Smoking in cars is likely to affect the health of any children present, even if the windows are open, a new study has shown.

According to UK scientists, the habit results in high levels of toxic fine particulate matter - a type of air pollution - from tobacco smoke being pumped into the car. These levels far exceed the safe levels recommended by the World Health Organisation (WHO) and occur even with open windows.

The team carried out a study involving 17 drivers and over 100 car journeys. Fourteen of the drivers were smokers. The average journey lasted just under a half-an-hour.

The levels of fine particulate matter were measured in the back seats of the cars every minute during any journeys taken. This took place over a three-day period.

The study found that levels of particulate matter were 10 times higher in ‘smoking ‘ cars and this appeared directly linked to the number of cigarettes that were smoked.

While the smokers did tend to open windows in the car if they were smoking during a journey, the levels of particulate matter found still exceeded the WHO's recommended limit of 25 µg/m3. In fact, they averaged 85 µg/m3 and during one journey, reached as high as 880 µg/m3.

The scientists noted that children who are exposed to second hand smoke have an increased risk of developing a number of health problems, including asthma and middle ear disease.

"Children are likely to be at greater risk from (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 explained.

The scientists said they support the introduction of bans on smoking in cars.

Earlier this year, the government confirmed that it is drafting new laws to ban smoking in cars when children are present. However, it is not yet known when this ban will come into operation or how it will be monitored.

Details of this study are published in the medical journal, Tobacco Control.