Researchers have backed a proposal to ban smoking in private cars after they found that the habit produces pollutants which could be harmful to passengers.
Even when smokers open their windows or use air conditioning, the concentrations of pollutants are three times higher than the World Health Organisation indoor air quality standards, scientists said.
Such exposure is likely to affect the health of child passengers, said researchers from the University of Aberdeen.
The data, published in Tobacco Control, examined 17 drivers, 14 of whom were smokers, who made a total of 104 journeys, with an average duration of 27 minutes.
Levels of fine particulate matter were measured every minute in the rear passenger area during typical car journeys made by smokers and non-smokers over a three-day period. The average particulate matter levels were 11 times higher in smoking cars compared with non-smoking cars.
The authors said that exposure to second-hand smoke is linked to several children's health problems, including sudden infant death, meningitis and respiratory conditions such as asthma and wheezing.
"Children are likely to be at greater risk from SHS (second hand smoke) exposure due to their faster breathing rates, less developed immune system and their inability to move away from the source in many home and car settings," they write.
The authors added: "We believe that there is a clear need for legislation to prohibit smoking in cars where children are present."
They conclude: "Exposure to PM2.5 (fine particulate matter) at the levels reported here is likely to be harmful to respiratory health, and measures to remove or reduce this exposure within the confines of vehicles should be considered both in terms of individual responsibility and via legislation."
The Smoke-free Private Vehicles Bill to ban smoking in private cars is being considered by MPs. The bill would outlaw smoking in cars which are carrying children, with a penalty of a smoke-free awareness course as a first offence, or a £60 fine.