The number of smoke-free homes in the United States has nearly doubled in the past 20 years, according to new data from the Centers for Disease Control and Prevention (CDC).

The CDC study found 83 percent of homes were smoke-free in 2010-2011, compared with 43 percent in the early 1990s, Reuters reports. More than 90 percent of homes without a smoker and almost half of those with at least one adult smoker did not allow smoking in the home, the study found.

“It’s a shift in social norms,” said lead study author Brian King. “People no longer see smoking around non-smokers as socially acceptable behavior.” Secondhand smoke from cigarettes continues to kill an estimated 41,000 non-smokers each year, he noted. “We know there is no safe level of secondhand smoke. The ultimate goal is to not expose people to a known carcinogen.”

In 2012, an estimated 18 percent of Americans smoked, compared with 42 percent in 1965. The percentage of smoke-free homes ranged from 69.4 percent in Kentucky and West Virginia to 93.6 percent in Utah in 2010-2011.