Leading the current conversation about autism rates is Health and Human Services secretary Robert F. Kennedy Jr., who in a press conference on Wednesday brought even more urgency to the rise in diagnoses—claiming the surge is caused by “an environmental toxin.”

“Somebody made a profit by putting that environmental toxin into our air, our water, our medicines, our food, and it’s to their benefit to normalize it. But that’s not good for our country,” Kennedy said.

Autism spectrum disorder (ASD) is a developmental disability caused by differences in the brain, according to the CDC. And while some people with ASD have an identifiable difference—like a genetic condition—other causes are not yet known. 

Scientists believe multiple causes could act together, but more research needs to be done. A recent study out of China, for example, established a connection between gestational diabetes—diabetes during pregnancy, which affects up to 9% of U.S. pregnancies—and autism (as well as other brain and nervous system problems).

At Wednesday’s press conference, Kennedy announced that the department would be looking at all “potential culprits.” Kennedy, a known vaccine skeptic, founded the organization Children’s Health Defense—whose mission is to “end childhood health epidemics by eliminating toxic exposure”—after mothers of kids with ASD begged him to look into the connection between mercury (in water supplies and in vaccines, through the preservative thimerosal) and autism. 

“You have to ask yourself: Why is it so pervasive?” the HHS secretary said on Wednesday.

Below are the five “toxins” Kennedy announced he would be looking into—and what science says about each.

Mold

One of the possible culprits Kennedy identified was mold. And while two systematic reviews of past studies—one, from 2021, looking at 11 studies, and another, from 2018—found possible links between the mycotoxins released from some molds and the development of autism, a definitive causal effect has not been established.

“The information reviewed indicates that exposure to mold and mycotoxins can affect the nervous system, directly or through immune cell activation, thus contributing to neurodevelopmental disorders such as autism spectrum disorder,” the authors of the 2018 review wrote.

Food additives

Kennedy said that food additives would also be investigated, and thus far the connection between such additives and ASD has been unclear. One meta-analysis from 2020 found that food dyes could be linked to ASD, but could not definitively establish it as a cause. And a 2019 study from the University of Central Florida potentially linked ASD to mothers who consumed processed foods containing the preservative propionic acid (PPA), which may alter how a fetus’ brain develops during pregnancy.

Pesticides

As for pesticides, also on Kennedy’s list, a study from last year found that areas in southern Spain with increased pesticide usage had greater prevalence of ASD, suggesting a connection between heightened environmental pesticide exposure and autism. 

A 2023 systematic review suggested that more research needs to be done in this area, but that maternal age could be an important factor to consider, as the concentration of pesticides in the body increases with age, and so higher maternal ages are more strongly associated with an increased risk of autism in their children. 

The authors also suggested that pesticide exposure could work in tandem with other factors that may be harmful or beneficial for nervous system development, such as lifestyle, socioeconomic or educational status, ethnicity, or gender.

Medicines

While Kennedy did not explicitly refer to vaccines as one of the potential causes of autism, he did say they would look into “medicines” and their connection to autism. 

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He recently appointed a vaccine skeptic to reexamine the findings of the debunked Wakefield studies that linked vaccines—specifically the measles, mumps, and rubella (MMR) vaccine—to autism. But neither common medications taken during pregnancy nor vaccines, including the MMR shot, have been shown to be associated with ASD. 

One of the most renowned such studies is from Denmark, and looked at 537,303 children, 82% of which had received the MMR vaccine. It found that the risk of autism among vaccinated children was the same as that in unvaccinated children, and there was no association between the age at the time of vaccination, the time since vaccination, or the date of vaccination and the development of autism.

Ultrasounds

Kennedy pointed to ultrasounds—a diagnostic tool that relies on energy in the form of sound waves that bounce off of tissues and bones to create images—as another area of exploration. He believes there is a “timeline” that aligns with the rise in autism rates and the proliferation of ultrasound technology, which became more common in prenatal care in the 1960s and 70s. However, studies have found that there is no correlation between ultrasounds and ASD. 

The American Academy of Obstetrics and Gynecology emphasizes that there is no evidence of ultrasounds being harmful to a developing fetus. Still, the academy recommends that pregnant women receive only medically-necessary ultrasounds because “it is possible that effects could be identified in the future.”

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