On the day smoking in cars when children are present became illegal, "shocking" new figures show the harm secondhand smoke poses to youngsters - even with the windows down.
The new law which protects children from the effects of tobacco smoke will see the driver and any smoker fined £50 if they have someone under 18 in the car too.
And Dr Anil Namdeo, of Newcastle University's Transport Operations Research Group, has carried out experiments on secondhand smoke in vehicles to test levels of dangerous chemicals - fine particles 100 times thinner than a human hair known as PM2.5 - which children sitting in the back of a car would breathe in.
His team tested having the windows open or closed, fans on or off and with the air recirculating or not.
Drivers tested 20 minute routes around Newcastle, replicating a school run, using volunteer smokers. Dummies were used in the back as no children were involved.
Despite common misconception, even driving with the windows open while smoking exposed those in the back to dangerous levels of chemicals.
The test found even with the window open, levels 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.
Levels of poisonous carbon monoxide were two to three times worse than on a busy road at rush hour.
Professor Kevin Fenton, National Director for Health and Wellbeing for Public Health England, added: "This experiment unearths shocking data, reinforcing that smoking in vehicles with a child is never safe. Even with the window down you are still putting your child at risk of developing serious health conditions."
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.
Experts think three million children are currently exposed to it, putting them at risk of serious conditions including asthma, bronchitis and infections of the chest and ear.
Dr Namdeo said: "People think that by opening the window they are clearing the air, but what actually happens is the air is sucked in from outside and pushes the smoke backwards, straight towards the passengers in the back seat.
"Within minutes of the driver lighting up we saw a rapid increase in the levels of these harmful chemicals - fine particles known as PM2.5 - not just around the driver but also around the child's car seat."
Dr Malcolm Brodlie, consultant in paediatric respiratory medicine at the Great North Children's Hospital in Newcastle, said: "Babies and children who breathe in smoke are more likely to have problems with asthma attacks and chest infections, and need more hospital care and doctors' appointments."