The dangers of secondhand smoke to people are well documented, but a new study has proven that it is dangerous to cats as well. A study published in 2002 in the American Journal of Epidemiology by Dr. Elizabeth Bertone, an epidemiologist, and Drs. Anthony Moore and Laura Snyder, both veterinarians, demonstrated a link between exposure to secondhand smoke and lymphoma, the most common form of cancer in cats. The study found that cats exposed to any level of household tobacco smoke were two and a half times more likely to develop lymphoma than cats that are not exposed to smoke. The incidence of lymphoma increased according to the duration and level of the cat's tobacco exposure, leaving those exposed five or more years at more than triple the risk of that of unexposed cats. Those cats that lived with two or more smokers had four times the rate of lymphoma.

Lymphoma is usually seen in middle age and older cats. It most commonly affects the intestines or the nasal passages, but it can also affect the chest, kidneys, or other organs. Chemotherapy and radiation can allow some patients to go into remission, but treatment can be expensive and many patients will not respond. Lymphoma has historically been believed to be caused by the feline leukemia virus. The incidence of leukemia has decreased over the years thanks to testing and vaccination, yet the incidence of lymphoma is increasing. This observation is what prompted the study to find what else may be causing this type of cancer.

Indoor cats seem to have a greater disadvantage than humans in terms of exposure to tobacco smoke because human household members often leave the home, whereas cats face continuous exposure. In addition, cats are exposed not only through inhaling but by swallowing particles of smoke and toxins which have settled on their fur and household surfaces when they lick their fur and paws. "We believe that feline exposure patterns to environmental tobacco smoke may mimic those of young children living in households where adults smoke," said Dr. Bertone. "Quitting smoking may not only reduce the risk of heart disease, cancer and diabetes in smokers but also of cancer in children and pets."