This book arose because three of my previous tomes were on water, blood and alcohol. As we would not be alive without the first two fluids and the exclusion of the third would, for many, make life onerous to cope with, I decided to keep the fluid theme going and to resolutely remain in the realm of substances absolutely crucial to our survival. Oxygen seemed the obvious choice. Fluids, incidentally, have relatively low viscosities, exhibit flow and display turbulence, possess mass and can be deformed and compressed. It is near-ubiquitous, life-sustaining oxygen which we take for granted that makes us (and the planet) tick, the four main categories of oxygen usage being medicine, metallurgy, chemical synthesis and rocketry. If you think life’s a gas, you would almost certainly be right and that gas is none other than oxygen, although nitrous oxide (factitious air) first laid claim to that accolade, owing to the euphoric effects of inhaling it - an attribute that subsequently led to its recreational use as a dissociative anaesthetic, laughing gas parties being all the rage with the aristocracy (the hysterical participants falling about cachinnating) from as early as 1799. But it’s oxygen in all its guises and the far-reaching effects this element has had (and continues to have) on practically everything on our planet (and elsewhere) that takes centre stage in this offering of mine which, hopefully, will seem like a breath of fresh air - an expression that sadly has become a contradiction in terms. In the sapient words of prolific author Dr. John Gribbin: “Oxygen exists in the atmosphere because there is life on Earth; and life exists on the land surface of the Earth because there is oxygen in the atmosphere.” Given the parlous state of our planet, we would be unwise not to heed this warning by astrophysicist Carl Sagan: “Anything else you're interested in is not going to happen if you can't breathe the air and drink the water. Don't sit this one out. Do something."