After a decade of delays and lawsuits, this is an important step toward achieving cleaner air.

For residents in the Harrisburg area, it means providing much needed relief from power plant emissions from neighboring states.

In the United States, coal-fired plants, which account for about 1 percent of the total global emissions, are the largest and last uncontrolled source of mercury emissions into the air. We all agree they must be cleaned up.

EPA took comments about the proposed rule through the end of March. Public hearings took place in major cities.

While there has been controversy about the proposal, there is no debating one thing - mercury is a persistent toxic substance. Methylmercury, the form of mercury that is of greatest concern, can accumulate in fish.

In pregnant women, methylmercury can be passed on to the developing fetus - at sufficient exposure levels, it may cause developmental problems.

As EPA moves this proposal forward, it is important that everyone concerned about this toxin keep some basic facts in perspective:

* There has never been an attempt by the federal government to control mercury from coal-fired power plants.

* About 40 percent of all mercury that enters the food chain in this country, primarily in fish, comes from sources outside the United States.

THAT DOESN'T mean we shouldn't control what goes into the air. It just means that when it comes to environmental issues, nothing is as simple as it seems.

* There is no technology available, real-world tested and proven, that we can say with assurance will reduce these emissions by 90 percent at all affected sources.

* EPA's plan sets a nationwide cap on mercury emissions and creates an emission allocation and trading system that will get big reductions in mercury sooner and faster than any other regulatory scheme.

THIS PROGRAM, working in combination with a new interstate air quality rule to further reduce power plant emissions of sulfur dioxide and nitrogen oxide, represents the largest and most expensive commitment to improving air quality in this country in more than a decade.

The American people want their government to shoot straight with them when it comes to issues of health and the environment. We believe the plan we have proposed for mercury keeps faith with our obligation to protect public health in this country.DONALD S. WELSH is EPA Regional Administrator, based in Philadelphia.