Los Angeles ? Blacks who smoke up to a pack a day are far more likely than whites who smoke similar amounts to develop lung cancer, suggesting genes may help explain the racial differences long seen in the disease, researchers say.

The largest study ever done on the subject also found that Hispanic and Asian smokers were less likely than black smokers to develop the disease – at least up to a point. The racial differences disappeared among heavy smokers, or those who puffed more than a pack and a half per day.

Doctors have long known that blacks are substantially more likely than whites to develop lung cancer and more likely to die from it. But the reasons for the disparity are unclear.

Some say the difference is a matter of genetics, while others contend smoking habits may play a role. For example, researchers say blacks tend to puff more deeply than whites, which may expose them to more carcinogens. Smoking rates are also slightly higher among blacks, but whites tend to smoke more cigarettes a day.