While scientists are still continually learning what causes autism, there's already considerable evidence linking environmental exposures to the condition, in addition to genetic factors.
In recent years, a number of environmental risk factors have been identified, and while scientists have yet to demonstrate that any specific agents cause the condition, it's imperative we keep learning more about the potentially toxic substances that seem to be linked to the disorder.
In this context, one challenge is that many studies measure chemical exposures by testing blood or urine samples, both of which only provide a short-term window – sometimes just hours – to identify chemicals in the sample before their signature vanishes.
In a new study published in Neurotoxicology and Teratology, scientists found a way around that: analyzing children's primary or 'baby' teeth as a long-term record of chemical exposures that build up as teeth form, which can preserve detectable chemical traces for decades.
"Teeth capture cumulative exposures over long periods of time, whereas blood or urine samples only show a specific window of time," says environmental scientist and first author Lynne Heilbrun from the University of Texas at San Antonio. "We need both."
In their pilot study, the researchers analyzed shed baby teeth donated by the families of 38 children, 22 of whom had autism, with the other 16 serving as controls.
Children's teeth, which are shed between approximately six to 12 years of age, primarily record chemical exposures from the fourth fetal month – when primary teeth start forming while in the uterus – up until about when a child turns three, at which point tooth formation completes.
"There are several critical windows of exposure during pregnancy, especially in the first trimester, but it doesn't stop there," Heilbrun says. "The central nervous system continues to develop after birth until around age 25."
After cleaning and preparing the donated teeth, the researchers analyzed the teeth using two-dimensional gas chromatography and time-of-flight mass spectrometry, looking for signs of chemical compounds used in pesticides, plastics, fragrances, flame retardants, and sunscreen products, among others.
On average, the results showed over 300 toxicants in each tooth sample, but not every toxic substance was found in each child.
Focusing their analysis on 60 compounds that were present in at least half of the children, the researchers found roughly two-thirds (39) were known substances classified by the EPA as developmental or reproductive toxicants or endocrine-disrupting compounds.
Of those 60 compounds, the vast majority (95 percent) were found in higher concentrations in the teeth from children with autism, although only 19 of the differences detected ultimately passed statistical significance.
Some of the differences observed were dramatic, however, with one chemical – benzyl benzoate, which is used in personal care products, as a food additive, and in lice and scabies treatments – appearing in the autism group at 722 percent higher concentration than in controls.
"Compounds significantly higher in cases included OPEs, benzoic acids and esters, phenylpropanes, phthalates, salicylates, ethers, aromatic ketones, and caprolactam," the researchers explain.
"Remarkably, we detected 11 compounds in deciduous teeth that have been previously reported to migrate from baby bottle components… These toxicants, along with other chemicals migrating from numerous baby products, food contact materials, PEX tubing, and indoor furnishings, were among the most frequently detected compounds in cases and controls."
The findings don't demonstrate that any of these chemicals are causing autism, and the researchers acknowledge that this is only a small pilot study.
Nonetheless, the evidence here is concerning, with the researchers noting that many of the toxicants they identified target the brain, liver, and mitochondria.
"Our study suggests that children are being chronically and repeatedly exposed to a wider range of developmental toxicants than previously identified in short-term exposure studies," the researchers write.
"Toxicants leaching from baby bottles, pacifiers, food packaging, and PEX water tubing expose infants through food preparation and daily use of plastic products. Given that pregnant and lactating women increase food and water intake, maternal exposure is likely substantial."
The authors say future studies are warranted to investigate further – including more research using tooth samples, to help us understand more about how these chemicals might be shaping children's autism risk.
"Precautionary avoidance of these chemicals, especially among pregnant women and young children, should be a public health priority," the researchers conclude.
The findings are reported in Neurotoxicology and Teratology.
This article was fact-checked by Rachel Garner and edited by Rebecca Dyer. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.