Feb. 29 (HealthDay News) -- While smoking is the leading risk factor for chronic bronchitis, genes also play a major role in the development of the disease, say Swedish researchers.

The study, which analyzed data on more than 40,000 twins born in 1958 or earlier, found that inherited genes accounted for 40 percent of the risk for chronic bronchitis and that 14 percent of the genetic risk was also linked to a genetic predisposition to smoke, whether or not a person actually smoked.

The findings are published in the first issue for March of the American Journal of Respiratory and Critical Care Medicine.

"(This) study on the population-based Swedish Twin Registry, showing a genetic effect for the development of chronic bronchitis that does not differ by sex, is the first to our knowledge to quantify heritability of the disease," Jenny Hallberg, of the department of public health sciences at Karolinska Institutet in Stockholm, said in a prepared statement.

Previous research had suggested that women were more likely than men to develop chronic bronchitis, so the findings that prevalence didn't differ by sex pointed to a number of intriguing possibilities, she noted.

"It is possible that women are more prone to report symptoms. Or, more likely, this could be an effect of smoking being more harmful for women due to their smaller lungs from start [exposure to cigarette smoke relative to body size]," Hallberg wrote.

The finding that genetic factors that contribute to chronic bronchitis are largely independent of smoking shouldn't be interpreted to mean that smoking has no effect on the disease, she cautioned.

"Although there was some genetic interplay, it is safe to say that smoking itself, and not the genes that predispose one to smoking, is a larger risk factor in developing chronic bronchitis of environmental exposures -- primarily smoking -- than genetic predisposition. This is true of both men and women," Hallberg wrote.

Chronic bronchitis and emphysema account for most cases of chronic obstructive pulmonary disease (COPD).

SOURCE: American Thoracic Society, news release, Feb. 29, 2008