Smoking in cars where children are passengers should be banned, according to an expert in children's health.
Professor Terence Stephenson says that cigarette smoke is made more dangerous in cars, because people breathe it in, in a small closed-in space.
Opening the car window reduced the effect but didn't get rid of it completely so Professor Stephenson is asking for it to be banned altogether.
The government says it is looking at its smoking laws again next year.
Expert calls for ban on in-car smoking
Second-hand smoke from adults smoking cigarettes is linked to chest infections and asthma in children.
Over the past few years the government has introduced new laws around tobacco - smoking in public places is now banned in England, Scotland, Wales and Northern Ireland.
Tobacco advertising has already been banned on television, in newspapers and magazines and on posters on the side the road.