Many perflourinated compounds have been phased out of manufacturing due to concerns about toxicity and damage to wildlife. Among them is perfluorooctane sulfonate (PFOS), which, until 3M began voluntarily phasing it out in 2002, was used in firefighting foams. 
Now scientists are worried that not enough is known about chemicals being used in firefighting foam in place of PFOS, many of which are not named by manufacturers.
The findings come after researchers examined blood samples from 20 firefighters and compared them to samples from students and office workers who had not been exposed to firefighting foam.
After using quadrupole time-of-flight tandem mass spectrometry to analyze the samples, they identified 3,000 organic and fluorinated chemicals. But there were nine fluorinated compounds in the data that stood out in only the firefighters’ blood, either exclusively or in higher concentrations — four of them are mystery compounds that have never been publically reported.
According to the researchers, the four compounds have been tentatively identified as sulfonic acids analogous to PFOS.
One scientist has pointed out that these new sulfonic acids haven’t been found in commercial foams and might be metabolites.
But one of the study’s researchers said the compounds should be examined further to determine risks.
“It is likely that the unknowns have similar properties to PFOS, such as toxicity and persistence in humans and environment,” said Maria Jose Gomez Ramos.