A new law that bans smoking in cars when children are present comes into force in Scotland this week.
Anyone caught breaking the law by smoking in private vehicles with someone under 18 on board faces an on-the-spot penalty of £100 or a fine of up to £1,000 if the case goes to court.
Health campaigners welcomed the move as they pointed to figures suggesting around one in six 15-year-olds are being exposed to second-hand smoke in the car.
Sheila Duffy, chief executive of anti-smoking charity Ash Scotland, said: "We know from speaking to parents that they want to protect their children from tobacco smoke, but often don't know enough about how smoke is harmful and lingers in the air even after you can't see or smell it.
"This legislation sends a clear message that children should grow up in a smoke-free environment, and who could disagree with that?"
The Smoking Prohibition (Children in Motor Vehicles) (Scotland) Bill, introduced by Liberal Democrat MSP Jim Hume, was passed unanimously by the Scottish Parliament in December last year and comes into force on Monday.
It was introduced to protect young people from the harm caused by second-hand smoke, which can cause serious conditions such as bronchitis, pneumonia and asthma.
Latest research shows the toxic particles in second-hand smoke can reach harmful concentrations within a minute of lighting a cigarette in a car, ministers said.
The British Medical Association (BMA) welcomed the ban but called on the Scottish Government to go even further and introduce a complete ban on smoking in vehicles.
BMA Scotland chairman Dr Peter Bennie said: "The ban on smoking in cars with children is an important first step and we welcome this move to protect our most vulnerable.
"Doctors witness first-hand the devastating effects of smoking-related harms on their patients.
"Children are still developing physically and, as a result, are more susceptible to the harmful effects of second-hand smoke.
"When someone smokes in a vehicle it creates a concentrated source of exposure to second-hand smoke.
"An outright ban on smoking in vehicles would ensure that adults, and particularly vulnerable adults, who may be unable to object to others smoking while they are present, are also protected. This would also be easier to enforce."
Responding to the call by the BMA for a complete ban on smoking in vehicles, director of smokers' group Forest Simon Clark condemned the idea as "needlessly illiberal".
He added: "There's no evidence that smoking while driving is a road safety issue.
"The only reason to ban it is to make life as awkward as possible for adults who smoke.
"We don't condone smoking in cars with children, which is inconsiderate. Adults, however, have a choice and if the driver is smoking we can choose not to travel in the same vehicle or we can speak up.
"If a driver is on his own there's no reason at all to ban smoking in cars. It's a private space and should be treated like someone's home."