Smoking is forbidden in nearly three out of four U.S. households, an increase from 43 percent a decade ago.
The Centers for Disease Control and Prevention, which conducted the survey in 2003, said it was the first study to offer a state-by-state look at the prevalence of smoking in American homes.
Utah led the nation, with people in nearly nine out of 10 homes saying smoking was never allowed. The state's large population of Mormons, who eschew tobacco, probably contributed to that statistic, the agency said.
Kentucky, a tobacco-growing state, was in last place, with a little more than half of the households sending smokers outside (or, at least, to the garage).
But even in Kentucky, smokers found fewer place to light up. Ten years earlier, only a quarter of the state's households barred smoking.
"That really says that people are starting to understand the hazards of secondhand smoke," said Dr. Corinne Husten, co-author of the study and chief of the epidemiology branch of the CDC's Office on Smoking and Health.
The report is based on a national survey done mostly by telephone every two years. For a household to be included in the results, everyone 15 and older had to respond, and they all had to agree on the smoking rules.
The survey covered 127,000 U.S. households in 2003, the most recent year for which such data was available. The study looked at 900 to 7,000 homes in each state. Similar numbers were surveyed in previous years.
Participants were asked whether smoking was allowed everywhere in the home, only in some places, or not at all.
Among households with at least one smoker, the national prevalence of take-it-outside rules rose from about 10 percent in the early 1990s to 32 percent in 2003. Among households with no smokers, the percentage with such rules rose from 57 percent to almost 84 percent.
The CDC said the increases were driven in part by scientific reports and other information in the last 15 years warning that secondhand smoke causes premature death and disease.
A growing number of state and local smoking bans in restaurants, bars and workplaces may also have been influential at home, Husten said.
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.
The choice of the city of Helsinki is not incidental as the capital of Finland had hosted US-Soviet negotiations on the limitation of nuclear stockpiles in 1969