Smoking is bad for pets too, the University of Glasgow said, citing an ongoing study into the effects of second-hand smoke on dogs and cats.
Research under way at the Scottish institution has found that pets living in a smoky environment have a higher risk of health problems including some animal cancers, cell damage and weight gain.
"Pet owners often do not think about the impact that smoking could have on their pets," said Clare Knottenbelt, professor of small animal medicine and oncology.
"Our findings show that exposure to smoke in the home is having a direct impact on pets.
"It risks ongoing cell damage, increasing weight gain after castration and has previously been shown to increase the risk of certain cancers."
While dogs can take in significant amounts of smoke, the university study shows cats are "even more affected".
"This may be due to the extensive self-grooming that cats do, as this would increase the amount of smoke (chemicals) taken in to the body," said Knottenbelt.
The research so far has found that while outdoor access made little difference for cats, smoking away from them did reduce the amount taken into the body.
It has found that when smokers light up fewer than 10 times a day, nicotine levels dropped significantly, but were nonetheless noticeably higher than in cats from non-smoking homes.
An examination of the testicles of castrated dogs found that a gene, which acts as a marker of cell damage, was higher in dogs living in smoking homes.
Dogs living with a smoker also gained more weight after neutering.
The finished research paper is expected to be published in 2016.
