Dr. Saverio Stranges of the University at Buffalo in New York and colleagues had set out to investigate the association between cumulative lifetime exposure to other people's cigarette smoke and heart attack risk. They found no such link, but they did find a "dramatic decline" in reported second-hand smoke exposure over time.

"In the absence of high levels of recent exposure to second-hand smoke, cumulative lifetime exposure to second-hand smoke may not be as important a risk factor for MI (heart attack) as previously thought," the team writes in the Archives of Internal Medicine.

But evidence is mounting that short-term exposure can indeed be harmful, they add, pointing to recent studies depicting sharp drops in hospital admissions for heart attack after smoking was banned in workplaces or other public settings.

In the current study, the researchers had study participants report their exposure to second-hand smoke at home, work and in public from childhood to the present time. They compared 1,257 healthy individuals to 284 people who had suffered heart attacks. All were non-smokers ranging in age from 35 to 70.

While heart attack risk was the highest among those with the most exposure to second-hand smoke, the relationship was only significant for men's exposure to second-hand smoke in the workplace or in public, the researchers found. And after they controlled for other risk factors, the relationship disappeared.

The researchers also observed that reported exposure to second-hand smoke has dropped "sharply" in recent years. For example, three-quarters of study participants said they were exposed to cigarette smoke at home before the age of 21, but just 13 percent of women and about 12 percent of men said they were exposed to smoking at home recently, while 10.8 percent of women and 23.5 percent of men reported recent exposure to second-hand smoke in the workplace.

"The fact that exposure to second-hand smoke is declining is a positive development that is likely to contribute to reductions in mortality from CHD ," the researchers conclude.