Smoke effect '11 times worse' in enclosed spaces
7 February 2014 Last updated at 07:26 GMT
Hundreds of doctors and health experts have signed a letter urging MPs in England to support a ban on smoking in cars carrying children.
Critics say the move would be an unnecessary intrusion into people's lives, but those supporting it say second hand smoke is a major cause of ill health in youngsters.
MPs are due to vote on the issue next week.
Peter Mackereth is a tobacco specialist at the Christie Hospital, which is one of the largest cancer centres in Europe.