Instead of following your trick-or-treaters down the block in the car, consider walking the...

How much is one company allowed to pollute? That might depend on how many credits it has. The practice of capping emissions levels (putting a limit on how much a company is permitted to pollute) and issuing credits to those companies who fall below the cap-credits that can then be sold to companies who are above it-represents an attempt to use the free market to regulate pollution and encourage better environmental practices. The Environmental Protection Agency (EPA) established one such system, specifically targeting acid-rain pollutants, as a part of the Clean Air Act of 1990. Just how effective pollution trading is, in the long run, is the subject of much debate; many environmentalists see it as a scheme that allows the worst polluters to continue their dirty ways without responsibility, as long as they have deep pockets. See also carbon market.