SMOKERS are being urged to stop lighting up in their cars after a "shocking" study found it had a similar effect on children as putting them in a smoke-filled bar.
The report by a Scottish health board also concluded that opening a window made hardly any difference, with youngsters still being exposed to dangerous levels of poisonous particles.
It has prompted NHS Greater Glasgow and Clyde to produce 15,000 "Our car is smokefree" stickers in an attempt to persuade the 15 per cent of people who still smoke in the car while carrying children to stop.
Doctors have already called for smoking in cars to be banned to cut the number of children suffering asthma and other diseases from passive smoking.
Children are at particular risk because they breathe faster than adults and their immune systems are not fully developed.
The study involved fitting a child-size doll with smoke monitoring equipment to a car seat.
It was conducted by the Scottish Centre for Indoor Air, a collaboration between Aberdeen University and the Institute of Occupational Medicine.
Dr Sean Semple, who led the study, said: "The concentration of fine particulates that children would breathe in during these journeys sometimes reaches levels that are similar to those measured in smoky bars prior to Scottish smoke-free legislation."
The study also showed that opening a window only reduced harmful air levels slightly, and not to a safe level.
Brenda Friel, the health board's senior health improvement officer, said: "No-one would think twice about the dangers of taking a child into a smoke-filled environment, yet many drivers don't realise the harm that can be done. Worryingly, 15 per cent of UK smokers smoke in the car with children.
"The study has revealed some shocking facts.
"I am sure that many people who smoke in a car in which a child is travelling believe that opening the window is enough to protect them from any harmful effects. Our tests prove that this is not the case."
Dr James Paton, a reader in paediatric respiratory medicine at Glasgow University, said: "This report about the exposure of children to small particles resulting from smoking in cars makes salutary reading and its message is very clear.
"Do not smoke in cars when your children are present. Do not allow them to travel in a car with anyone who will smoke in the car. It is potentially damaging to their respiratory health."
Dr James Cant, head of the British Lung Foundation in Scotland, said: "This is a powerful piece of research. Children's lungs are so easily damaged by exposure to second-hand smoke."
Simon Clark, director of the smokers' group Forest, said: "We wouldn't encourage anyone to smoke in a car if small children are present. It's a matter of courtesy if nothing else, but this study borders on scaremongering."