It seems reasonable to assume that the proportion would be similar for heart attacks, fatal or not. So even if a city completely eliminated secondhand smoke (which Helena's ban did not do, since it did not apply to smoking at home), how could that possibly cut heart attacks in half?
Sargent and Glantz note that smoking bans also encourage smokers to cut back or quit. Inconveniently for them, that point suggests that any drop in heart attacks could be due to less smoking rather than less exposure to secondhand smoke.
In any case, the numbers still don't add up. According to the U.S. Centers for Disease Control and Prevention, smoking accounts for about one-fifth of heart disease deaths. So even if every smoker in Helena quit (which no one claims happened), you would not get anything like the drop that Sargent, Shepard and Glantz attribute to the ban.
If smoking bans cut heart attacks in half, it's odd that no one has noticed it before, especially in big cities such as Los Angeles and San Francisco, where an effect of such magnitude should have been obvious. Indeed, why didn't Glantz study hospital data in places within his own state where the samples would have been much bigger and the results more meaningful, instead of focusing on what Sargent calls "a tiny little community in the middle of nowhere"?
California's ban on smoking in workplaces took effect in 1995; it was extended to bars in 1998. Yet according to CDC data, the number of heart disease deaths in California did not drop substantially in either year. If smoking bans cut heart attacks in half, surely the effect would have shown up in these numbers.
Likewise, Delaware should see a sharp drop in heart attacks now that it has banned smoking in all workplaces. So should New York City, where a smoking ban took effect this month, and New York state, which recently passed a ban that takes effect in July. Ditto Boston, Chicago, Dallas and Florida. Or is there something special about hearts in Helena?