We've all heard the dangers of second-hand smoke to humans, but what about the risks to our pets?

Cats living in homes where people smoke cigarettes are more than twice as likely as other cats to develop a deadly form of cancer known as feline lymphoma, according to researchers at Tufts University School of Veterinary Medicine and the University of Massachusetts.

The researchers said the findings, which are published in the Aug. 1 issue of the American Journal of Epidemiology, offer a compelling reason for further study of the relationship between second-hand smoke and non-Hodgkins lymphoma in humans, which is similar to lymphoma in cats.

"It has long been believed that the major cause of feline lymphoma was feline leukemia virus," said Antony Moore, a veterinary oncologist and director of Tufts' Harrington Oncology Program. "The results of our study clearly indicate that exposure to environmental factors such as second-hand tobacco smoke has devastating consequences for cats because it significantly increases their likelihood of contracting lymphoma."

Several recent studies have suggested that people who smoke tobacco may have an increased risk of contracting non-Hodgkins lymphoma. And other studies have suggested that children of parents who smoke may have an increased risk of developing lymphoma. However, the results of these studies are often hard to prove due to myriad other risk factors that people face, the researchers said.

In this study, researchers studied 180 cats that were treated at Tufts Veterinary School's Foster Hospital for Small Animals between 1993 and 2000. Eighty of the cats were treated for lymphoma and 100 were treated for renal failure.

The researchers found that the risk of lymphoma for cats exposed to any household environmental tobacco smoke was more than double (2.4) that of cats not exposed to tobacco smoke. Cats that were exposed for five or more years had a risk of more than triple (3.2) that of other cats.

The number of smokers in the home had an effect on the results. Cats living with one smoker had about 1.9 times the risk of lymphoma than cats with smoke-free owners. But cats living with two or more smokers had a four-fold increase in risk of the disease.

In sharing their homes with humans, cats are exposed to many of the same environmental contaminants as their owners, including tobacco smoke. And exposure levels in cats continuously kept indoors may actually be higher than those of human household members, who often spend extended periods of time outside their homes.

Cats may become exposed by inhaling the smoke or by ingesting it when they groom themselves and lick particulate matter off their fur.

"We believe that feline exposure patterns to environmental tobacco smoke may mimic those of young children living in households where adults smoke and where the children inhale tobacco smoke or ingest particulate matter by mouthing contaminated objects," said lead researcher Elizabeth Bertone from the Department of Biostatistics and Epidemiology at the University of Massachusetts in Amherst.

"Our findings offer another reason for smokers living with pets and children to try to 'kick the habit,'" Bertone said. "Quitting smoking will not only reduce their risk of cancer, heart disease and diabetes, but may reduce the risk of cancer in their children and pets as well."

Feline lymphoma is the most common cancer in cats, and often involves their intestinal tracts. Cats that contract lymphoma are usually about 10 years old.

The typical treatment for the disease involves chemotherapy and possibly radiation therapy for a course of about six months. The cost of treatment is $2,000 to $3,000. Approximately 65 percent of cats that receive treatment go into remission, and about 25 percent of them survive more for more than two years.