Copper is used as bottom-coat on sailboats, it's also used as a non-hormonal contraceptive IUD material. I don't think it's terribly harmful for humans if it's

Copper is used as bottom-coat on sailboats, it's also used as a non-hormonal contraceptive IUD material. I don't think it's terribly harmful for humans if it's safe for a women's uterus. California tends to be exceptionally concerned for the environment in general. All that copper dust making it into the coastal marine environments isn't going to be good for it, considering the same stuff is used to keep marine life off boats. edit: "The issue has been under consideration since the 1990s, when cities south of San Francisco were having trouble meeting Clean Water Act requirements to reduce copper in urban run-off flowing into San Francisco Bay. Preliminary studies indicated that brake pads were a significant source of copper in that runoff. Tiny amounts of copper fall onto the streets and parking lots every time drivers step on their brakes." from https://www.copper.org/environment/impact/copper-brake.html https://www.copper.org/environment/impact/copper-brake.html As I suspected, it doesn't appear to be primarily a human toxicity concern. It's to protect the marine environment.