Derbyshire drivers and passengers are being warned they will soon be breaking the law if they smoke in a vehicle carrying a person under 18.
From October 1, they face fixed penalty fines of £50 – with drivers at risk of being fined twice if they have failed to stop a passenger from lighting up and are smoking themselves.
The Government says surveys suggest about three million children are exposed to smoke in vehicles.
Officials also believe about 200 children a week visit GPs because of the effect of secondhand smoke in cars.
Dame Sally Savies, England’s chief medical officer, said: “Children breathe faster than adults so they are much more exposed to the dangers of second-hand smoke.
“Their airways, lungs and immune systems are still developing so are much more at risk from harm.
“We want children to grow up free from harm and we need parents to understand why smoking in vehicles is so dangerous.
“Eighty per cent of smoke is invisible so even if you think you are being careful you cannot see where the smoke is going.”
Simon Clark, director of smokers’ lobby group Forest, said: “The Government is taking a sledgehammer to crack a nut.
“The overwhelming majority of smokers know smoking in a car with children is inconsiderate and they don’t do it.”
He added the law would be difficult to enforce.
A Department of Health spokesman explained: “The rules don’t apply to e-cigarettes.
“The legislation covers any private vehicle which is enclosed wholly or partly by a roof.
“A convertible car, or coupe, with the roof completely down and stowed is not enclosed and so isn’t covered by the legislation.
“But a vehicle with a sunroof open is still enclosed and so is covered by the legislation.
“Sitting in the open doorway of an enclosed vehicle is covered by the legislation.
“The rules apply to motorhomes, campervans and caravans when they are being used as a vehicle but don’t apply when they are being used as living accommodation.
“The rules don’t apply to boats, ships and aircraft, as they have their own rules, nor do they apply to work vehicles and public transport, as they are already covered by smokefree legislation.”