Passive smoking causes lasting damage to children's arteries， prematurely ageing their blood vessels by more than three years， say researchers.

The research， carried out in Finland and Australia， appears to reveal the physical effects of growing up in a smoke-filled home - although it is impossible to rule out other potentially contributory factors entirely.

Hidden damage

Ultrasound scans showed how children whose parents both smoked developed changes in the wall of a main artery that runs up the neck to the head.

While the differences in carotid intima-media thickness were modest， they were significant and detectable some 20 years later when children had reached adulthood， say the investigators.

Study author Dr Seana Gall， from the University of Tasmania， said: \"Our study shows that exposure to passive smoke in childhood causes a direct and irreversible damage to the structure of the arteries.

\"Parents， or even those thinking about becoming parents， should quit smoking. This will not only restore their own health but also protect the health of their children into the future.\"

The results took account of other factors that might otherwise explain the association， such as whether the children went on to be smokers themselves， but the findings remained unchanged.

However， if only one parent smoked the effect was not seen - possibly because exposure was not as high.

Dr Gall said: \"We can speculate that the smoking behaviour of someone in a house with a single adult smoking is different. For example， the parent that smokes might do so outside away from the family， therefore reducing the level of passive smoking. However， as we don't have this type of data， this is only a hypothesis.\"

Regardless， experts say all children should be protected from second-hand smoke.