While heavier exposure to secondhand smoke is worse, there's no safe level…even less than an hour's exposure might be enough to push someone already at risk of a heart attack over the edge.

That's because within minutes, the smoke's pollution-like small particles and other substances can start constricting blood vessels and increasing blood's propensity to clot — key heart attack factors. Yet many people don't know they have heart disease until their first heart attack, making it important for everyone to avoid secondhand smoke, Benowitz said.

"Even if you think you're perfectly healthy, secondhand smoke could be a potential threat to you," he said.

Many of the IOM committee members initially were skeptical they'd find much benefit from the bans, said statistician Stephen Feinberg of Carnegie Mellon University. He proclaimed himself "the resident skeptic" who changed his mind. "There was a clear and consistent effect of smoking bans," he said.