Opening a window will not protect children from passive smoking, parents are being warned.

The smoke from one cigarette lingers in the air for more than two hours, say doctors at London's Royal Brompton Hospital.

The warning is part of a campaign to highlight the dangers of children inhaling their parents' cigarette smoke.

Passive smoking has been linked with an increased risk of meningitis, asthma, cot death and chest infections.

Declare your home and car a smoke-free zone

Judith Watt, SmokeFree London

Children are at more risk than adults because their lungs are smaller and not fully developed, says paediatric respiratory fellow Dr Liz Edwards.

"The short and long-term damage to children's health caused by passive smoking should not be underestimated," she said.

'Smoke-free zone'

In an attempt to tackle the problem, parents attending the Royal Brompton chest clinic are to be given written advice about the dangers of passive smoking.

This will be extended to other London hospitals and eventually to all hospitals in the UK.

Judith Watt, head of SmokeFree London, an alliance of NHS Trusts and other agencies, said parents misunderstand the scale of the problem of passive smoking.

"You can't just stand by the window and blow in the opposite direction," she told BBC News Online.

"If you choose to smoke, smoke outside, away from kids.

"Declare your home and car a smoke-free zone," she added. "Don't ever ask kids to buy cigarettes or fetch an ash tray for you."

Chemical cocktail

Statistics suggest that in London alone, up to 650 new cases of childhood asthma a year arise from passive smoking.

There are thought to be five hospital admissions each day in London for children under five because of passive smoking.

Over the course of a year, the children of smokers can inhale up to 150 cigarettes' worth of nicotine.

The smoke contains more than 4,000 chemicals and poisons, including arsenic, ammonia, the insecticide DDT and formaldehyde.