They are referring to crossing these thresholds (from Fed 87-118):
> EPA’s health advisories, which identify the
concentration of chemicals in drinking
water at or below which adverse health
effects are not anticipated to occur, are:
0.004 parts per trillion (ppt) for PFOA,
0.02 ppt for PFOS, 10 ppt for GenX
chemicals, and 2,000 ppt for PFBS.
Health advisories are non-regulatory
and reflect EPA’s assessment of the best
available peer-reviewed science.
0.004 ppt is also more commonly known as the number zero. It's probably just the currently best limit of detection for PFOA in water.
> EPA’s health advisories, which identify the concentration of chemicals in drinking water at or below which adverse health effects are not anticipated to occur, are: 0.004 parts per trillion (ppt) for PFOA, 0.02 ppt for PFOS, 10 ppt for GenX chemicals, and 2,000 ppt for PFBS. Health advisories are non-regulatory and reflect EPA’s assessment of the best available peer-reviewed science.
0.004 ppt is also more commonly known as the number zero. It's probably just the currently best limit of detection for PFOA in water.