We need oxygen so that our body can generate energy. This gas comprises 21 per cent of the atmosphere: if we are not to suffocate, it must constitute at least 17 per cent of the air we breathe; if we are not to burst into flames, it must remain below 25 per cent. We can breathe oxygen-enriched air, as many sick people do, but if surrounded by it, we are in danger. The three astronauts destined for the first manned Apollo flight were burnt alive in minutes in their spacecraft on 27 January 1967, when fire started in the oxygen-enriched air of the cabin.
In October 1969 at South Shields, the same fate befell ship repairers in the hold of the Lady Delia. They were using a drill, normally worked by compressed air, which had inadvertently been connected to a supply of oxygen. The critical 25 per cent was exceeded and then one man lit a cigarette. It burst into flames which spread to his overalls; as his mates rallied to help, they too ignited. Within minutes four men lay dead, and seven were badly burnt.
But it is too little oxygen that is generally the threat to life. That was what brought the Biosphere project in Arizona to a premature end in January. Eight people had been sealed into the glass-walled ecosystem last December to test whether it was possible for humans to sustain life on a space station or the Moon. Within a month they were gasping for breath as the oxygen level of the air fell below 17 per cent. Thirty tonnes of it had disappeared, probably in reaction with iron in the soil.
Oxygen (02 ) is also attracted to the iron in haemoglobin in our blood, and is thereby transported to where our bodies need it. Thanks to haemoglobin, one litre of blood can dissolve 200cc of oxygen, 50 times as much as the same volume of water. (Not all species use iron as the oxygen carrier: spiders and lobsters use copper, which is why their blood is blue.)
The haemoglobin passes its 02 to an enzyme, mono-oxygenase, which also has an iron atom at its active centre, and is a catalyst for vital oxidation processes in the body, such as synthesising new molecules and detoxifying others. Dr John Lindsay-Smith of York University is seeking a simpler molecular model of mono-oxygenase so that oxygen from the air could be used as an industrial oxidation catalyst; this would be much less polluting than existing nitrate and chromate processes.
Oxygen gas consists of two atoms, but the bond between them still puzzles chemists. The gas will liquefy at minus 183C and the liquid is magnetic, as Michael Faraday discovered in 1848 when he spilled some and watched it stick to the poles of a magnet; it behaves like this because it has two free electrons. In theory these should make it react instantly with anything it touches, yet 02 is so stable that in our body it needs an enzyme catalyst to make it react.
There are 1 million billion tonnes of the gas circling the globe, all of it produced from photosynthesis in plants. The 7 billion tonnes of fossil fuel we burn each year consumes about 24 billion tonnes of 02 ; the 100 million tonnes of 02 that industry extracts is trivial in comparison. It would take more than 2,000 years at the present rate of depletion for oxygen to fall from 21 per cent to 20 per cent of the Earth's atmospere.
Without oxygen the brain begins to die within minutes; however, too much poisons it. This threat is not appreciated by many sports divers, according to Kenneth Donald, Emeritus Professor of Medicine at Edinburgh University, who has made a life study of the subject. In his book, Oxygen and the Diver, he warns against breathing pure oxygen at a depth of more than 25ft, since this can lead to convulsions.
Amateur divers have taken to using so-called nitrox mixtures, with a boosted oxygen content, in place of compressed air; but it, too, can be dangerous.
Nitrox was developed by the Navy in the Second World War for divers disposing of mines, because it allowed more time under water without causing oxygen poisoning and decompression sickness. Today professional divers breathe a costly mixture of oxygen and helium that enables them to work safely at depths down to 2,000ft.
Oxygen is produced industrially by distilling liquefied air. In the UK we use 4.5 million tonnes of it a year. More than half goes to making steel, about 25 per cent to ethylene oxide that is turned into antifreeze or polyester for bottles and fabrics, and the rest is used in medical care or to purify sewage - and so prevent disasters such as the one in Paris last year. A violent storm caused raw sewage to flood into the Seine, where it used up the oxygen in the water and killed all the fish. Huge pumps now bubble 15 tonnes of 02 a day into the Seine; on the Thames, a 'bubbler' is employed when necessary.
The author is science writer in residence at the Department of Chemistry, Imperial College, London.