[mikeash]

A lot of pollution is stuff where a little bit of it is more or less harmless, but it gets bad when lots of people do it. It can make sense to grant special case exemptions rather than lowering standards as a whole.

For example, it's common for old cars to be exempt from emissions regulations, and small aircraft are still allowed to use leaded gasoline. (Arguably the leaded gas use by small aircraft is still big enough that it should be stopped, but regulators don't seem willing to actually do it.)

Of course, granting it to one company just because they're building a factory you want is naked favoritism, not the sort of at least vaguely sense-based decisions we'd want to have.