Every year, three million children are exposed to second hand smoke in a car. And every week, 200 of these children are made so unwell that they have to visit their GP with health complaints that are entirely preventable.
We know that children are more susceptible to the damaging effects of second hand smoke than adults. Growing youngsters breathe more rapidly, and their lungs, airways and immune systems are still developing.
At the same time, research has also shown that around a third of children who are exposed to smoke in vehicles don't feel able to ask the person to stop because they feel frightened or embarrassed.
I have met some of those children. On local school visits, when asked about my job I often talk about this legislation as it's so directly relevant to young people. And I ask children whether they've been in cars with an adult who is smoking - it varies by school but as many as a third of the class in some primary schools say they have, and that, although they found it unpleasant and uncomfortable, many felt they could not say anything. One little girl even danced around the classroom when she heard the law was changing, telling me how asthma affected her.
A common misconception is that smoking in a car with the window open means toxic fumes will disappear. This is not true. In fact, 80% of smoke is invisible, so even with a window ajar, young people and children are still badly affected.
We want to support people like Kelly and Steven Johnson, who, like most parents, want to do the very best for their children. Kelly was horrified when she saw her seven year old son, Daniel, picking up cigarette butts from the garden and mimicking her and her husband's smoking habits.
Kelly says: "Before I quit smoking, I would always try not to smoke in front of the kids but there were times when I'd think nothing of lighting up in the car. I'd always put the windows right down to make sure the car was fully ventilated, but I now realise that even by putting the window down my kids would still have been able to breathe in my smoke.
"I feel really guilty as a parent looking back, because my smoking in the car wasn't fair on my children. It was my choice to smoke, not theirs. Thinking about the harm that my smoking might have been doing to their health now upsets me because I understand the dangers.
"I was shocked one day last year when I saw my seven-year-old, Daniel, picking up our cigarette butts in the garden and pretending to smoke them. No-one wants their child to start smoking. As a mum, I want to set a good example for my children and be a positive role model for them which is one of the main reasons I decided to quit.
"I'm really proud of me and my husband for quitting. We have been smokers since we were teenagers and smoked more than 20 roll-ups a day. It's taken me a few attempts, but I feel so much better now that I've quit. All the money I'm saving from not spending on tobacco can now go on spoiling my new baby."
Kelly has now successfully given up smoking for nearly a year, and has never felt better. She was initially given support through the Smoking Association, and used patches. Like most smokers she wanted to quit, but she got her inspiration to finally do it from her children.
We want all children to grow up free from the burden of disease that tobacco brings. This is why we're making it illegal for anyone to smoke in a car with under 18s from 1st October. After this date, there will be a £50 fine for anyone caught but I would much rather see more people follow Kelly and Steven's example.
After all, there are three million reasons to quit.