Smoking in cars carrying young passengers is "reprehensible" given the dangers of second-hand smoke, but Ontario would have to think hard before becoming the first province to pass "invasive" legislation against it, Premier Dalton McGuinty said Monday.

The Liberals didn't promise to ban smoking in cars with kids in the recent election, and McGuinty ruled out the idea just a few months ago, calling it a slippery slope.

But as health advocates rally around a private member's bill which would give police the right to pull people over for smoking in a car with passengers under the age of 16, McGuinty now says he's open to the idea.

"To one extent, it's pretty invasive," McGuinty said Monday after decorating his office Christmas tree with children from the legislature daycare.

"The car is a kind of personal space. On the other hand, as a parent, I think the idea of smoking inside a car is reprehensible. Kids have no choice in the matter at all so I think it's an important debate to have."

Jurisdictions in the United States and Australia, as well as the town of Wolfville, N.S., have all banned smoking in cars where children are present. In B.C. and Nova Scotia, opposition politicians have tabled private member's bills recently that would also ban the practice across those provinces.

Private member's bills rarely become law unless they are adopted by the government, but McGuinty said he's going to stay neutral and hear what Ontarians have to say.

"If nothing else, this debate will help to reinforce what we all understand — that smoking is harmful to all of us but it can be particularly harmful to children," McGuinty said. "All those who become aware of the debate will hopefully understand that you shouldn't smoke in a car."

While tobacco lobby groups say any legislation would be a violation of people's rights, health experts say the risk to children's health is good enough reason to force people to butt out after they buckle up.

George Habib, CEO of the Ontario Lung Association, said the law would be no different than compelling people to buckle their seatbelt or properly install a carseat.

Exposing a child to an hour of smoke in a car is like giving them a pack of cigarettes, Habib said. Second-hand smoke has been linked to a host of health problems including ear infections, lung cancer, asthma and heart problems, he added.

A recent Ipsos-Reid poll found the majority of Ontarians were in favour of a ban, including 66 per cent of smokers surveyed, Habib said. Ontario municipalities don't have the power to outlaw the practice, he added.

"We need the province to do this."