CDC lists smoke-free laws for public places and work spaces, an overall decrease in smoking rates, and a social stigma against smoking around nonsmokers as likely reasons for this decline.
The home is the primary source of secondhand smoke exposure in children. In 2006 the U.S. Surgeon General reported secondhand smoke is never safe. More than 7,000 chemicals are found in secondhand smoke, hundreds of which are toxic and about 70 of which cause cancer. Secondhand smoke kills an estimated 41,000 nonsmoking American adults each year, and causes 150,000-300,000 new cases of bronchitis and pneumonia in children annually. Since 1964, 2.5 million nonsmokers have died from exposure to secondhand smoke.
Read the NPR story on the report.