Ontario should become the first province to ban smoking in cars that contain young passengers, health advocates said Thursday as they rallied around a private member's bill that would outlaw the practice, which critics liken to child abuse.

Although Premier Dalton McGuinty has said such a ban would be a dangerously slippery slope, health activists say the likelihood of children developing cancer, asthma and heart problems is good enough reason to force people to butt out after they buckle up.

``Second-hand smoke is a killer,'' said Rocco Rossi, CEO of the Heart and Stroke Foundation. ``Therefore we should be protecting our children from it.''

``We already regulate in the car _ we require seatbelts and child seats to protect our children. We're not breaking new ground. We're not going down a slippery slope, because the state is already in the car.''

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

The private member's legislation introduced Thursday by Liberal David Orazietti faces a steep battle, since such bills rarely become law unless they are adopted by the government.

If the bill did become law, Orazietti said it would allow a police officer to pull over anyone smoking in a car with kids and fine them between $200 to $1,000.

``This is a reasonable step to take,'' Orazietti said. ``It's a responsible step to take. We've got an obligation to protect children and youth.''

Health Promotion Minister Margarett Best said Ontario already has one of Canada's toughest tobacco laws, which ban smoking in bars, restaurants and workplaces. But she was noncommittal about whether the province would support the latest proposal.

``I expect that parents will take responsibility for those kinds of decisions relating to their children and that's the direction I am going in at this point,'' Best said.

But Michael Perley of the Ontario Coalition for Action on Tobacco said the province already has all kinds of other laws protecting children from abuse, so a ban on smoking in cars with kids should be no different.

``These are very young people who are not in a position, in that environment, to do anything to protect themselves,'' Perley said.

``They can't stand up and step out of the car at 60 miles an hour. The youngest ones aren't even in a position to know that anything bad is being done to them.''

Health experts say second-hand smoke is extremely detrimental to a child's health _ particularly in a car. Cynthia Callard, executive director of Physicians for a Smoke-Free Canada, said smoking one cigarette in a car is worse for a child's health than taking them into the smokiest bar.

The exposure can cause a whole host of illnesses, from ear infections to cancer, she said.

``Parents do not have a blanket right to harm their children, and putting a child in a car with smoke is certainly harming the child,'' said Callard, adding areas that have brought in a ban have seen people voluntarily obey the law.

The government's reluctance to adopt a ban seems to say that the Liberals are more concerned about interfering with parents that they are about the health of children, she added.

But Arminda Mota, spokesperson for the industry-funded smokers' rights organization mychoice.ca, said such legislation is an infringement on the rights of parents. No one recommends smoking in a car with kids in the back seat, Mota said.

``But common sense cannot be legislated,'' she said. ``Tobacco control has gone too far.''

Irene Gallagher, with the Ontario division of the Canadian Cancer Society, said it would be nice if parents voluntarily refrained from smoking around their kids or kicked the habit altogether.

``We feel that when they buckle up, they should butt out,'' she said. ``They should be thinking about the effects of second-hand smoke.''

But until that happens, Gallagher said children need to be protected by law.