Researchers found 42 'mystery chemicals' in the blood of pregnant women - and their newborn babies

• San Francisco researchers identified 42 "mystery chemicals" in the blood of pregnant women.
• The chemicals weren't linked to any known compounds and hadn't been reported in people before.
• Research showed that mothers passed the chemicals to their newborn babies.

When researchers collected blood samples from 30 pregnant women in San Francisco, they expected to find evidence of common environmental chemicals.

Chemicals known as PFAS (per- and polyfluoroalkyl substances) are found in the bloodstreams of 99% of Americans. Other toxic substances, like flame retardants and pesticides, often show up in human blood samples as well.

But to their surprise, the researchers discovered 55 chemicals never before reported in people.

A few of those chemicals contained recognizable compounds: Two belonged to the PFAS family, one was a pesticide, and 10 more were plasticizers - substances that make plastic durable and flexible.

The remaining 42 substances were labeled "mystery chemicals," since the researchers couldn't find a way to categorize them. The chemicals were identified in all 30 pregnant woman - as well as their babies after they were born, according to the researchers' new study.

"We're finding them, but we don't know where they're coming from and we don't have any information about their potential toxicity," Tracey Woodruff, the study's senior author, told Insider.

The researchers were particularly concerned by evidence that the chemicals could pass from one generation to next.

"The majority of the chemicals we see are able to cross the placenta, suggesting that the placenta is not efficient at preventing these exposures and it's not efficient at removing these chemicals from the fetus," Dimitri Panagopoulos Abrahamsson, the study's co-author, said.

He added: "Because they appeared to be both in the moms and in the babies, these chemicals would be expected to remain in the population for a very long time."

Some mystery chemicals may be linked to consumer goods

The Environmental Protection Agency (EPA) maintains a database of nearly 900,000 chemicals and their uses, but chemical manufacturers aren't legally required to disclose every compound they create. That makes it difficult to hunt down substances that could potentially pose a risk to human health.

Even when the EPA prohibits the use of a certain chemical, manufacturers have been known to develop spin-off chemicals that aren't subject to the same regulations. For instance, companies must seek EPA approval to manufacture or import products that use PFAS with eight carbon atoms, but they're free to manufacture or import PFAS with six carbon atoms. (Research suggests that both versions might be linked to cancer.)

The San Francisco researchers found four types of PFAS that weren't previously identified in human blood. In general, such chemicals are found in food packaging, clothing, carpets, and cookware.

The researchers think several of the "mystery chemicals" they found may hail from consumer goods as well, since items like furniture, electronics, and cosmetics are known to contain chemicals.

"There are some chemicals that appeared to be at higher levels in people with a higher socioeconomic background," Abrahamsson said. "Our best educated guess about this is that when you can afford more products, when you have a higher buying power, you introduce a lot more products to your home."

He added that some of the mystery chemicals his team identified may be impurities - chemicals either purposely or accidentally added to common substances used by manufacturers.

"In these cases, it's even trickier to know where these chemicals are being used because they're not the main chemical used in the product," Abrahamsson said.

Potential threats to fetal development

In general, chemicals pose a greater health risk in higher doses, or when people are exposed more regularly. But Woodruff said it will take a while before scientists know what levels of these mystery chemicals, if any, are potentially hazardous to humans.

"Given that they're mystery chemicals, they're probably not even on EPA's radar in terms of identifying their potential health risk or setting any type of levels that would be of more or less concern," she said.

Already, pregnant women in the US are widely exposed to environmental chemicals like pesticides or flame retardants, which may threaten the development of a fetus. In some cases, this exposure can lead to birth defects, childhood cancer, or health problems in adulthood such as reproductive issues, obesity, and diabetes.

For that reason, Woodruff said, it's important for scientists to keep studying unidentified substances in people's blood. But these studies are bound to hit a wall, she added, if companies don't report all the substances they're using.

"We are only covering the tip of the iceberg on chemicals that we need to be focused on," Woodruff said. "There are many of them, and we anticipate that there's reason to be concerned."