls (PCBs) - that were widely used as pesticides, lubricants, and insulators in the U.S. between the 1950s and 1970s.
Use of DDT in the U.S. was banned in 1973, while production of PCBs was halted in 1977, after evidence mounted that the chemicals can harm wildlife, the environment, and human health.
With regard to the latter, exposure to the chemicals in pregnancy has been linked with impaired neurodevelopment in offspring, as well as abnormal thyroid hormone levels in both mother and child, which can impact fetal development.
While organochlorine use ceased almost 40 years ago in the U.S., the chemicals can persist in the environment for decades; human's primary source of exposure to these chemicals is through diet - mainly consumption of meat and fish - as the compounds accumulate in soil, lake sediment, and animal fat.
80 percent greater autism risk with high prenatal exposure to PCBs
For the new study, co-author Kristen Lyall, assistant professor at the A.J. Drexel Autism Institute at Drexel University in Philadelphia, PA, and colleagues set out to investigate whether prenatal exposure to organochlorines in the U.S. could be linked to autism development - a possible association that they say has received little attention.
The team drew data from the Expanded Alphafetoprotein Prenatal Screening Program, involving 1,144 children who were born in Southern California between 2000-2003 and their mothers.
- Around 1 in 68 children in the U.S. have autism
- Between 2002-2010, autism prevalence in the U.S. increased by 6-15 percent each year
- Autism services cost the U.S. around $236-262 billion every year.
Of these children, 545 were diagnosed with autism, 181 had intellectual disabilities, but no diagnosis of autism, while 418 were free of autism and intellectual disabilities.
Children's prenatal exposure to PCBs and other organochlorine pesticides (OCPs) - including DDT - was determined through blood samples taken from the mother during the second trimester of pregnancy.
Compared with children with the lowest prenatal exposure to PCBs, those with the highest prenatal exposure to the chemicals were found to be at 80 percent greater risk of autism.
"Adverse effects are related to levels of exposure, not just presence or absence of detectable levels," notes Lyall. "In our Southern California study population, we found evidence for modestly increased risk for individuals in the highest 25th percentile of exposure to some of these chemicals."
The team identified two compounds - PCB 138/158 and PCB 153 - that appeared to have the strongest link with autism; children with the highest prenatal exposure to these PCBs were found to be at 79-82 percent greater risk of autism, compared with those with the lowest prenatal exposure.
A further two PCBs - PCB 170 and PCB 180 - were associated with a 50 percent increased risk of autism among children with the highest prenatal exposure, the team reports.
Higher prenatal exposure to PCBs, OCPs linked to intellectual disabilities
When it came to intellectual disabilities, the team found high prenatal exposure to PCBs doubled the risk of such problems, compared with those who had low prenatal exposure.
While prenatal exposure to OCPs was not associated with increased risk of autism, the researchers did identify a link between mid-range - rather than high - prenatal exposure to the compounds and increased risk of intellectual disabilities.
"The results suggest that prenatal exposure to these chemicals above a certain level may influence neurodevelopment in adverse ways," says Lyall.
Overall, the researchers say their findings indicate higher exposure to organochlorines in pregnancy may raise the risk of autism and intellectual disabilities for offspring:
"The overall pattern of our results suggests increases in risk of ASD [autism spectrum disorder] and ID [intellectual disability] with prenatal exposure to higher levels of a number of OCCs, and in particular, PCBs.
Future work should further consider genetic background in the role of these exposures on neurodevelopmental outcomes. Continued investigation of OCCs [organochlorine compounds] in association with ASD and ID is needed, given our findings and the dearth of studies investigating this topic."