Hazardous 'forever chemicals' stay in your blood for years, longer than lead or arsenic. It still helps to cut them out.

• Hazardous "forever chemicals" called PFAS are in most Americans' blood, and they don't break down.
• The EPA just set strict limits for PFAS levels in drinking water, which can be a source of daily exposure.

Hazardous "forever chemicals" have probably been flowing through your veins for years.

Per- and polyfluoroalkyl substances (PFAS) are a class of thousands of human-made chemicals, which are likely in your food, water, clothes, and furniture. They're linked to multiple cancers, thyroid disease, liver damage, decreased fertility, asthma, allergies, and reduced vaccine response in children.

That's why the US Environmental Protection Agency announced new, strict limits on the levels of six PFAS in drinking water on Wednesday. It's the first time the agency has passed a regulation to limit the day-to-day PFAS exposure that many people in the US get from simply staying hydrated. The EPA is giving water officials five years to test their systems, remove PFAS from the water, and comply with the new limits.

"It is easily the most consequential and difficult decision to protect drinking water in the past 30 years," Ken Cook, president of the watchdog Environmental Working Group, told reporters of the new EPA decision.

Scientists suspect PFAS are in every American's bloodstream — and they stay there. That's how these substances earned the "forever chemicals" nickname. They don't break down.

"Once they get into your body, they stick around for a really, really long time," Carmen Messerlian, a professor of reproductive environmental epidemiology at Harvard's TH Chan School of Public Health, who studies PFAS, told Business Insider in March 2023.

Just how long they stick around is straightforward to calculate using a rule of chemistry, called "half-life."

In humans, half-life is the amount of time it takes for your body to expel half the amount of a substance from your blood, by urinating or absorbing it into other tissues.

The half-life of a substance can vary widely between individual people, but studies have calculated averages. That research tells us how long PFAS can linger in our blood, compared to toxic heavy metals or everyday substances like caffeine.

That means if you completely cut PFAS out of your life — a feat that scientists say is virtually impossible — in four years, seven years, or maybe even 10 years, your body will have only expelled half the chemicals.

That's not to say getting PFAS out of our bodies is a lost cause.

After a lot of bad press around two of the most notorious PFAS — perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) — US manufacturers phased them out of production in the 2000s.

You can see the results in American's blood. From 1999-2000 to 2017-2018, blood levels of PFOA declined by 70% and levels of PFOS declined even more, by 85%, according to data from the Centers for Disease Control and Prevention.

"That's an indication that when something is done, and when we stop using the chemicals, stop releasing them into the environment, the concentrations [in our bodies] do go down," David Andrews, a senior scientist studying PFAS at the watchdog organization Environmental Working Group, told BI in February 2023.

There are still thousands of other PFAS widely manufactured and used in products across the US, and the entire planet.

Everything from dental floss and menstrual products, to food packaging and furniture, is spreading PFAS throughout the environment and our bodies — from the day the chemical is first manufactured, all through the time you use the product, and even after you throw it away.

Regulating six of these chemicals in US drinking water is just the start of solving the problem.

"We're really just covering the very, very tip of the iceberg," Messerlian said.

"We need these chemicals to stop circulating in our environments by stopping them from being in the food production line, and the product production line," she added.

Catherine Boudreau contributed to this story.