Unlike some spots in the state, the oysters found in Biscayne Bay aren’t good for eating — they’re typically too scarce, too small and too tainted with pollution to be safe.

But for researchers at Florida International University, the shellfish that grow in the briny bay also can serve an important and counterintuitive purpose. They’re actually indicators of the safety of fresh drinking water pumped from Miami-Dade’s wells into household taps.

A newly published FIU study of oysters in three coastal Florida areas — Miami-Dade, Tampa and Naples — found they were contaminated with potentially harmful chemicals known as PFAS, short for perfluoroalkyl and polyfluoroalkyl substances. And Biscayne Bay’s shellfish had, by far, the highest levels of what are also known as “forever chemicals.”

PFAS include thousands of man-made chemicals used in everything from nonstick pans to fast-food packaging to waterproofed clothing to firefighting foams. Though research is ongoing, the U.S. Environmental Protection Agency says high exposures have been linked to different types of cancers, thyroid issues and disruption to the reproductive system.

These chemicals have bled into drinking water in every state, and experts say nearly everyone has been exposed to at least some PFAS. But multiple studies, including the latest from FIU, suggest that Miami-Dade’s water has particularly high levels of contamination.

As recently as 2020, the county shut down three drinking water wells over unsafe levels of PFAS, including several chemicals that posed serious enough public health risks that production was halted years ago.

“PFAS has been known to be toxic for many years. This is an ongoing problem,” said Natalia Soares Quinete, an FIU assistant professor of chemistry and one of the authors of the paper, published in the journal Science of the Total Environment.

She and other researchers collected oysters from Biscayne Bay, Marco Island near Naples and an oyster farm near Tampa Bay. They found PFAS in oysters from all three spots. Oysters, “filter feeders” that constantly pump water through their systems, are particularly vulnerable to coastal pollution. Chemicals can build up in their meat, which makes them valuable to water quality researchers.

Decades ago, when more fresh water from the Everglades flowed into Biscayne Bay, oysters were common in some areas. They have largely disappeared since, although a grassroots restoration effort is underway.

In the study, the Biscayne Bay oysters were the tiniest of the bunch, which initially led the researchers to assume that they were younger and might have absorbed the least amount of PFAS.

“But what we found was exactly the opposite. Biscayne Bay oysters were the smallest, and they had the most contamination,” said Leila Lemos, a postdoctoral scientist at the FIU Institute of Environment.

Biscayne Bay oysters also had thinner shells than they should have for their age. Lemos suspects it’s because Biscayne Bay is far more polluted than the other two spots, so struggling oysters must contend with higher concentrations of PFAS and other toxins.

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The study wasn’t intended to assess the health risks of actually eating oysters. None of the farmed Tampa oysters, for instance, exceeded recommended PFAS levels, but Lemos warned that her team only tested for a handful of known compounds linked to health issues. And there is no commercial harvest of oysters in Naples or Biscayne Bay, so they’re rarely, if ever, eaten.

The health problems posed by PFAS are still being assessed but are potentially wide-ranging. Unlike more well-known contaminants, like lead, where one dose can cause immediate problems, PFAS build up in the body over time and are linked to long-term impacts like birth defects or certain cancers. A Harvard University study even suggested that high exposure to PFAS could decrease the efficacy of COVID-19 vaccines.

So far, there isn’t even a firm standard for a “safe” level of PFAS for water utilities. The EPA has never set mandatory regulations for the chemicals, but in June, the agency did dramatically toughen recommendations for some chemicals identified as the worst of the bunch.

For one of the most common and harmful forms, called PFOA, the agency dropped its recommendation for a safe daily dose from 70 parts per trillion to 0.004 parts per trillion. That’s thousands of times stricter — and a sign that PFOA in particular is more problematic than initially believed.

That’s such a minuscule amount — think a fraction of a grain of sand in an Olympic-sized swimming pool — that most water utilities don’t have sensitive enough equipment to even measure a near-zero level.

“We shouldn’t be seeing those compounds. The goal is to not see them in our water,” said Soares Quinete.

In Miami-Dade, PFAS levels in some past sampling have far exceeded the new recommendations — enough to shut down some wells. The county has explored suing some of the chemical companies who originally made the toxic PFAS chemicals to help pay for future cleanups, an approach Tampa is also considering.

Florida beat them to it with a state lawsuit filed in May accusing several major chemical companies of failing to warn consumers of the dangers of their products.

Miami-Dade has been under fire for high PFAS levels even before the EPA tightened its recommendations for safe levels.

When an environmental advocacy group named Miami’s water number three on the nationwide list of most PFAS contaminated in 2020, Miami-Dade pushed back and declared its water safe and within the EPA guidelines it uses as a yardstick.

The Environmental Working Group said it found PFAS levels of 56.7 parts per trillion in Miami’s drinking water, the third-highest of the 31 states it tested. The nonprofit advocacy group did not say where it sampled that water.

At that time, the EPA suggested limit for PFOA — one of the most infamous chemicals — was around 70 parts per trillion, so the county argued that it was still well within safe limits. And when water levels exceeded those safe limits, the county acted.

Around the same time, Miami-Dade found unsafe levels of PFAS in six drinking water wells and shut down three of them. Two of the wells, in Hialeah, registered about double the 70 ppt limit then suggested by the EPA.

Since then, the average PFAS levels reported by the county have been far below that older 70 ppt threshold.

Miami-Dade County’s latest annual drinking water report showed a PFOA concentration of up to 10 ppt in the main system and South Dade water supply system and undetectable levels elsewhere.

But under the new EPA suggested threshold of 0.004 ppt, Miami-Dade’s water contains thousands of times more PFOA than it should.

And more recent one-time samples, like those in a newly published paper from Soares Quinete and Lemos examining PFAS levels in surface and drinking water across Florida, are even higher.

They found South Florida’s tap water had the highest levels of PFAS compared to central and southwest Florida, and in some spots, they were several times higher than even the old EPA safety guidelines suggested.

The average total for all tap water was 86.3 ppt, with an even higher concentration near airports, where it used to be common to use a firefighting foam with extremely high levels of PFAS chemicals. Tap water in the Grapeland Heights neighborhood near Miami International Airport was a staggering 242 ppt, and Dania Beach, near the Fort Lauderdale-Hollywood International Airport, had the second-highest levels at 124 ppt.

In a statement, Miami-Dade Water and Sewer Department Spokesperson Jennifer Messemer-Skold said the department hasn’t made any changes around the new standards yet.

“WASD is in the process of reviewing the EPA’s modified recommendations and continuing to collaborate with County, State, and Federal agencies, and industry experts to address these advisory changes. We will take all steps necessary to comply with any updated requirements as we continue to work to safeguard the public’s health by providing safe and reliable drinking water.”

The researchers also were surprised to find that PFAS levels were higher in people’s tap water than in surface water, like canals and Biscayne Bay. Soares Quinete thinks it might be due to the degradation of the plastic PVC pipes carrying our water supply, but she said more research is needed.

She said that PFAS levels in drinking water are not researched enough, but what’s been found so far suggests that the government needs to take action to protect people.

“I think people should be worried. They should advocate for changes,” Soares Quinete said.

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