A growing number of state and local smoking bans in restaurants, bars and workplaces may also have been influential at home, Husten said.

Loyd Silberstein, a retired school teacher in California, said he smokes at home — but not when his children or grandchildren come over. On those occasions, he goes out to the back yard or garage.

"My wife says I don't care about her, just the kids," laughed Silberstein, 75, of San Mateo.

The study was published the CDC's Morbidity and Mortality Weekly Report.

In another MMWR article this week, surveys of nearly 750,000 teens in 137 countries and territories showed that students exposed to smoking at home were most likely to take up the habit themselves.

The study found that more than 71 percent of nonsmoking students surveyed in Europe said they were exposed to cigarette smoke at home. The exposure was much lower in other parts of the world — particularly in Africa, where the statistic was just 23 percent.