Peeing in the Pool

Peeing in the pool is the classic example of an externality.  If I pee in the pool, I gain all of the benefit (I don't have to get out of the pool and go to the bathroom), and pay almost none of the cost (my pee bothers me much less than it bothers everybody else).  And so, we have a rule against peeing in the pool, and its one that republicans and democrats alike can agree on.  The air we breathe and the water we drink and fish in is fundamentally like a big pool, and so it ought to be common sense that there are regulations about how much you can pee in the pool.  Of course I'm not really talking about pee (which is gross but fundamentally harmless), but instead about Mercury (which makes kids stupid, or dead), soot (which makes them asthmatic, or dead), as well as any number of other things.

Democrats and Republicans agree that we shouldn't be peeing in public pools, so why do we disagree about whether we should have rules about the poisons you can let out into the air?