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Are Dogs Who Live with Smokers More Likely to Get Cancer?

Dogs, like humans, can suffer from health problems because of tobacco smoke.

Modified from a KleinYan photo — Creative Commons License
Source: Modified from a KleinYan photo — Creative Commons License

I was walking across the campus of my university when I when I saw one of my faculty colleagues sitting on a bench next to his wife. She waved me over, and I noticed a concerned look on her face. When I got there she told me in a rather worried voice, "You have got to help me convince Dan that he should stop smoking. If not for himself — at least for Lilly." It turns out that Lilly was a new spaniel puppy that they had just brought home from the breeder. The woman's concerns had to do with secondhand smoke from her husband's cigarettes. This is the tobacco smoke that people breathe in when they are near smokers. It has been shown that such secondhand smoke is associated with an increased incidence of lung cancer which was her concern for the pup.

Dan shook his head and waved one hand in dismissal, "Dogs don't get lung cancer. Remember that they are down at floor level and the smoke that gets breathed in is high up around the level where a smoker's mouth is."

While lung cancer is relatively rare in dogs it does happen. In fact, the 1970 study which served as conclusive proof that smoking tobacco causes lung cancer involved 36 Beagles that had been trained to smoke heavily. Of these 12 developed lung cancer. Prior to that study the tobacco industry could claim that there was no evidence that cigarette smoking caused lung cancer. The fact that heavy smokers were 20 times more likely to die of lung cancer than non-smokers was dismissed as merely "a statistical association" that did not prove a cause and effect relationship.

Of course, the fact that a dog who has been trained to smoke develops lung cancer is a lot different than presuming that the same problem will occur in dogs who are merely living in a household with a smoker. Nevertheless there is a small but growing body of research which suggests that secondhand cigarette smoke may harm pets as well as humans. For example there was a study published in the American Journal of Epidemiology* reporting work done by a group of investigators headed by John S. Reif, a professor of environmental health at Colorado State University. His team looked at 51 dogs with lung cancer and 83 dogs with other cancers and found that dogs in households which contained smokers had a 60% higher risk of lung cancer.

It turns out that the shape of the dogs head is also a factor, since the risk was much higher for dogs with short muzzles (like Pugs) or medium-sized muzzles (like Labrador Retrievers). Still the long nosed dogs were not immune to the cancerous effect of smoking. Reif was also the principal investigator in a larger study published later in the same journal**. This study involved 481 dogs with cancer and found that long nosed dogs, like greyhounds and collies, were twice as likely to get nasal cancer if they lived with smokers. Reif speculated that the reason for this was that carcinogens from tobacco smoke become trapped in their elongated nasal passages.

My colleague felt that the issue of secondhand smoke affecting his dog was unlikely since his argument was that the dog is short and the smoke is concentrated high in the air above the dog, where the smoker's mouth is. However his argument overlooks what researchers call "thirdhand smoke". Even if a person chooses to smoke outside of their home, or only smokes when their dog is not in the room with them, they are still exposing their pets to toxins. The toxic particulate matter from tobacco smoke gets into the person's hair and clothing so that when you later come into contact with your dog he is exposed to these toxins. In addition the smoke will eventually settle and contaminate surfaces such as cushions, carpets, and the floor. The dog may lay on these and when he later grooms himself he ends up consuming some of these toxins. And there are a lot of these toxins. Researchers have found around 250 poisonous gases, chemicals, and metals in tobacco smoke, including such nasty things as hydrogen cyanide, arsenic, and lead. This can certainly contribute to your pet's toxic load.

A more recent study published in the journal Nicotine and Tobacco Research*** demonstrates how secondhand and thirdhand smoke is basically turning our dogs into "passive smokers". Marcello Roza and Carlos Vegas of the Department of Pneumology at the University of Brasilia in Brazil tested 30 Yorkshire Terriers, half of whom lived in a home containing cigarette smokers. One of the ways in which you can determine whether an individual is a smoker or has been exposed to significant environmental amounts of tobacco smoke over a long period of time is to use what researchers call "biomarkers". One of these is cotinine which is an alkaloid found in tobacco and also appears when we metabolize nicotine. The level of cotinine in the urine of dogs was significantly higher in those animals who lived with smokers. Furthermore the dogs in smoking homes had increased levels of macrophages and lymphocytes which seems to be evidence of prolonged inflammation of the airway of these dogs, and might well be interpreted as precursors for nasal, throat, and lung cancer. In effect these dogs are already showing some of the negative effects associated with tobacco smoke that they might have developed if these themselves were the active smokers.

All of this data suggests that exposure to tobacco smoke is putting dogs at risk for various forms of cancer. Although smoking has been on the decrease, according to the World Health Organization a significant number of people still smoke. Their data shows that the percentage of smokers varies by country. For example:

Australia 17%
Canada 20%
France 30%
Germany 26%
Japan 23%
United Kingdom 21%
United States 27%

Overall this means that on average more than one out of five dogs will end up living in a house that contains someone who smokes tobacco. The evidence suggests that these animals may begin to show the same kinds of health problems that their cigarette smoking owners are at risk of developing. Thus I am inclined to agree with my colleague's wife in the suggestion that if you care about your dog's health (even if you don't worry much about your own) you should stop smoking.

Stanley Coren is the author of many books including: Gods, Ghosts and Black Dogs; The Wisdom of Dogs; Do Dogs Dream? Born to Bark; The Modern Dog; Why Do Dogs Have Wet Noses? The Pawprints of History; How Dogs Think; How To Speak Dog; Why We Love the Dogs We Do; What Do Dogs Know? The Intelligence of Dogs; Why Does My Dog Act That Way? Understanding Dogs for Dummies; Sleep Thieves; The Left-hander Syndrome

Copyright SC Psychological Enterprises Ltd. May not be reprinted or reposted without permission

Data from:

* Reif, J. S., Dunn, K., Ogilvie, G. K., & Harris, C. K. (1992). Passive smoking and canine lung cancer risk. American Journal of Epidemiology, 135, 234–239.

** Reif, J. S., Bruns, C., & Lower, K. S. (1998). Cancer of the nasal cavity and paranasal sinuses and exposure to environmental tobacco smoke in pet dogs. American Journal of Epidemiology, 147, 488–492.

*** Roza, M.R. & Viegas, C.A.A. (2007). The dog as a passive smoker: Effects of exposure to environmental cigarette smoke on domestic dogs. Nicotine and Tobacco Research, nine, 1171-1176