Six years ago, when I asked an epidemiologist about a report that a smoking ban in Helena, Mont., had cut heart attacks by 40 percent within six months, he thought the idea was so ridiculous that no one would take it seriously. He was wrong.

Since then, 10 other studies have attributed substantial short-term reductions in heart attacks to smoking bans, and last week an Institute of Medicine (IOM) committee endorsed their findings. But a closer look at the IOM report, which was commissioned by the U.S. Centers for Disease Control and Prevention, suggests its conclusions are based on a desire to promote smoking bans rather than a dispassionate examination of the evidence.

Thousands of jurisdictions around the world restrict smoking. Some of them are bound to see significant drops in heart attacks purely by chance, while others will see no real change or significant increases. Focusing on the first group proves nothing unless it is noticeably bigger than the other two groups.

The largest study of this issue, which used nationwide data instead of looking at cherry-picked communities, concluded that smoking bans in the U.S. "are not associated with statistically significant short-term declines in mortality or hospital admissions for myocardial infarction." It also found that "large short-term increases in myocardial infarction incidence following a workplace ban are as common as the large decreases reported in the published literature."

That study, published by the National Bureau of Economic Research (NBER) in March, suggests that publication bias -- the tendency to report positive findings and ignore negative ones -- explains the "consistent" results highlighted by the IOM committee. But even though the panelists say they tried to compensate for publication bias by looking for relevant data that did not appear in medical journals, they ignored the NBER paper, along with analyses that found no declines in heart attacks following smoking bans in California, Florida, New York, Oregon, England, Wales and Scotland.

If smoking bans reduce heart attacks, the effect could be due to declines in smoking, declines in secondhand smoke exposure or both. The IOM report settles on that last explanation, quite a leap given that "only two of the studies distinguished between reductions in heart attacks suffered by smokers versus nonsmokers."

In any case, when people stop smoking, their heart attack risk declines gradually over several years, and it's hard to see why the risk would fall any faster for people exposed to secondhand smoke. Furthermore, estimates from the CDC and the American Heart Association indicate that smoking and secondhand smoke together account for about 25 percent of heart disease deaths. So how could a smoking ban, even one that eliminated all smoking, cut heart attacks by 40 percent (or 47 percent, as another study claimed)?

The IOM panelists dodge these issues by declining to estimate the size or the timing of the impact from smoking bans, citing the limitations of the studies and the wide variations between them. But "if you can't even estimate the magnitude of an effect," notes Michael Siegel, a Boston University public health professor who was one of the report's reviewers, "you are hardly in a position to conclude that (it) exceeds random variation combined with the known secular decline in heart attack rates."

Even while taking refuge in imprecision, the IOM committee tries to make transparently absurd claims seem plausible by intimating that spending a half-hour in a smoky bar just might kill you, even if you were completely healthy when you went in. If so, where are the bodies? The report concedes that "there is no direct evidence that a relatively brief exposure to secondhand smoke can precipitate an acute coronary event."

Siegel, who faults the IOM committee's "sensationalistic" approach, is a longtime supporter of smoking bans who nevertheless tries to separate his political advocacy from his scientific analysis. It's too bad the authors of the IOM report, who immediately used it as an excuse to demand strict smoking regulations throughout the country, did not follow his example.