Ontario was under growing pressure Thursday to become the first province to ban smoking in cars containing young passengers as health advocates rallied around a private member's bill that would outlaw a practice 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."

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, a New Democrat politician tabled a private member's bill last month that would also ban the practice.

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

Ontario has already banned smoking from bars, restaurants and workplaces, Rossi said. Protecting children in cars from second-hand smoke is a "natural" next step, he said.

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

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.

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 in law.