Has all the air in the world been breathed before?

Dave - It rather depends what you mean by not being breathed before. If you're only interested in the molecules not having been breathed in before - and specifically if you're interested in the oxygen and nothing else - then any oxygen molecule you're breathing in either hasn't been breathed before at all - or it can't have been breathed more that a few times, because otherwise it wouldn't be oxygen anymore it would be carbon dioxide! But, if you're thinking about the actual atoms and whether any of the atoms have ever been breathed before, that's a whole different kettle of fish.

Chris - Because they're being recycled around the Earth all the time, aren't they? In the sense that a plant makes some oxygen, you breath it in and mix it with some sugar to make it into carbon dioxide, that comes back out and the plant breathes that back in and recycles all these atoms again and again.

Dave - That's totally right. I was just doing some calculations. It takes about 2,000 years for all of the plants in the world to work their way through all the oxygen in the atmosphere. So, some of the molecular oxygen you're breathing in won't have been breathed in before. But because the Earth is about 3.8 billion years old, you would have thought that pretty much all of it will have gone through plants and creatures in the past. But then again, I wouldn't say for definite that all of the atoms, all the molecules that you breath in have been breathed before, because there are 6 with 23 zeros (6 x 1023), or maybe as low as maybe 3 with 23 zeros (3 x 1023), molecules of gas in every lung full of air, and some of those might well have just been sitting there since the beginning of the Earth.

Also, some atoms have been coming out of volcanoes or degassing out of the centre of the Earth, and some have been raining in from space. So, in a normal breath, a few atoms won't have been breathed before, but I think most of the air will, wherever you are.

Chris - When you say Earth is 3.9 billion years old, you mean life on Earth is 3.9 billion years old because the Earth itself is a little bit older than that?

Dave - Yeah, definitely. So, creatures have only been breathing stuff for 3.9 billion years.