By JOEL SCHWARTZ

Americans spend tens of billions dollars each year on measures to reduce emissions of smog-forming pollutants from motor vehicles, power plants, and a host of other sources. Costs continue to rise over time, as the Environmental Protection Agency (EPA) tightens its ozone standards and adopts ever more stringent regulations.

These costs might be worth bearing if ozone were exacting a large toll on people’s health. But even as environmental activists have become more strident in raising health alarms, evidence has mounted that ozone at current levels is causing little or no harm, even in the most polluted areas of the country.

For instance, the prevalence of asthma nearly doubled during the past 25 years — but at the same time, levels of ozone and other air pollutants sharply declined. Emergency room visits for asthma are at their lowest in July and August — when ozone levels are at their highest.

The government’s own research even undermines ozone regulations. A government-funded study of thousands of children in California found children who grew up in the highest-ozone areas had a 30 percent lower risk of developing asthma, when compared with children in low-ozone areas.

More recently, epidemiologists claimed thousands of Americans die prematurely each year due to ozone. The claim is based on small statistical correlations between daily ozone levels and daily deaths. But correlation doesn’t necessarily mean causation, as recent embarrassing medical reversals have shown.

For example, based on correlation studies, medical experts presumed that hormone-replacement therapy (HRT) and Vitamin A supplements prevent heart disease, calcium supplements prevent osteoporosis, and a low-fat diet reduces cancer risks. But randomized trials showed these claims are either greatly exaggerated or downright false. In fact, Vitamin A turned out to increase cardiovascular risk.

The ozone-mortality claim should be treated even more skeptically. First, it is based on the same unreliable methods that have led medical authorities astray in other areas. Second, even though ozone was correlated with higher premature mortality on average, ozone seemed to protect against death in about one-third of cities. How could ozone kill people in some cities and save them in others? More likely, both results are chance correlations rather than real effects.

According to an EPA fact sheet, “ozone can irritate lung airways and cause inflammation much like a sunburn ... People with respiratory problems are most vulnerable, but even healthy people that are active outdoors can be affected when ozone levels are high.”

But these alarming claims stand in stark contrast to EPA’s own estimates of the actual health effects of ozone. In the journal Environmental Health Perspectives, EPA scientists predicted that going from ozone levels during 2002, which were by far the highest of the past several years, down to national attainment of the stringent new federal eight-hour ozone standard would reduce respiratory-related hospital admissions and emergency room visits by no more than a few tenths of a percent.

Likewise, researchers from USC followed nearly 1,800 California children for eight years. They found ozone had no effect on lung development, even though the study included areas that exceed the federal ozone standard more than 100 days per year.

Despite the evidence, in its State of the Air report, the American Lung Association claimed, “Almost half of all Americans are living in counties where [ozone] places them at risk for decreased lung function.”

Regulators and environmental activists don’t maintain their jobs, power, and funding by admitting we’ve solved the problems that justify their existence. Rather, they stay in business by creating an appearance of serious and pervasive harm from air pollution, regardless of whether such alarms are justified. Unfortunately, journalists pass along the false health alarms with little or no critical review.

Even many health experts and medical researchers contribute to air pollution scares. Despite the evidence against air pollution as a cause of asthma, public health authorities around the country continue to cite air pollution as a key factor in rising asthma prevalence.

None of this would matter if reducing air pollution were free. But Americans will have to spend more than $100 billion per year — about $1,000 per household — just to attain the current eight-hour ozone standard. EPA is now considering an even tougher ozone standard that will be even more costly.

That money — or more correctly, the labor, capital, and know-how that money represents — would otherwise go to health care, food, housing, education, leisure, and other things Americans value and that would genuinely improve their welfare. Instead, for this stupendous sum we will eliminate at best a few tenths of a percent of all respiratory disease and distress.

Joel Schwartz (joel@joelschwartz.com) is a visiting fellow at the American Enterprise Institute and a policy advisor to The Heartland Institute in Chicago (www.heartland.org).