Parts per trillion (ppt)

Parts per trillion is a very small unit. One ppt means one part of PFAS in a trillion parts of water. EPA sets PFAS limits at this scale, so lab reports must be read in the right unit.

EPA's drinking water table gives PFAS limits in ppt and notes the same figure is also expressed as ng/L. If a lab reports 4 ng/L, that equals 4 ppt. This is the number to look for in your report.

When you read a lab report, check three things. Look at the unit, the compound name and the detection limit. A result marked "ND" means not detected at that lab's limit, which is not the same as zero.

Compare results with the MCL for PFOA and PFOS, which is 4.0 ppt each on EPA's table. A private well is not bound by that limit, but the number is a useful reference.

If the report confuses you, ask the lab or consultant to explain it in writing.