Another factor is the drop in the number of adult smokers: It has now inched below 20 percent, according to 2007 CDC data.
The new study found about 46 percent of nonsmokers had signs of nicotine in their blood in tests done from 1999 through 2004. That was a steep drop from 84 percent when similar tests were done in the late 1980s and early 1990s.
But health officials stopped short of celebrating.
"It's still high," said Cinzia Marano, one of the study's authors. "There is no safe level of exposure."
Cigarettes cause lung cancer and other deadly illnesses not only in smokers, but also in nonsmokers who breathe in smoke, studies have shown.
For nonsmoking adults, secondhand smoke increases their lung cancer risk by at least 20 percent and their heart disease risk by at least 25 percent. Children exposed to secondhand smoke are at increased risk of asthma attacks, ear problems, acute respiratory infections and sudden infant death syndrome, health officials say.
The new CDC report drew its data from the National Health and Nutrition Examination Survey, a unique government study that sends mobile trailers out to communities. Participants are asked about their health, get blood tests and physical exams.
The blood tests check for cotinine -- a byproduct of nicotine that usually is detectable for up to four to five days.
The blood tests are important, because many people underestimate their exposure to secondhand smoke, said Terry Pechacek, associate director for science in the CDC's Office on Smoking and Health.
The new report focused on data collected on about 17,000 nonsmokers in the years 1988 through 1994, and about the same number in the years 1999 through 2004. People ages 4 and older were included.
The decline is secondhand smoke exposure was not as dramatic in black nonsmokers as it was in whites and Mexican-Americans. The proportion of blacks with a recent exposure to tobacco smoke dropped from 94 percent to about 71 percent, for whites it dropped from 83 percent to 43 percent and for Mexican-Americans, 78 percent to 40 percent.
Also troubling -- the exposures for children did not decline as dramatically as it did for adults. More than 60 percent of children ages 4 through 11 had recent exposure to cigarette smoke in the 1999-2004 period, the researchers found.
"Obviously, the exposure is at home," said Thomas Glynn, the American Cancer Society's director for cancer science and trends.
It's not clear if adult smokers are smoking more at home or in their cars because of the bans. But they're probably not smoking much less in those places, which would explain why their kids' exposure to tobacco smoke didn't decline as much as their friends' and co-workers', CDC officials said.
"Parents need to be aware that this is very dangerous, and they need to take actions to ensure that their children are not exposed," Pechacek said.
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