Cavities are associated with low socioeconomic status, but the reasons why are unknown. Aligne theorized that second-hand smoke might be a risk factor. Although cavities have decreased in the entire population during the last few decades, there is one subgroup - young children who are poor - who are still particularly vulnerable.

Aligne and his colleagues analyzed data from the third National Health and Nutritional Examination Survey, which provided a nationally representative sample of 3,873 children.

"When people hear about the results of our study, their gut reaction is to say, All you did was test for poverty," Aligne says. "But this relationship between cavities and second-hand smoking persisted after we controlled for many variables, including age, sex, race, region, dentists visits, nutritional status and blood lead levels." There was also a dose-response effect. That is, the higher the exposure to smoke was, the more cavities the children had.

The children in the study had dental examinations and a blood test measuring their cotinine levels. An objective, quantitative marker of tobacco-smoke exposure, cotinine can reveal whether someone is a smoker or is often subjected to second-hand smoke. When people are exposed to tobacco smoke, they absorb nicotine into the body. In order to excrete the nicotine, the body turns it into cotinine.

According to a report last year from the U.S. Surgeon Generals office, many people underestimate the prevalence of cavities in children. Forty-seven percent of the children involved in Aligne's study had cavities in deciduous (baby) teeth and 26 percent had cavities in permanent teeth. Second-hand smoke was most associated with cavities in deciduous teeth. That makes sense because children who have not entered school are more dependent on their parents, spending more time with them and increasing their exposure if the parents smoke.

According to the Surgeon Generals report, dental problems can have substantial consequences. These include pain and suffering, problems associated with eating and speaking, and difficulty learning. It is estimated that dental problems cause children to miss more than 50 million hours of school time each year.

Aligne hopes this study will encourage more dentists to discuss the ill effects of smoking with their patients.

"Dentists want people to understand what a big problem cavities are, and I think they're right," Aligne says. "I didn't appreciate that early in my career. If dentists want to take the next step in the fight to prevent cavities, they should educate their patients about the harmful effects of smoking.

"If a child has a cavity, the dentist should explain to parents that smoking may be the cause," Aligne adds. "Im sure they say, Dont eat too many sweets, but perhaps they should also say, "Do you know what causes cavities? New research shows that second-hand smoke may cause cavities. Maybe that's another reason you should try to quit."