Do you know you can now get "oxygen bars"? You just buy a can of oxygen, take a few deep breaths, and it's supposed to do you good. Sounds great. Unless you know a bit about how the body works - and then it's a bit puzzling.

Because if you have normal lungs, a normal heart and normal red blood cells - and most of us have - then taking in a few breaths of extra oxygen will make absolutely no difference to you. Simply breathing in the atmosphere around you gives you all the oxygen you need.

Naturally, if the air is polluted with chemicals and dust, then breathing in pure oxygen for a minute or two will give you a brief respite from them - but that could be done by putting on a mask or breathing from a can of fresh air, which would be a lot cheaper. However, it's a lot easier persuading people to buy cans of oxygen than of fresh air!

Just over a fifth of the air we breathe is oxygen. Our lungs transfer it to the red cells in our blood, which carry it round the body (pumped by the heart), to be used as fuel by every tissue. Without a good constant supply of oxygen, there isn't a cell in our bodies that can survive for more than a few minutes.

Happily the systems we use to pick up, transport and distribute the oxygen are near-perfect, so that even breathing in air consisting of only one-fifth oxygen and four-fifths nitrogen (which we don't use and just washes in and out of our lungs), is enough for all our needs. In a healthy person, adding extra oxygen to the mixture won't add any more oxygen to a single extra red blood cell - and that's what matters. There are people, though, who do need more oxygen than normal air can supply. They can't get enough air deep into their lungs, so that there's not enough contact between the red cells and the air. The red cells pass through the lungs without being able to pick up enough oxygen - and that makes them breathless, blue, and exhausted by the slightest exercise.

Most adults with this condition are in the later stages of chronic bronchitis - and only two in every hundred of them are non-smokers. Their lungs have been so scarred by tobacco that they are stiff, instead of elastic. They come to depend on breathing in high concentrations of oxygen just to stay comfortable at rest. But they can't do it just by taking an occasional whiff - they need to use oxygen constantly for more than 16 hours a day to get any real benefit. A can or two is no use to them.

I'm constantly amazed by how strong the need to smoke is. A year ago, I was called to a man in the final stages of lung failure, who had been provided with an oxygen tent at home. There he was, in his tent, the oxygen full on, smoking! The cigarette end was glowing a brilliant red, and with one puff, half of it burnt immediately away. He was putting himself and his family in terrible danger - but that was nothing to him, compared to the need for a cigarette.

Oxygen is needed at the other end of life. Premature babies often have lungs that are not yet mature enough to breathe normal air - so they need more oxygen than normal air contains. That's given in a special incubator, but it's never pure oxygen: it's always mixed, like air, with other gases. In the past, when near-pure oxygen was given to babies like this, they became blind. Too high an oxygen level damaged the immature eyes. Sadly the change was permanent, so the children were blinded for life.

Our real need isn't for more oxygen - it's for fresh, clean air. The air in our cities is fouled up by the exhaust gases from vehicles - acid gases from petrol engines and tiny particles of soot from diesel engines. They are being blamed, at least partly, for the big rise in asthma cases in our cities over the last ten years. They especially harm children.

Anyone with sensitive lungs should avoid traffic fumes if they can, or wear a mask if they can't. The Japanese do it - why don't we?

In the meantime, vehicle owners should be made to make sure that their cars and lorries are clean. Of course, none of that care and attention to the air matters if you smoke. Smoking raises the pollution levels inside your lungs millions of times higher than walking down a traffic-filled street.

If you smoke, there's no point in complaining about pollution - it can only ever be a tiny fraction of the pollution you force upon yourself.

Converted for the new archive on 30 June 2000. Some images and formatting may have been lost in the conversion.