I get the vehicles sprayed by Rust Check, at least annually. I believe the product is mineral-oil based. I presume it's not environmentally harmful.I feel most suggestions here environmentally irresponsible. Depositing wheel bearing grease all over the roads can’t be good for water run off. Most greases, like high temp disk brakes wheel bearing grease are harmful to aquatic life as per material safety data sheets. It disappoints me people have such little regard for these sorts of things.
Bear in mind, as well, that it's only a very small percentage of people who brother to apply any sort of rust-preventative product.
In contrast, a significant percentage of vehicles leak motor oil, ATF, gear oil, and brake fluid. I'm sure the damage done by these leaks dwarfs any damage done by rust-proofing products.
I think the biggest single thing we can do as DIYers is to take our used fluids to a proper recycling/disposal facility, and to encourage others to do likewise.