Activists have rushed to embrace a new study that seems to prove, at long last, that secondhand smoke is life-threatening. The British Medical Journal reported that after a smoking ban in Helena, Montana, the heart-attack rate dropped by almost half. In an accompanying editorial, researchers from the Centers for Disease Control heralded the finding. And that prompted the Washington Post to blare, "Second Hand Smoke Poses Heart Attack Risk, Warns the CDC."

In truth, the study is woefully unreliable; but its compatibility with widespread anti-tobacco animus gave it status as "fact." Meanwhile, other solid data that tobacco products can improve the health of smokers go largely ignored.

In June 2002, Helena banned puffing in public places. The prohibition was overturned in December. In the six months it was in effect, there were four heart attacks per month. From June to December of the pre-ban years, the rate was about seven per month. After December 2002, when the ban was lifted, the monthly average jumped back to seven in June-December 2003.

The study has the ring of plausibility. We know that in the laboratory, components of cigarette smoke can cause blood clotting and damage to the lining of blood vessels.

But the drop in Helena heart attacks was too dramatic to be explained by reduced exposure to environmental smoke. The reason is that airborne smoke has been shown to have only a weak relationship to heart disease. Moreover, it is difficult to draw meaningful inferences from just a handful of adverse events like heart attacks in a small town over just a half-year.

Still, it would be worth replicating these observations on a much larger scale -- in New York or LA. My guess is that a big-city study would not show such a massive decline in heart attacks. If smoking bans produced such a rapid fall, wouldn't we have noticed by now? With secondhand smoke having declined 75% between the early '90s and 1999, why didn't the national heart-attack rate plummet as it did in Helena?

The stampede to embrace the flimsy Helena study provides an illustration of the double standard surrounding scientific evidence about tobacco. One striking example concerns the virtues of smokeless tobacco. Smokers can dramatically reduce health risks by switching to smokeless products. Consider a Swedish experience: Although 40% of men in Sweden use tobacco products, Swedes have the lowest rate of lung cancer in the EU. Why? Largely because of moist snuff (or snus), which represents half of all the tobacco that Swedish men use. (The other half smoke.) Risks of mouth cancer, depending on the smokeless product used, range from negligible (with snus) to half the risk associated with smoking, for products like chewing-tobacco.

Compare this level of evidence with the study from Helena. Yet anti-smoking advocates embrace that idiosyncratic study while rejecting the growing scientific consensus -- based on replicated, large-scale studies -- that smokeless tobacco can be an effective strategy for harm reduction. A researcher at the National Cancer Institute wrote that "we know little" about "transition to other tobacco products (such as smokeless tobacco) as methods for reducing cancer risk." The surgeon general told Congress that "there is no significant scientific evidence that suggests smokeless tobacco is a safer alternative to cigarettes." Both are patently false.

Assuming that steering people toward better health is the goal here, we need a clear-eyed view of the data -- not a wishful guide to policy that overlooks strong evidence.

Dr. Satel is a resident scholar at the American Enterprise Institute.