*We answered this question on the show...* *Chris* - But first, just a ball park estimate for you two, what weight of rubber do

you think gets worn out on the road every year around the world?

*Dave* - It must be a kilo or two per car per year. So, in the UK, that's sort of 20 million kilos or about 20,000 tons or so off the top of my head.

*Chris* - Well, when this question came in, I was just playing with the numbers. So, I looked up how many cars do we think there are on the roads and in 2011, they announce they think, alongside the 7 billionth person, there were 1 billion cars on the world’s roads. So, if you assume that each of those has got 4 wheels, so there are 4 billion tyres.

So, how much tread is on a tyre? So I said, okay, if you imagine that a tyre is about 3 metres in circumference and let’s make a really ball park estimate. Let’s pretend it’s 10 centimetres wide, so pretty narrow, but some are going to be bigger, some are going to be smaller, all the numbers will come out in the wash. Let’s assume the tread is a centimetre deep to make the maths easy. That means that the thickness of tread on your average tire must be 3,000 cubic centimetres because I'm multiplying 3 metres, 300 centimetres by 10. So that in other words, it’s 3 litres of rubber on a tyre. So, on all the cars in the world, that must be 4 billion tyres times 3 litres, that's 12 billion litres of rubber. If we assume that a rubber tyre wears out in a year, which in some cases, with a bit of hard driving it’s going to, then you could be rubbing 12 million cubic metres of rubber onto the roads around the world every year, which is quite a lot, isn’t it?

So, then I looked up the density of rubber. It’s 1200 kilograms per meter cubed which means that there are 14.4 million tonnes of rubber being rubbed off on roads all the way around the world every year. That's extraordinary, isn’t it?

*Dave* - You can see it on my bike tyres, if you cycle a lot and everything gets covered in a black gunk which I think is mostly tyre.

*Ian Burgess* - If you live in a city and you have your windows open, you find a black grimy powdery mess inside your windows, particularly in summer because that's when people have their windows open of course, and those little drop bits of powder are actually ground up tyres and bits of road tar.

*Chris* - So, in other words, there's 14.4 million tons of rubber littering people’s living rooms, and the air, all breathing that in. What about the road surface Dave because that's the other point of 'I live in hope’s' tweet? Where does all this stuff go? I mean, Ian is saying that the roads and the tyre goes in your windows, but presumably, same place up in the air, we breath it in?

*Dave* - I guess most of it gets washed into the rivers. It either gets washed directly into rivers or gets caught in rain and gets pushed back down through the drains.

*Ian** Burgess* - Through the drains and front gardens and so on.

*Dave* - I think it’s quite a pollution problem near big motorways and things.