The CDC disclosed its new advisory in a commentary to a study published in the British Medical Journal yesterday, saying doctors need to warn people with heart problems that secondhand smoke can significantly increase their risk of a heart attack. The agency said that as little as 30 minutes' exposure can have a serious and even lethal effect.

The commentary accompanied a study showing that the number of heart attacks in Helena, Mont., decreased substantially after the city banned indoor smoking, then rose quickly to its former level after the law was struck down in court.

That study found that during the six-month period in 2002 when the ban was in effect, the number of heart attacks reported by Helena's heart hospital fell by 40 percent.

In his commentary, Terry Pechacek, associate director of science at CDC's Office on Smoking and Health, wrote that the research underscores evidence that secondhand smoke rapidly increases the tendency of blood to clot, which can restrict flow to the heart.

Pechacek said the new study strengthens the growing body of research pointing to potentially fast and acute reactions to secondhand smoke, in addition to the long-term damage done to nonsmokers who live with smokers. The CDC has estimated that secondhand smoke causes 35,000 heart disease deaths a year in the United States, but Pechacek said that estimate is likely to be revised upward.

"We've said before that secondhand smoke increases the risk of heart disease in nonsmokers, but this is our first recommendation that clinicians advise their patients with heart disease to avoid indoor settings where smoking is allowed," he said in a telephone interview.

"We don't make these kind of statements lightly," he said. "What we are seeing in the data is a substantial biological change that occurs with even 30 minutes of exposure to secondhand smoke."

The new CDC recommendation is bound to become part of the often acrimonious national debate over whether smoking in public places should be banned. Public health advocates say the bans will save many lives, while cigarette makers say the decision should be left to individual choice.

Just yesterday, the Kentucky Supreme Court upheld a ban on smoking in bars, restaurants and other public places in Lexington, ruling that the city had acted within its authority to "promote and safeguard public health."

That ban has drawn national attention because Kentucky has the highest smoking rate in the nation -- about one-third of adults there are smokers, according to the CDC -- and is the second largest producer of tobacco.

As both the CDC and authors of the new study acknowledge, the Montana data are limited by the relatively small number of people involved. Pechacek said that similarly dramatic reductions in heart attacks are unlikely to be found in larger populations, but he said the study is nonetheless important because it offers the best real-world information to date on the connection between indoor smoking and serious heart problems. He said studies have been proposed or begun into how the indoor smoking bans in California, New York City and Delaware have affected heart attack rates.

The study's authors, Richard P. Sargent and Robert M. Shepard of St. Peter's Community Hospital in Helena, Mont., and Stanton A. Glantz of the University of California at San Francisco, collected information about the number of heart attacks from St. Peter's hospital records.

During the six-month period in 2002 when the indoor smoking ban was in effect, 24 Helena residents suffered acute heart attacks. For the five years before and after 2002, the average number of heart attacks reported for Helena residents during the same six months was 40. The authors found through St. Peter's records that the number of heart attacks suffered by people living in the area outside Helena -- where there was no smoking ban -- did not experience the same 2002 dip as Helena.

Of the patients followed in the study, 38 percent were current smokers, 29 percent were former smokers and 33 had never smoked.

Pechacek wrote: "If future studies replicate the positive results from the Helena study, the public health implications would be dramatic: thousands of acute [heart attacks] among non-smokers in countries around the world could potentially be prevented each year."