Women may be especially susceptible to the toxic effects of cigarette smoking, U.S. researchers said on Monday.

They said women who smoke develop lung damage earlier in life than men, and it takes less cigarette exposure to cause damage in women compared with men.

"Overall our analysis indicated that women may be more vulnerable to the effects of smoking," said Dr. Inga-Cecilie Soerheim of Brigham and Women's Hospital in Boston and the University of Bergen in Norway.

Soerheim, who presented her findings at the American Thoracic Society meeting in San Diego, California, said researchers had suspected this but until now had lacked proof.

Her team analyzed 954 people in Norway with chronic obstructive pulmonary disease, which includes lung problems from chronic bronchitis to emphysema.

COPD affects an estimated 210 million people worldwide. The most common symptoms include shortness of breath, coughing and a limited ability to exercise.

In the study, about 60 percent were men and 40 percent were women. All were current or former smokers.

Overall, both groups had similar lung impairments. But when they looked at younger people — those under age 60 — or those who had been lighter smokers, they found women had more severe disease and worse lung function than men.

"This means that female smokers in our study experienced reduced lung function at a lower level of smoking exposure and at an earlier age than men," Soerheim said in a statement.

Soerheim suspects the differences may be related to anatomy. Women have smaller airways than men, making each cigarette potentially more dangerous, she said. Hormones may also play a role, she said.

"Many people believe that their own smoking is too limited to be harmful — that a few cigarettes a day represent a minimal risk," she said in a statement. "However, in the low exposure group in this study, half of the women actually had severe COPD."

An estimated 12 million people in the United States have COPD, the fourth-leading cause of death.