A small number of the chemicals have been well-studied, with the most known about the two oldest, and now discontinued, compounds — PFOS and PFOA.

Source: https://m.startribune.com/forever-chemicals-drinking-water-death-mortality-health-effects-pfas-epa/600265362/

Michigan bans sludge spreading if PFOS levels exceed 125 parts per billion, which is 24 times greater than Maine’s old limit.

Source: https://www.centralmaine.com/2023/03/19/from-maine-to-california-states-struggle-with-sludge-solutions/

PFOS once was used widely in firefighting foam and PFOA sometimes was an unintentional byproduct of foam manufacturing.

Source: https://www.chicagotribune.com/news/environment/ct-forever-chemicals-illinois-well-water-20230625-7yjoupnh3zdbfkhls6ln52ms3m-story.html