Just when you thought it was safe to breathe, Pittsburgh is ranked as having the sootiest air in the nation. To locals and visitors alike, that's hard to believe, given the visible cleanup the former Steel City and region have achieved over decades.

But anyone who is informed on the subject knows that what you can't see can hurt you.

That's the message behind the latest report by the American Lung Association on metros with the worst 24-hour levels for soot -- fine-particle pollution that contains ash, diesel exhaust, chemicals, metals and aerosols. In the same study, which covers 2004-06, Pittsburgh came in second to Los Angeles for the worst annual readings on this unhealthful airborne cocktail.

The ALA is concerned about particle pollution because coughing and sneezing cannot keep such microscopic threats from burrowing deep within the lungs, where they can lead to breathing problems, asthma, lung cancer and even heart attacks or strokes.

Pittsburgh earned the report's headline because it's the first time that a metro outside California tops any of the group's rankings. The ALA said the region's notorious shift is not so much due to southwestern Pennsylvania's air getting much worse, but to California's getting much better.

Improvement will come to Pittsburgh, too, but not overnight. A major factor in its soot readings are emissions from the Clairton Coke Works. Fortunately, U.S. Steel is embarking on a $1 billion upgrade, which will replace batteries of coke ovens, and reduce pollution significantly, beginning in late 2011.

That's a welcome and necessary step, but it's only part of what must be a full-court press.

We're glad, for instance, that Allegheny County Chief Executive Dan Onorato decided to upgrade the county's Air Quality Program rather than turn it over to the state. But we're concerned that his plan will also weaken local air standards. His timing for that, given Pittsburgh's latest pollution rankings, is all wrong.

Coal-fired power plants must continue to reduce emissions, and smokestacks in other states must be accountable for the fallout they dump here. Mass transit should be kept strong and convenient, otherwise the public, even with record gasoline prices, won't use it. Individual Pittsburghers need to make "green" choices, and not just on Air Quality Action Days.

On a broader level, the state Department of Environmental Protection must keep fighting the federal government so that it maintains standards and enforcement for clean air and clean water. And the United States needs a president who will revive the Environmental Protection Agency so that it safeguards public health, rather than putting politics over science.

If reports like this make one thing clear, it's that lungs don't care where toxic pollutants come from. In Pittsburgh, clean air should be everyone's business. Getting tough and staying tough is the only way we can all breathe easier.