Smoking in the car 'breaks toxic limit'
Smoking in the car, even with the windows open or the air conditioning on, creates pollution that exceeds official "safe" limits, scientists say.
Any child sitting in the back of a car when someone in the front is smoking would be exposed to this.
A Scottish team who took measurements during 85 car journeys found readings broke World Health Organization limits, Tobacco Control journal reports.
The British Medical Association says all smoking in cars should be banned.
Currently, it is legal in the UK.
Children are particularly susceptible because they have faster breathing rates, a less developed immune system and are largely unable to escape or avoid exposure to second-hand smoke, says Dr Sean Semple, of the University of Aberdeen.
Using a device strapped to the back seat of the car, the researchers logged and then analysed air quality data during a number of journeys ranging from about 10 minutes to an hour in duration.
In 49 of the 85 journeys in total, the driver smoked up to four cigarettes.
During these 49 smoking journeys, levels of fine particulate matter averaged 85µg/m3, which is more than three times higher than the 25µg/m3 maximum safe indoor air limit recommended by the World Health Organization.
Even if the driver smoked only one cigarette and had the window wide open, particulate matter levels still exceeded the limit at some point during the journey.
On average, the level of second-hand smoke was between one-half and one-third of that measured in UK bars before the ban on smoking in public places came into force.
Levels averaged 7.4µg/m3 during the 34 smoke-free journeys.
The research authors say: "The evidence from this [research] paper is that second-hand smoke concentrations in cars where smoking takes place are likely to be harmful to health under most ventilation conditions.
"We believe that there is a clear need for legislation to prohibit smoking in cars where children are present."
But Simon Clark, director of the smokers' lobby group Forest, says: "We don't encourage adults to smoke in a car if small children are present, out of courtesy if nothing else, but we would strongly oppose legislation to ban smoking in cars.
"According to research, 84% of adults don't smoke in a car with children present so legislation to ban it would be disproportionate.
"In terms of civil rights we are entering difficult territory. For most people a car is their private space. If you ban smoking in cars with children, the next logical step is to ban parents from smoking in the home.
"Parents must be allowed to use their common sense, and most of the time they do. There is no need for further regulation."
But Prof John Britton, chair of the Royal College of Physicians Tobacco Advisory Group, says a ban is necessary to protect children.
He said estimates suggested that each year passive smoking in children accounted for more than 20,000 cases of lower respiratory tract infection, 200 cases of bacterial meningitis, and 40 sudden infant deaths.
And last November the British Medical Association said an outright ban - even if there were no passengers - would be the best way of protecting children as well as non-smoking adults.