The researchers suggest that sharp decline may be due to the near-elimination of the rapid and harmful effects of secondhand smoke on blood platelets and the arteries that supply blood to the heart.
Also, when public places are declared smoke-free, smokers are forced or encouraged to quite or reduce smoking. That also lowers their heart attack risk.
The public smoking ban went into effect in Helena on June 5, 2002, and was suspended six months later due to a legal challenge.
The UCSF researchers analyzed the records at St. Peter's Community Hospital, which serves most of the region's cardiac patients. During an normal six-month period, the average number of people living or working in Helena admitted to the hospital with acute myocardial infarction (AMI) is just under seven per month.
During the smoking ban, that dropped to less than four per month, a 60 percent reduction in AMI admissions. The study found no significant change in the number of admissions for people living outside the Helena area.
This is the first empirical evidence to indicate that smoke-free policies have an immediate impact on heart attack rates, the authors note.
"This striking finding suggests that protecting people from the toxins in secondhand smoke not only makes life more pleasant; it immediately starts saving lives," Stanton Glantz, UCSF statistics expert, says in a news release.
"This work substantially raises the stakes in debates over enacting and protecting smoke-free ordinances," says Glantz, a professor of medicine at the UCSF Cardiovascular Research Institute who provided the study's statistical analysis.
Here's where you can learn more about secondhand smoke.