Everyone knows that secondhand smoke is bad for the lungs, but a new study suggests it might also increase the risk of diabetes.

The long-term study of more than 4,500 American men and women found the incidence of glucose intolerance -- a precursor to diabetes in which the pancreas can no longer produce enough insulin to regulate blood sugar -- was directly related to exposure to tobacco smoke.

After 15 years of follow-up, the researchers found smokers had the highest risk of glucose intolerance, with 22 percent of them developing the condition. But 17 percent of those who never smoked but were exposed to secondhand smoke developed the condition, a rate higher than the 14 percent found in smokers who gave up the habit. Only 12 percent of people who never smoked developed glucose intolerance.

The researchers also found that whites were more susceptible to this effect than blacks.

The findings appear in the April 8 issue of the British Medical Journal.

Exposure to secondhand smoke was measured in two ways, said study author Dr. Thomas Houston, an assistant professor of medicine at the University of Alabama and a researcher at the Birmingham Veterans Affairs Medical Center. "One was self reports of being around secondhand smoke," he said. "The second was measurements of blood levels of cotinine, a breakdown product of nicotine."

For smokers, there was a direct relationship between the number of cigarettes smoked and the incidence of glucose intolerance. "For every increase of 10 pack years of smoking, the risk of developing glucose intolerance increased by 18 percent," the report said.

Because it was an observational study, without strict controls on the participants' behavior, it was "the first step toward a conclusion, not the final answer," Houston said. But he noted that every effort was made to account for other factors associated with the development of glucose intolerance, such as age and body weight.

"I haven't heard about this before," said John Banzhaf, executive director of Action on Smoking and Health. "So far as I know, this is the first study to make this connection."

Secondhand smoke could be more dangerous for the pancreas, where insulin is produced, than the fumes inhaled directly by smokers, the researchers said. The toxic substances in secondhand smoke are produced at different temperatures and under different chemical conditions. So some toxins that damage the pancreas might be at even higher concentrations in secondhand smoke than in smoke that is inhaled directly, they said.

That stands to reason, Banzhaf said. "I know that the secondhand smoke is different from, and in many ways more dangerous than, directly inhaled smoke," he said. "It's the difference between a well-ventilated fire in a fireplace and one where there is not enough ventilation to produce proper oxygenation."

If tobacco smoke -- whether secondhand or inhaled directly -- is a risk factor for diabetes, "that would be very important and frightening because of the other trend of increasing obesity," Banzhaf said. "When you put the two together, you have a really deadly combination."

Dr. Robert Rizza, president of the American Diabetes Association, said the new study provides "one more reason why it is best not to be exposed to passive smoke."

That warning probably applies most urgently to young people, Rizza said. It's possible that there are critical periods in development when toxic substances are more dangerous. So, he added, "the prudent thing is to be sure your children aren't exposed to passive smoke."