A leading hospital says up to a third of the children it treats for certain conditions are ill because their parents smoke in front of them.
Alder Hey Hospital says parents often lie about smoking
Dr Steve Ryan, medical director of Liverpool's Alder Hey Hospital, says bronchitis, asthma and ear infections could be cut if parents quit smoking.
He said parents often lied about whether they smoke near their children.
The British Lung Foundation says 17,000 under-fives are treated every year for exposure to second-hand smoke.
Speaking to BBC Radio Five Live, he said out of the 35,000 children the hospital treats every year, 2,000 are there because they have been exposed to their parents' smoke.
He said between a quarter and a third of those suffering from certain conditions such as chest infections and asthma were the victims of passive smoking.
Parents often know the health implications of smoking around their children, he added.
"People feel guilty," he told BBC News. "If it was easy, they would give up. Looking after children is good fun but it can be stressful and for some, cigarettes are a way of relieving that stress."
He does not think legislation is the answer but believes parents should be aware of the various levels of risk.
The top level, he said, was parents smoking in cars, where children were "trapped" and exposed to a "high intensity" of fumes.
Mothers smoking is a greater risk than fathers smoking, and smoking in the same room as your child is also high risk, he added.
"Having smoke on your clothes is a lower risk," he said.
"But a good tip for parents is always put on another layer of clothes when smoking outside. Our staff are made to put a coat on when they go out to smoke during their breaks."
Amanda Sandford from smoking campaign group Ash said an estimated half of all children are exposed to smoke in the home.
"It is clearly a widespread problem and I don't think people realise that so many children are regularly breathing in smoke," she said.
"About a quarter of adults smoke and there are more among young adults, people between 25-34, the age at which they are more likely to be parents with younger children."
She added that now smoking was banned in enclosed public places, parents should treat their home like the workplace, and smoke outside.
Research published in 2005 suggested children exposed to their parents' smoking were three times more likely to develop lung cancer later in life.
The government's independent Scientific Committee on Tobacco and Health concluded in 2004 that exposure to second-hand smoke can cause a number of serious medical conditions, including lung cancer, heart disease and childhood respiratory disease.
'Banging the drum'
Martin Birchall, an Ear Nose and Throat (ENT) surgeon and professor at Bristol University, said the childhood development period was "precious".
He said: "Passive smoking at home, exposing children to smoke they cannot escape from, increases the risk of them getting ear disease, sticky runny noses and sore throats, and further down the track, some of these ENT symptoms can in due course led on to worse diseases such as asthma."
He added: "We need to keep banging the drum. We need to publicise the fact that every cigarette you smoke is a cigarette that your child is smoking also."
A Department of Health spokesperson said: "Second-hand smoke kills. We must continue to help people understand the dangers of second-hand smoke, especially for the health of their children.
"The £56m we invested in NHS Stop Smoking Services last year was money well spent - we are well on track to meet our target to reduce the proportion of smokers in England to 21 per cent by 2010."