It's now illegal to light up a cigarette in a vehicle carrying a child in Ontario, the second province to institute such a ban.

The new law, effective Wednesday, protects those under age 16 from second-hand smoke.

Nova Scotia's law took effect April 1, and prohibits smoking in cars carrying children under 19. B.C. passed a similar law, though it's not yet in force. New Brunswick and Prince Edward Island are also considering a ban.

Ontario drivers and passengers who refuse to butt out face fines of up to $250.

But officials are hoping for voluntary compliance, Dr. Isra Levy, medical officer of health for the City of Ottawa, told a news conference Wednesday to publicize the new law.

Levy said the concentrations of cigarette smoke in the vehicles of smokers is close to 30 times higher than in their homes, making it the main source of exposure to children. That leads not just to childhood health problems such as asthma, bronchitis and ear infections, he said, "but also in subsequent years, heart and lung disease and cancers."

Exposure to second-hand smoke is also associated with sudden infant death syndrome.

Dr. Tom Kovesi, a children's lung specialist at the Children's Hospital of Eastern Ontario, estimated that about 25 per cent of children in Ottawa are exposed to cigarette smoke.

He said he has seen many cases where children with asthma were driven to his clinic by parents who smoked on the way there.

"[They] will end up in my clinic wheezing by the time they get there," he said.

Children of smokers might be exposed to thousands of toxic compounds during a car ride of 30 minutes to an hour, Kovesi said.

Peace of mind for parents

Anne-Josée Marion, mother of a three-year-old son, said the new law provides peace of mind to parents like herself who have worked hard to prevent their children from exposure to second-hand smoke.

"Luca eventually will be older and will be participating in sports or school events, and now, I don't have to worry about him getting into a car with another parent who might be car pooling."

The Canadian Cancer Society said it is pleased with the legislation.

"This law reinforces the important message to parents and caregivers that when you buckle up, butt out," Peter Goodhand, CEO of the Ontario division of the Canadian Cancer Society said in a press release Wednesday.

Dr. Ken Arnold of the Ontario Medical Association said doctors have been calling for a ban on smoking in vehicles with children present because the kids can't protect themselves.

"Certainly putting people with young, healthy lungs in a tin box and having someone light up just seems so unfair to those children," said Arnold.

Health Canada said opening a window doesn't clear smoke from a car.

With files from the Canadian Press