Economists are not real big fans of command and control pollution regulations--you know the kind where the regulatory agency says here is how much pollution is allowed and here is how you are going to do it. The reason? Because there are usually cheaper ways to meet the same goals. Requiring everyone to do the same thing means that no one has the flexibilty to figure out the cheapest way to meet the goal on their own. The only way a command and control regulation is efficient (least cost) is if the required method of reduction is the least cost method for all polluters. But since it is unlikely that the regulatory agency can possibly know the least cost reduction technology for all polluters, it's unlikely that command and control regulation are the cheapest.
That's why the EPA has built in some flexibility for states to meet Clean Air Act ground level ozone--smog--standards.