Secondhand smoke isn't just a health threat to people. It can also hurt dogs and cats, veterinarians say.

According to the Centers for Disease Control and Prevention, more than 126 million Americans who don't smoke are exposed to secondhand smoke in their homes, vehicles, workplaces, and public places. This exposure causes thousands of lung cancer and heart disease deaths among nonsmokers every year, according to the California Environmental Protection Agency.

Making the leap from the effects of secondhand smoke on humans to their effects on pets isn't a big one, says veterinarian Carolynn MacAllister of Oklahoma State University.

"There have been a number of scientific papers recently that have reported the significant health threat secondhand smoke poses to pets," MacAllister said. "Secondhand smoke has been associated with oral cancer and lymphoma in cats, lung and nasal cancer in dogs, as well as lung cancer in birds."

Mouth cancer in cats

MacAllister cited a study done by the Tufts College of Veterinary Medicine that showed that the number of cats living with mouth cancer (also known as squamous cell carcinoma) was higher for those living in homes with smokers than those who lived in smoke-free environments.

"One reason cats are so susceptible to secondhand smoke is because of their grooming habits," MacAllister said. "Cats constantly lick themselves while grooming, therefore they lick up the cancer-causing carcinogens that accumulate on their fur. This grooming behavior exposes the mucous membranes of their mouth to the cancer-causing carcinogens."

Cats living with smokers are also twice as likely to develop malignant lymphoma, a cancer that occurs in the lymph nodes and that is fatal to three out of four cats within 12 months of developing it.

Lung and nose cancer in dogs

Studies have also shown that dogs living in a smoking household are susceptible to cancers of the nose and sinus area, particularly if they are a long-nosed breed, because their noses have a greater surface area that is exposed to carcinogens and a greater area for them to accumulate. Dogs affected with nasal cancer normally don't survive for more than one year.

Short and medium-nosed dogs are more susceptible to lung cancer, "because their shorter nasal passage aren't as effective at accumulating the inhaled secondhand smoke carcinogens," MacAllister said. "This results in more carcinogens reaching the lungs."

Birds are also at risk for lung cancer, as well as pneumonia, because their respiratory systems are hypersensitive to any type of air pollutant.

To help prevent animals from being adversely affected by smoking, pet owners who smoke should have a designated smoking area that is separated from the home or stop smoking altogether, MacAllister said.