Dr Steve Ryan, Medical Director of Liverpool’s Alder Hey Hospital, says bronchitis, asthma and ear infections could be cut if parents quit smoking.
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, but they often lied about whether they smoke near their children.
“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.”
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.
Research published in 2005 suggested children exposed to their parents’ smoking were three times more likely to develop lung cancer later in life.
The British Lung Foundation says 17,000 under-fives are treated every year for exposure to second-hand smoke.
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.