Toxic PFAS compounds are contaminating the air inside homes, classrooms and stores at alarming levels, a new study has found.

Researchers with the University of Rhode Island and Green Science Policy Institute tested indoor air at 20 sites and detected the “forever chemicals” in 17 locations. The airborne compounds are thought to break off of PFAS-treated products such as carpeting and clothing and attach to dust or freely float through the indoor environment.

Experts previously considered food and water to be the two main routes by which humans are exposed to PFAS, but the study’s authors note that many humans spend about 90% of their time indoors, and the findings suggest that breathing in the chemicals probably represents a third significant exposure route.

“It’s an underestimated and potentially important source of exposure to PFAS,” said Tom Bruton, a co-author and senior scientist at Green Science.

PFAS, or per- and polyfluoroalkyl substances, are a class of about 9,000 compounds used to make products water-, stain- or heat-resistant. Because they are so effective, the chemicals are used across dozens of industries and are in thousands of everyday consumer products such as stain guards, carpeting and shoes. Textile manufacturers use them to produce waterproof clothing, and they are used in floor waxes, nonstick cookware, food packaging, cosmetics, firefighting foam and much more.

PFAS are dubbed “forever chemicals” because they do not naturally break down. They accumulate in animals, including humans, and are linked to cancer, birth defects, liver disease, thyroid disease, decreased immunity, hormone disruption and a range of other serious health problems.

A February Guardian analysis of household products found fluorine, an indicator of PFAS, present in 15 items. The chemicals are so widely used that it is difficult to say with precision where all the airborne PFAS are coming from, though the new study also detected their presence in carpets and clothing at some sites.

The study, published on Tuesday in Environmental Science & Technology, used a new PFAS measurement technique for checking air. It found particularly high levels in several kindergarten classrooms and also checked the supply room of an outdoor clothing store, offices, several university classrooms, university labs and an elevator.

A 2017 study found a correlation between high levels of PFAS in the air and in human blood serum, and the new study used modeling that found that kindergarteners were probably exposed to more PFAS by breathing them in than by ingesting the compounds.

“This reinforces that as long as there are PFAS in products that we have surrounding us in our homes and in our lives, there’s going to be some amount that ends up in the air, ends up in dust, and we are going to end up breathing it in,” Bruton said.

Also notable are the types of PFAS that the study detected. Among the most prevalent was 6:2 FTOH, a compound used in floor waxes, stain guards and food packaging. Industry previously claimed that 6:2 FTOH was safe, but in May the Guardian revealed that two major PFAS producers had hidden studies that suggested that the compounds are highly toxic at low doses in lab animals and stay in animals’ bodies for much longer than was previously known.

Science from industry, federal agencies and independent researchers now links 6:2 FTOH to kidney disease, cancer, neurological damage, developmental problems, mottled teeth and autoimmune disorders, while researchers also found higher mortality rates among young animals and human mothers exposed to the chemicals.

The new study also found high levels of 8:2 FTOH, a type of compound that major PFAS manufacturers in the US claimed to have phased out of production because it is so dangerous. Its presence suggests that not all companies have phased it out, or that it is in products made in countries where the chemical has not been phased out.

“To me, this is one more reason to turn off the tap on PFAS production and use,” said Bruton.