My mother always said “David, fresh air is good for you.” If I’d known more chemistry as a kid, I would have had an excuse to stay indoors.

There really is no such thing as fresh air – it’s all very old. Every molecule of oxygen and water is the same as it always has been. That means that every atom in your body is so old that it has been places you don’t even want to think about!

Your body is about 70% water. These water molecules have been to some interesting and rather nasty places. Some of it will have been in snow on the mountains, and in muddy sludge in sewers. It will have certainly been inside many different animals, on its continuous trip around the planet.

So you are not the first to use that life-giving air. The millions and millions of oxygen molecules in your lungs are sure to include some that were exhaled when Julius Caesar breathed his last.

Recycling is not a new idea. Nature has been recycling atoms since the moment of creation. Our wonderful, warm wet planet, revolving in space, is like a huge spaceship, with all the doors and windows closed. All the molecules circulating on it stay around, whatever happens to them. Even when you die, the global recycling continues, and your atoms move on to other fascinating places.

So next time you take a deep breath, remember that so-called fresh air has been around for a long time!