Smoking accounts for most of the 16 million cases of chronic obstructive pulmonary disease (COPD), but workplace exposure to dust or fumes might account for as many as five million cases of the disease, according to a new study from the University of California, San Francisco.

COPD involves chronic lung inflammation, a narrowing of the airways and increasingly severe breathing difficulties. Researchers note that COPD kills more than 100,000 people a year.

"Although smoking prevention remains paramount, controlling hazardous exposures can also have an important role in reducing COPD," said Paul Blanc, senior author of the study.

The study is based on interviews of more than 2,000 people. The National Institutes of Health funded the study. Researchers found that chronic bronchitis, emphysema and related diseases are twice as frequent in people who have been exposed to toxic airborne substances in the workplace, regardless of whether they smoke.

More than half of the people with COPD reported that they had been exposed to airborne toxins in the workplace, while only one in three of those free of respiratory disease reported such exposure. Researchers say their analysis shows that exposure to various workplace toxins doubles the risk of COPD, and that workplace exposure is a contributing cause of 20 percent to 31 percent of the diagnosed COPD cases.

In November 2002, NIOSH reported that more than 20 million workers in the U.S. are exposed to gases, vapors, fumes and dusts that can cause COPD.