MPs have voted to make it illegal in England and Wales to smoke while children are in the car.
Emerging eight years after smoking in public places was banned, the new legislation has been praised as a landmark in protecting children, aiming to shield youngsters from the effects of passive smoking.
Professor Dame Sally Davies, the Chief Medical Officer, told Sky News: "Smoking just a single cigarette in a vehicle exposes children to high levels of air pollutants and cancer-causing chemicals like arsenic, formaldehyde and tar, and people often wrongly assume that opening a window, or letting in fresh air, will lessen the damage."
A Newcastle-based research team found that, even with the window open, the concentrations of carcinogenic chemicals in the car was higher than previously thought.
Leader of the research, Dr Anil Namdeo, said: "With the window closed the levels peaked at several hundred times the safe limit but even with the window open we saw a significant rise to well above the safe recommended limits."
Although drivers could be fined £50 for breaking the new law, police officers are more likely to issue offenders with a warning.
"As the existing smoke-free law extends to vehicles, police forces will be taking an educational, advisory and non-confrontational approach when enforcing the new legislation," said a spokeswoman for the National Police Chief's Council.
"This would see people being given warnings rather than being issued."