A common 'forever chemical' known as PFOS (perfluorooctanesulfonic acid) has been linked to liver cancer in humans in a worrying new study.

Once a key ingredient in the water-repelling product commercially known as Scotchguard, PFOS was finally phased out soon after the turn of the century following concerns over its toxicity and environmental impact.

Still, it didn't earn its label of 'forever chemical' for nothing, with environmental levels of this and closely related substances remaining alarmingly high around the globe.

Now a study by researchers from the University of Southern California and the Icahn School of Medicine at Mount Sinai in the US have confirmed an association between PFOS and the development of a particularly deadly form of liver cancer.

Hepatocellular carcinoma (HCC) accounts for more than four out of five cases of liver cancer in the world. With a five-year survival rate of less than 20 percent, it's also regarded as one of the most deadly of cancers any of us could get.

Although the total incidence of HCC has declined over the past decade in the wake of dropping hepatitis infections, a rise in non-alcoholic fatty liver disease – a condition exacerbated by obesity and high cholesterol – could confound efforts to keep cases down.

On the back of research like this new study, we might also add contaminated drinking water to that list of risk factors.

The long-chains of synthetic compounds known as perfluoroalkyl substances (PFAS) are now widely recognized as particularly nasty saboteurs of our body's hormonal and liver systems.

In spite of a succession of bans of PFAS in jurisdictions around the globe over recent years, a regrettable amount of damage might already be seeded.

Along with substances like perfluorooctanoate (PFOA), and perfluorohexane sulfonate (PFHxS), PFAS and PFOS take their time breaking down in the environment, with half-lives of up to seven years.

That means in spite of efforts to slowly wind down their production and replace their use in anything from cosmetics to fabric protection to fire-fighting foam, today's population continues to be exposed to whatever was being dumped into waterways decades ago. And will for some time yet.

With more than 98 percent of the adult US population having detectable concentrations of these compounds in their blood, researchers are now turning their attention to questions of what might be considered a 'safe' level of contamination.

Animal studies have demonstrated clear links between PFAS and liver damage. But what was really needed was a population-scale analysis of exposure and risk of ill health.

"Part of the reason there has been few human studies is because you need the right samples," says Veronica Wendy Setiawan, a cancer epidemiologist from the Keck School of Medicine at the University of Southern California.

"When you are looking at an environmental exposure, you need samples from well before a diagnosis because it takes time for cancer to develop."

As part of a collaboration with the University of Hawai'i called the Multiethnic Cohort Study, the researchers analyzed blood taken from 50 individuals a diagnosis of non-viral HCC.

These were compared with a carefully matched sample of bloods taken from 50 volunteers without a diagnosis.

Measuring levels of various types of PFAS in blood samples taken prior to the development of liver cancer, the researchers identified a strong association between PFOS and HCC.

Those in the top 10 percent of blood-PFOS levels, in fact, were 4.5 times more likely to develop HCC than those with lower blood-PFOS levels, providing the strongest evidence yet that we're capable of absorbing dangerous levels of these notorious substances.

"This study fills an important gap in our understanding of the true consequences of exposure to these chemicals," says the study's lead author, Keck School of Medicine public health researcher Leda Chatzi.

Knowing where we can draw the line on a safe level of exposure will go a long way to refining regulations and supporting measures on monitoring environmental levels, without resorting to panic or risking the spread of misinformation.

Forever chemicals might be with us for a while to come, but the sooner we can learn just how bad they are, the better off future generations will be.

This research was published in JHEP Reports.