If passive smoking harms humans, new study tells it causes more negative effects on animals. Experts found a causal relationship between a smoking environment and animal sickness including cancer, cell damage and weight gain.

Most of the time, pet owners are unaware of the potential risk second-hand smoking poses to their animal's health, according to Clare Knottenbelt, small animal medicine and oncology professor from the University of Glasgow. "It risks ongoing cell damage, increasing weight gain after castration and has previously been shown to increase the risk of certain cancer," she said.

Pets are more exposed to the carcinogenic particles since they usually stay more often indoor and are close to carpets, where these microscopic materials settle. While dogs carry the same risk, cats are reportedly more affected, suggesting that significant amount of smoke is inhaled because of their extensive self-grooming where they are likely to inhale smoke particles from their fur. 

Meanwhile, castrated dogs tend to gain more weight when placed in a smoking environment. Furthermore, because of their low height, they are most likely to take in "third hand" smoke, which experts highly believe is more toxic compared with second hand smoke.

"Owners who consistently smoked away from the cat did not protect their cat from exposure but did reduce the amount of smoke that was taken into the body," Victoria Smith MRCVS, an investigator studying the causal relationship between second hand smoking and lymphoma in cats, said.

In addition, researchers pointed out that even though owners smoke outdoor for the sake of their pets have not totally eliminated carcinogenic particles but have reduced its amount. And owners who smoke at home less than 10 times a day although have minimized exposure of nicotine to pets, it is still higher compared with in a non-smoking environment.