Using data from several cycles of the California Health Interview Survey, the study's authors estimate that 561,000 children are directly exposed to secondhand smoke in the home. Another 1.9 million are at risk because they live in a home where another family member is a smoker, even though smoking may not be allowed in the home itself.

Secondhand smoke exposes children to a greater risk of developing asthma, respiratory infections and countless other ailments. Research shows that children raised by smokers have a greater risk of becoming smokers themselves.

"The next frontier in the campaign against smoking is to reduce smoking at home," said Sue Holtby, the study's lead author and a senior researcher at the Public Health Institute, which works with the UCLA Center for Health Policy Research in conducting the California Health Interview Survey. "California's fight against tobacco has been a major public health success story, but we still need to spread awareness and ensure that every family knows the dire consequences of addiction."

Among the findings:

African American children three times more likely to live with smokers

Nearly 12.6 percent of African American children live in homes where smoking is permitted, three times the rate of any other racial or ethnic group. Both African American (13.4 percent) and white children (12.2 percent) are significantly more likely than other groups to have an adult or teen smoker in their household.

Income level and smoking

Children living in households at or above 300 percent of the federal poverty level (FPL) are far less likely to be exposed to secondhand smoke than children from lower-income levels.

Rural children at greater risk than urban

Approximately 19.4 percent of households in California's Northern/Sierra region and 14.6 percent of those in the San Joaquin Valley region have someone in the home who smokes. Comparatively, only 9.5 percent of households on the Central Coast contain a smoker. The Central Coast also has the lowest rate of households that permit smoking in the home. In contrast, close to 5 percent of homes in the San Joaquin Valley and Northern/Sierra regions permit smoking indoors.

Los Angeles paradox

Although Los Angeles doesn't have the highest percentage of smoking households (10.8 percent) it has a surprisingly high percentage (4.1 percent) of households with children where smoking in the home is allowed, relative to other regions.

The authors noted that the data can help identify communities that may benefit from targeted messages concerning the adverse health effects of secondhand smoke. In particular, media campaigns aimed at African American families, as well as families in the Los Angeles area, may effectively communicate the potential risks that secondhand smoke poses to the health of young children, the researchers said.

Service providers and case workers also have an opportunity to deliver smoking-prevention messages to low-income families eligible for a variety of state and federal assistance programs, such as Medi-Cal and the WIC (Women, Infants and Children) program. Since Medi-Cal and WIC providers usually screen patients and clients for smoking status, they are ideally positioned to identify those who are most at risk and to point parents toward information about smoking-cessation programs in their areas, the authors said.

The study was funded with a grant from First 5 California.

Read the policy brief: "Children's Exposure to Secondhand Smoke: Nearly 2.5 Million Affected in California" (http://healthpolicy.ucla.edu/NewsReleaseDetails.aspx?id=88).

Note: If no author is given, the source is cited instead.

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