Doctors in Britain measured concentrations of fine particles in cars driven by 17 people, 14 of them smokers, using an electronic monitor on the back seat.

The volunteers were asked to follow their normal smoking habits as the smoke levels in their car were monitored over three days.

Out of 104 journeys – average time 27 minutes – 63 were smoke-free.

During smoking journeys, levels of fine particles were 85 microgrammes per cubic metre on average, compared to guidelines of 25 mcg/cu. metre for indoor pollution set by the UN's World Health Organisation (WHO).

Even when when the driver opened the window or turned on ventilation to remove the smoke, particulate levels were still above the WHO benchmark at some point during these journeys.

The average peak during smoking trips was 385 mcg/cu.metre, with the highest being more than 880 mcg/cu. metre.

In contrast, particulate levels during non-smoking journeys averaged only 7.4 mcg/cu. metre.

The type of fine particulate that was measured is less than 2.5 micrometres in diameter.

These very small particles are considered dangerous because they can lodge deep in the lung, causing irritation.

"Children exposed to these levels of fine particulate are likely to suffer ill-health effects," says the study, led by Sean Semple of the Scottish Centre for Indoor Air at the University of Aberdeen.

"There are increasing numbers of countries legislating against smoking in cars and such measures may be appropriate to prevent the exposure of children to these high levels of second-hand smoke."

The investigation appears in the journal Tobacco Control.