Lung cancer appears to run in families, researchers said on Tuesday, though exposure to tobacco smoke is still the dominant cause of the disease even for those who may be genetically predisposed.

The strongest family link was found in the relatives of patients who developed the disease at age 60 or younger. The parents of such people had nearly a three and one-half times higher risk of also developing the disease compared to the general population, the study said.

For siblings in such cases the risk was more than three times higher and for children slightly less than that.

The genetic risk also extends beyond the immediate family, said the report from Landspitali-University Hospital in Reykjavik, Iceland. Aunts and uncles, cousins, nieces and nephews of lung cancer victims of any age also run a higher risk, though at lower levels than the immediate family.

Lung cancer is the leading cause of cancer deaths among men and women in many Western countries, said the study, published in this week’s issue of the Journal of the American Medical Association, and smokers comprise about 90 percent of all cases.

The Icelandic researchers said they traced the genetic links by looking at all 2,756 patients diagnosed with lung cancer in the country from 1955 to 2002 and linking them with an extensive genealogical database containing all living Icelanders and most of their ancestors since the settlement of the country.

“The nationwide genealogy database used in our study provided a means for uncovering the familial component by revealing more connections between patients, missed in most other populations,” the study said.

While the higher risk in immediate families may be related to second-hand smoke as well as genetics, it said, the higher risk found in persons outside the immediate family provides further evidence of a genetic link.

“Although the results presented here support a role for genetics in the risk of lung carcinoma, it should be emphasized that tobacco smoke plays a dominant role in…this disease, even among those individuals who are genetically predisposed,” the study concluded.

In another study published in the same journal, researchers in Taiwan reported that residents of that country who consumed drinking water with high levels of arsenic had a higher risk of lung cancer, with cigarette smoking adding to the risk.

Arsenic, a known cancer-causing agent, occurs naturally in soil and can contaminate drinking water. The study said people living on the southwestern and northeastern coasts of Taiwan had been exposed to arsenic in high concentrations in well water before a public tap water system was established.

Between 32 percent and 55 percent of lung cancer cases were estimated to be caused by the combined effect of cigarette smoking and ingested arsenic, depending on the levels of both exposures, the study said.

The study came from the College of Public Health, National Taiwan University, Taipei. It covered more than 10,000 people who were tracked for eight years.

SOURCE: Journal of the American Medical Association, December 22/29, 2004.