On some day in December, as the clock passes midnight, air that was fresh and clean in scores of places will become, in an instant, unconscionably filthy. Millions of people will sleep on unaware of the harm done them.

The harm done by federal air regulators, that is.

The regulators mean well. What the Environmental Protection Agency expects to do in December is to declare definitively which counties do not meet the new rules on how much soot — fine particulate matter, as it’s called — can be floating around.

The agency has been coming up with the new rules for several years, and in August, it named its preliminary list of 215 counties that don’t make the grade. You’ve got until October 2 to chime in with your comments on the rules at www.regulations.gov.

Before you do, there’s something you should know: The air in those 215 counties isn’t getting dirtier. The air in most places is getting cleaner. What’s changing is the definition of clean.

Take a spot in New York, for instance, an air monitor on Canal Street in Manhattan. According to the state Department of Environmental Conservation’s 2007 air report, on particularly bad days, the monitor measured 38 micrograms of soot per cubic meter of air, neither best nor worst in the city and well under the 65-microgram limit.

The EPA is cutting that threshold to 35 micrograms. The Canal Street monitor will exceed that, meaning the air in that part of Manhattan, on that magic day in December, will suddenly become illicitly filthy.

The new rules will also snare places that hadn’t been in trouble over soot before: Grand Rapids, Mich., the Quad Cities, swaths of industrial Wisconsin. Nor is it just soot: Earlier this year, the EPA changed its limits on ozone, saying scores of counties previously clean were now, by definition, polluted. Queens was added to the dirty list. Much of Connecticut and Massachusetts, clear under the old standard, were snared. So was Buffalo.

But ozone pollution isn’t worsening. It’s generally easing — down 10% in Chicago, for instance, over the past decade. Same with particulates: Levels fell 15% in the first four years after the EPA started monitoring them nationally in 1999.

“We have the cleanest air that human beings have ever breathed in cities,” an environmental consultant who studies pollution policy for the American Enterprise Institute, Joel Schwartz, says. Take a slightly larger sort of particulates, the kind known as PM10: In early August, New York was recording between 10 and 20 micrograms per cubic meter. In Beijing, the readings ranged from 110 to 278.

The EPA says America’s low levels of pollutants are still too high, saying new research shows both soot and ozone to be more unhealthy than regulators once believed. Critics, however, take issue: The prevalence of asthma is up 75% even as ozone levels fell 20%. Other studies suggest soot is dangerous, but only for men or those with high school degrees, not women or those better educated. That, Mr. Schwartz says, signals statistical noise and biases in the research.

This counts, since the argument for imposing tighter limits is that the benefits are worth the costs. The costs are considerable.

Once counties are declared dirty, industries must buy much costlier pollution control equipment. Getting permits takes much longer. Growth is discouraged. Drivers must buy costlier fuels and submit to regular tailpipe tests. This revision of “clean” has real, quantifiable costs: The EPA estimates its new ozone rules cost Americans between $10 billion and $22 billion a year. Independent estimates peg the cost of the soot rules even higher, at between $70 billion and $150 billion a year.

Were that money to stay in the productive economy, would people be better off? That depends on the credence you give to the statistically noisy studies, though even the EPA estimates its ozone rules will cut asthma by only a few tenths of a percent. The problem, Mr. Schwartz says, is that even as pollutants fall to near natural levels and further cleanup grows ever costlier, regulators lack any kind of feedback mechanism to signal when enough is enough.

“EPA’s mission,” he said, “is to make sure the air couldn’t possibly hurt anyone, any time, anywhere.” The agency sets the rules and controls the research to see whether they must be tightened more. Even if it’s not a case of keeping the agency in business, if regulators are moved by a good-faith attack on pollution, “it’s a string of conflicts of interest that no one would tolerate in the private sector.”

Which you could say is the tolerable indulgence of a rich society, only it costs lives. The U.S. Office of Management and Budget reckoned that every $7.5 million to $12 million in regulatory expense also costs a life, lost as controlling pollution displaces people’s other priorities, like better food, housing, education, or medicine. By that calculation, the ozone rules could mean as many as 3,000 premature deaths a year.

That’s why it’s important to alter the cycle. Cleaner air is good, but our air has been getting cleaner. At some level, the process passes from necessary cleanup to costly, even fatal, obsession. Society as a whole needs to watch for that, since regulators and activists are not temperamentally equipped to do so. “Their power, their prestige, and their funding,” Mr. Schwartz said, “come from keeping people scared. They need a crisis to save us from.”

Even if it’s not a crisis.

Mr. McIlheran is a columnist for the Milwaukee Journal Sentinel.