In a step meant to control the windblown smog that drifts across state lines toward the Northeast, the Environmental Protection Agency on Thursday ordered 22 states to significantly reduce their emissions of nitrogen oxides, the main pollutant that causes smog.

The rule is the broadest regional attack ever mounted on nitrogen oxides, which come mainly from burning fossil fuels. Although the results are not expected to be dramatic or immediate, over the long term they are intended to bring some relief to many smog-afflicted states, especially in downwind states whose pollution comes partly from power plants and other sources in the Midwest.The EPA ruling comes after a year of bitter feuding among governors over how much the upwind states - most of them in the Midwest - should be held accountable for pollution that affects the downwind states, which are located in the Northeast.

The new regulation calls for reductions in emissions by 28 percent over the next nine years, with more than half the reductions expected in just six states: Ohio, Indiana, Illinois, West Virginia, Michigan and Pennsylvania. To hold down the costs, the agency encouraged states to set up a pollution trading system for electric utilities similar to the approach that has been used to control sulfur dioxide, which causes acid rain.

The EPA estimated that the rules would cost an average of $1.7 billion annually over the next 15 years, but the agency said the resulting health benefits were worth the expense, which is likely to fall most heavily on electric utilities. The agency estimated that this cost would amount to about one dollar on an average household's monthly electric bill, but it said that this might not be noticeable while electricity rates decline as the industry restructures.