You are a male house dust mite. Somewhere nearby, in your little curl of goose feather, someone is giving off the most incredibly exciting smell. Your immediate response is to produce a handy bag of sperm and leave it lying about in the old pillow you call home.

You are a female house dust mite. What is that interesting smell? A bag of sperm? For me? You shouldn't have.

Raunchy scenes like this are played out in a shag pile near you each night - never more so than in autumn when the nation's dust mite population begins to breed in unthinkable numbers to celebrate National Asthma Week.

When did you last launder your pillow? The goose-down filling is only 90 per cent feathers. The other 10 per cent? Dead skin, mould, mites, dead mites and dung. How do I know? A few moments with Dr John Maunder of the Medical Entomology Centre and you, too, will be a mine of disgusting information. "If you have a feather pillow you've never washed, put it in the dustbin," he says. A new one, however, is fully washable. "It's a British Standard. Has been for 24 years."

Mattresses, too, play host to vast populations of mites, their excreta bathed by the 45 gallons of moisture you lose each year during sleep. After vacuuming the mattress, cover it in a mite-proof sheet (pounds 19.50 single, pounds 24.50 double), and "never make a bed in the morning," advises Dr Maunder. "Make the bed after lunch. Your grandmother did."

But she didn't vacuum the mattress, as the mite-conscious housewife is now urged to do. Ideally, from the manufacturer's point of view, she should use an expensive, maximum-filtration appliance. The National Asthma Campaign, however, is reluctant to recommend these. "There is no scientific evidence that one vacuum cleaner is better than another."

Dr Maunder is a fresh air fanatic. "Open all your windows wide and leave them open." He is merciless in condemning those who imagine the air to be too polluted for children to breathe: "The child that sleeps in an unventilated room will find the room depleted of oxygen and filled with carbon dioxide, house dust mite dung and water vapour... The air quality in the average child's bedroom is so poor that if it were recorded in a factory the factory would be prosecuted." You have been warned.