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The big sneeze;Science

David Bodanis gets right up our noses to find the answer to an age-old question.

Why do we sneeze? Well, there's a great warehouse of odd microscopic items floating in the air, just waiting to get inside our noses.

There are 300,000 or more of these particles in every cubic foot around us, barely held up by the pattering air molecules around them. In an ordinary school or home these can include clothing fibres, insect limbs, factory ash, tire rubber fragments, pollen grains, shiny metallic blobs, skin flakes, micro-sand and all the rest of what we, with remarkable nonchalance, call dust.

Stepping outside doesn't necessarily help much. Trees can help soak up a great amount of the microscopic items floating around in the air. They're especially good at soaking up city pollution, such as the nitrogen dioxide fumes from cars. Even a small oak might have 40,000 leaves on it. If each leaf is six inches by three inches, that's 12,000 square feet of leaf surfaces, just ready to intercept floating pollution - and send much of it safely downwards towards the trees' roots. Just having a spreading lawn can help, for lawns tend to make the air wetter than it would be otherwise. That moisture coats the smallest pollution particles and makes them stick together. This is the worst possible fate for airborne travellers: they become too heavy to stay up, and crash to the ground.

However, going outside is also likely to expose you even more to pollen in the air. Pollen is tough. It doesn't make you sneeze because of the bulky Zeppelin-like air bags that are often inside it, nor even because of the bad habit it has of trying to mate with whatever object - be it nostril or flower - it happens to crumple into after its long flight. We sneeze because of the sticky proteins that are spread on the surface of pollen grains.

The proteins are there to ensure a good, tight landing - like a sticky mooring cable. But for many humans, such sticky proteins also start a full-scale chemical response as our body cells squirt out histamine to try to get rid of the intruder. This can lead to nose-twitching, eye-running and, ultimately, high-velocity bursts of sneezing.

If there were only a few hundred of these mating-desperate mini-Zeppelins around, your nose would probably be able to defend itself. But on a midsummer day there can be tens of millions of pollen grains disengaging from your shrubbery, or more distant plants, and bobbing in your direction.

One way to avoid the problem is to wear a miniature scuba face-mask, though this may provoke disheartening comments. Another possibility is to go back in time and make sure you are the last-born child in your family. Last-borns are four times less likely to get hay-fever than the oldest children. This could be because last-borns pick up more colds - as the family had more members around when they were young - and this made their immune system more practiced at moderating the histamine response with which we defend ourselves, but nobody knows for sure.

In lieu of scuba gear or rebirth, another possibility is to make muttered apologies to your friends and go back indoors. Pollen is relatively large as the floating micro-world goes, and if you wait absolutely still inside a room, much of it will float to the floor within an hour.

Night-time can be dangerous, though, for pollen bobs up in the hot air currents of midday. When those currents fade away at night, down, down, down - seemingly to the level of your nose - come the lonely-heart pollen hordes.

Rinsing your hair before you go to bed is one way of making sure you don't cross-contaminate yourself with the pollen that accumulated in the evening. Another good precaution is keeping a light sheet over your bed during the day. This will collect the pollen as it lands till bedtime, when gently rolling away the sheet will expose a pure and pollen-void zone underneath. With the windows closed you should be fine... up to a point. For in addition to dust and pollen, there are cold viruses hovering around at various times of the year. Pollen grains are perhaps 30 microns in diameter (a micron is a thousandth of a millimetre) but at 80 nanometres (a nanometre is one thousand-millionth of a metre), cold viruses are smaller still.

Cold viruses are little more than fine threads of genetic instructions, surrounded by a thin casing. But they've evolved to live best at about three degrees below body temperature - which is the temperature inside our noses - and they've also evolved to irritate our nasal lining so much that we conveniently help project them on their way, to boldly go and seek out yet more wondrous nasal caverns to colonise.

Whatever the source, when the tell-tale itching and twitching gets too much, there arises what certain horrified Northern European peasants in the era of the Bubonic Plague called the "pfnusen", which we - updating their word but keeping their distaste - call the "sneeze".

It jets out at 40 miles per hour or more - a force-eight gale on the Beaufort scale. This compares with the gentle five miles per hour - Beaufort force two - of normal breathing. Even if you manage to stuff a tissue up your nose, Beaufort force eight is capable of breaking twigs off trees, and the smallest nasal spray globules easily blast through gaps in the tissue's fibres. In the spray globules are the living virus "creatures".

One second after someone sneezes in your direction the globules will be 12 to 15 inches in front of their nose, and since they're whirling with a 40mph wind, they quickly dry out from the friction as they gush through the air.

All that's left of the soft water globules is a flotilla of dart-shaped mucus residues. On these hardened carriers the viruses travel far, gliding horizontally, quite as alive as they were in the nose. Even if the tissue has been wadded tight, a certain amount of the sneeze will spurt out around the sides. The gliding darts quickly disperse through the entire volume of your room, a rain of aerial cold viruses, a massed bomber attack.

For humans lucky enough to live in heavily populated countries, viruses spread on salivary darts are not all bad. Everyone gets peppered with the stuff, and indirectly that leads to almost everyone being fairly immune to most of them.

For individuals from more sparsely populated regions, it's not so good. The conquistadors who landed in South America in the early 1500s and proceeded to destroy great civilisations that were centuries old did so not because of the great superiority in Spaniards' weapons, but because each time they sneezed they released great numbers of contaminated gliding micro-darts that the Aztec or Mayan locals had no defence against.

Had the situation been reversed - and the South Americans had lived in more crowded, sneeze-ridden urban quarters than the Europeans - then the conquistadors would not merit entry in our history books, or they might appear only as "defeastadors", whose ships and unhidden maps had allowed the Aztec invasion and conquest of Western Europe to take place.

David Bodanis is the author ofThe Secret Family (Simon and Schuster, pound;18.99), which is the winner of the Alex Award, granted by the American Library Association for adult books that especially appeal to young adults aged 12 to 18.

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