Dr. Tim McAfee, director of the Office on Smoking and Health at Centers for Disease Control and Prevention, said the National Adult Tobacco Survey respondents were classified as having smoke-free rules if they never allow smoking inside their homes or vehicles.
The study, published in the journal Preventing Chronic Disease, found almost 11 million non-smoking adults continue to be exposed to secondhand smoke in their home, and almost 17 million non-smoking adults continue to be exposed to secondhand smoke in a vehicle.
The study's state-by-state data showed the highest prevalence of smoke-free rules in homes and vehicles occurred in many states with comprehensive smoke-free laws and longstanding tobacco control programs.
"We have made tremendous progress in the last 15 years protecting people in public spaces from secondhand smoke," McAfee said in a statement. "The good news is that people are applying the same protection in their homes and vehicles. However, millions of non-smokers, many of whom are children, remain exposed to secondhand smoke in these environments."
Eighty-nine percent of non-smokers report having smoke-free home rules, but 48 percent of smokers have them.
Eighty-five percent of non-smokers report having smoke-free vehicle rules, but only 27 percent of smokers do, the study said.