Pet owners have not yet got the message that their dogs and cats can get cancer from their second-hand smoke, according to TV vet Noel Fitzpatrick.
Professor Fitzpatrick, who features in Channel 4 series The Supervet, said the smoke can give dogs cancer in their nose and cats in their intestine "because they lick you".
But the vet, who added that dogs also get cancer from herbicides in grass, said warnings do not seem to sink in until owners see the problems "in real life" on TV.
Professor Fitzpatrick, who is known for designing bionic limbs for injured dogs and cats, was giving a lecture at the Hay Festival exploring the idea of One Health, a single shared medicine linking human and animal health.
He claims cancer in a dog is almost identical to that in a human and should be treated in a similar way using similar drugs.
A University of Glasgow study showed pets were at greater risk from passive smoking than humans because they spend more time in the home.