Chicago ? Heart attacks in Helena, Mont., fell by more than half last summer after voters passed a broad indoor smoking ban, suggesting that cleaning up the air in bars and restaurants quickly improves health for everyone, a study found.

Doctors said their study, which they described as a kind of “natural experiment,” is the first to examine what happens to public health when people stop smoking — and breathing secondhand smoke — in public places.

The doctors, themselves backers of the ban, acknowledged the effects need to be demonstrated in a larger locale. But despite the small numbers involved, they said Helena’s experience offers a clear hint that the change reduces the risk of heart attacks for smokers and nonsmokers alike from virtually the moment it goes into effect.

People who worry about secondhand smoke often fear lung cancer most, but that takes years of exposure.

Smoking is also a powerful trigger of heart attacks and it works quickly to increase the risk by raising blood pressure, increasing the tendency of blood to make clots.

The American Heart Assn. estimates 35,000 nonsmokers die each year from the effects of secondhand smoke on the heart. It said the Helena data should encourage more cities to limit smoking.

Helena’s smoking ban was adopted by voters in June and lasted for six months, until enforcement was suspended after a legal challenge. It “led to an immediate and dramatic decline in the number of heart attacks we saw,” Dr. Richard Sargent said.

Heart attacks climbed back to their usual level after smoking returned to bars, restaurants, casinos, bowling alleys and other public places in December.

Sargent, who with co-author Dr. Robert Shepard encouraged passage of the ordinance, presented the data Tuesday to applause at the annual scientific meeting of the American College of Cardiology.

“It is an extremely important study with implications for all cities,” said Dr. Guy Reeder of the Mayo Clinic.

The doctors kept track of all heart attacks in Helena, population 26,000, and compared that with the previous four years, adjusting the figures for seasonal ups and downs. During the six months the ordinance was enforced, heart attacks in Helena dropped from an average of seven a month to three.

They found the smoking ban appeared to have its biggest effect on smokers. Their incidence of heart attacks dropped by three-quarters, compared with two-thirds for former smokers and half for nonsmokers.

“This is a tiny, little community in the middle of nowhere,” Sargent said. “This study needs to be replicated in New York City.”

A rule that took effect Sunday in New York bans smoking in city workplaces. Last week, New York passed a statewide smoking ban, joining California and Delaware with stringent rules on indoor smoking. A Florida ban passed by voters last year takes effect in July.

Next month, indoor smoking becomes illegal everywhere in Boston except private homes, hotel rooms and some cigar bars. Seventy-four other Massachusetts communities have banned smoking in bars and restaurants.

Dallas also recently banned restaurant smoking, as have a scattering of other smaller cities around the country.

The Helena study is likely to provide ammunition for those backing these bans. “This gives us real data and proof that this needs to be done,” said Karla Cutting, a spokeswoman for a Chicago campaign to prohibit workplace smoking.