Fresh evidence of the potential harm caused by parents who smoke in front of their children has been revealed by a scientific study.

Children who were exposed to second-hand smoke had much higher levels of chemicals in their bloodstream which suggested their blood vessel walls could already be under attack.

Anti-smoking campaigners say the findings, presented at a heart conference in the US, reinforce the need for parents to protect their children from the effects of cigarette smoke.

Evidence that passive smoking in children is causing harm is growing all the time

Amanda Sandford, ASH

A research team from the University of Vienna took blood and urine samples from 158 children aged between three and 15.

They looked for the presence of a chemical called 8-epi-PGF2alpha.

This is formed when other chemicals called "free radicals" come into contact with arachidonic acid, a key body chemical which controls the opening and constricting of blood vessels and the prevention of blood clots.

If the activity of this acid is restricted, in theory it could open the door for the layer of cells lining the blood vessels to stop working properly, which in turn makes them more vulnerable to hardening - and the person to heart disease.

Accelerated damage

This is damage which is believed to accumulate over a lifetime - but which is accelerated by cigarette smoking.

However, even young children who were exposed to second-hand smoke from their parents had elevated levels of 8-epi-PGF2alpha.

Amanda Sandford: "Evidence is growing"

If parents were smoking more than 40 cigarettes a day, levels in the blood were as much as 130% higher than those coming from smoke-free homes.

Smoking by the mother had a far more pronounced influence.

Lead researcher Helmut Sinzinger said: "We speculate that mothers may have closer contact with their children at home.

"It's too early to speculate on measures other than recommending parents not to smoke when their children are present.

"In the US and western Europe nearly half of all children are exposed to second-hand smoke in some way. Later vascular disease might be triggered early in childhood by exposure to second-hand smoke."

Driven home

Amanda Sandford, from the anti-smoking pressure group Action on Smoking and Health, urged parents not to smoke while their children were around.

She told BBC News Online: "Evidence that passive smoking in children is causing harm is growing all the time.

"Their bodies appear to be far more vulnerable to the damaging effects of tobacco smoke.

"Ironically, it may be that the clampdown on smoking in so many public places means that smoking in the home is actually increasing, as smokers feel it is the one place in which they can smoke without interference."