The lead researcher on the work said that decline was likely due to less secondhand smoke exposure in restaurants and bars, as smoke can trigger heart problems due to its effects on arteries and blood clotting.
Researchers from the Mayo Clinic in Rochester, Minnesota found that in the 18 months before the first ordinance was enacted, the rate of heart attacks in Olmsted County was 151 for every 100,000 people. By the 18 months following the second ordinance, that fell to 101 per 100,000 people.
Dr. Richard Hurt said a few other studies, including one from Montana, have also suggested smoke-free workplace laws could impact heart attack rates.
But, he told Reuters Health, "There have been lingering doubts among some people about whether or not this was a real finding. We think we have produced the most definitive results that anyone can produce related to smoke-free laws and heart attacks."
Hurt, who led the research, said other predictors of heart attacks - including cholesterol levels, blood pressure and diabetes and obesity rates - all held steady or increased in Olmsted County over the study period.
"The only thing that really changed here was the smoke-free workplace laws," he said.
About 3,600 municipalities have laws on the books that restrict where people may smoke, according to the American Nonsmokers Rights Foundation, with more than 1,000 including a smoke-free provision of some kind.
According to Hurt, the findings also make sense biologically. Exposure to secondhand smoke can cause immediate changes in the lining of the aorta, and can make blood platelets stickier - so they're more likely to form a dangerous clot.
The study is in line with the Institute of Medicine, which advises the U.S. government and said in a 2009 statement that, "data consistently demonstrates that secondhand-smoke exposure increases the risk of coronary heart disease and heart attacks and that smoking bans reduce this risk."
Hurt's results are published in the Archives of Internal Medicine.
He and his colleagues tried to account for general changes in heart attack rates that might have happened regardless of smoke-free laws. They found heart attacks had been declining even in the years before the ordinances - but fell much faster once they were put in place.
But that analysis wasn't convincing enough for Dr. Michael Siegel, a tobacco control researcher from the Boston University School of Public Health.
"The main problem with this study is that there's no control group or comparison group. This is a look only at what happened before and after an ordinance went into effect in one particular county," Siegel, who wasn't involved in the research, told Reuters Health.
"We have no idea whether heart attacks may also have gone down in other counties in Minnesota that didn't have this smoke-free law. Without knowing that, I don't see how you can make the conclusion that this (decline) was due to the law."
Siegel said there is more-convincing evidence that smoke-free workplace laws can help protect against asthma attacks and other lung problems.
"We don't need to prove that they actually decrease heart attacks over a short period of time," he said. "There's lots of reasons to support (these laws)."
SOURCE: http://bit.ly/MbBLbb Archives of Internal Medicine, online October 29, 2012.