The new law banning smoking in cars when children are present is "landmark" legislation, the Chief Medical Officer has said.
Professor Dame Sally Davies described the ban, which would see drivers and smokers in England and Wales fined £50 if they have someone under 18 in the car, as "landmark" legislation.
Dr Penny Woods, chief executive of the British Lung Foundation, said that after years of campaigning, the law means the UK has "stopped playing catch-up and can now stand tall beside other countries".
On the day the ban came into force, research has revealed the harm secondhand smoke poses to youngsters - even with the windows down.
And Dr Anil Namdeo, of Newcastle University's Transport Operations Research Group, carried out experiments on secondhand smoke in vehicles to test levels of dangerous chemicals.
The test found even with the window open, levels of dangerous chemicals were more than 100 times higher than recommended safety guidelines.
And with windows closed and the fans on, levels were more than 200 times the safe limits.
Children are more susceptible to the effects of secondhand smoke - 80% of which is invisible - as they breathe more frequently than adults and their respiratory systems and immune systems are still developing.
Secondhand smoke contains 4,000 chemicals, more than 50 of which cause cancer.
But despite the research, smokers' group Forest said the law "stigmatises" millions of decent people.
What the law says:
From today it is illegal to smoke in a car or other vehicle with anyone under 18 present.
Both the driver and the smoker could be handed a £50 fixed penalty.
The ban does not apply to a 17-year-old driving on their own.
The law applies to private vehicles enclosed wholly or partly by a roof, so if the car is a convertible and the top is down and stowed, then it is legal to smoke even if a child is in the back. But if the sunroof is open, the car is still classed as being enclosed.
The legislation applies to motorhomes, campervans and caravans when they are being used as a vehicle but does not apply when they are being used as living accommodation.
The new law does not forbid smoking e-cigarettes in a car when children are present.