FOR FIVE YEARS nothing was done about a federal law that was supposed to herald an attack on smog. From 1997 until last Wednesday, that law was tied up in legal wrangling. Finally, the "preliminary" jousting has concluded and now the U.S. government can begin thinking about putting it into effect.

It's great news, but don't hold your breath. Well, if you can hold your breath, that's not such a bad idea, because smog is a killer. But the next thing that happens is that the Environmental Protection Agency has until April 2004 to decide what areas of the nation fail to meet the new standards. After that, those areas -- and Baltimore with its suburbs will surely be among them -- will need to come up with state-devised plans to set things right.

At issue is ozone, which is one of the two principal components of smog. Ozone is cooked up by the exhaust of cars, trucks, stationary engines, power plants and other polluters. The United States already has an ozone standard, which Baltimore already fails. The new law tightens it considerably.

So what does this mean for Maryland? There are three ways to cut ozone here, and all three may come into play:

The first is the easiest. A good portion of Maryland's ozone drifts in from power plants in the Midwest, and the new law is designed to sharply reduce their emissions. No pain here in that.

The second is to require cleaner exhaust through tougher emissions standards. A lot of that, by law, would have to be done on the federal level.

The third is to find ways to emit less exhaust. More hybrid cars and fewer SUVs. More money for rail and less for new highways. Shorter commutes -- through more compact development. All of that is long-term, but it's hard to see how it can be avoided forever.

But let's say Maryland draws up a plan and the EPA accepts it -- and maybe the state believes it can find a way to build the Intercounty Connector and offset the new traffic volume with cuts elsewhere -- but after a period of years the ozone levels are still unacceptable. What happens then?

Conceivably, there could be federal sanctions on the state. Highway money could be withdrawn, for starters. But for now that's more than a little hypothetical, and very distant.

The real cost to Maryland if the standard isn't met would be the continuing assault on its citizens' health. The EPA says that, nationwide, smog contributes to 15,000 premature deaths a year, and 350,000 cases of asthma. The good news is that, however slowly it may work, the agency is serious about cleaner air. Under both the Clinton and Bush administrations, it fought in court against attacks on the ozone law, and won a major battle with a unanimous Supreme Court verdict last year. The way for real action is finally clear.