It's true. Secondhand smoke affects pets as well as people. Just take a look at these study results.
A 2001 Colorado State University study, published in the American Journal of Epidemiology, showed secondhand smoke can cause cancer of the sinus in dogs. The study stated dogs with long noses were twice as likely to develop cancer of the sinus than those with short or medium noses. However, dogs with short or medium noses were more likely to develop lung cancer from secondhand smoke than those with longer ones.
A 2007 University of Minnesota study showed that cats who live with smokers have nicotine and other toxins in their urine.
A 2007 Tufts School of Veterinary Medicine study showed a connection between secondhand smoke and mouth cancer in cats, called squamous cell carcinoma.
A recent study from the University of Massachusetts showed that cats exposed to smoke are twice as likely to develop lymphoma, a cancer of the lymph nodes, than those that aren't.
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