We breathe in millions of dust particles every hour - and that means breathing in all sorts of substances. Iva Pocock reports
Few things in life are as unavoidable as dust. From the cosmos to the kitchen counter it swirls around us, mixing mineral, vegetable and animal microparticles. It contains everything from the remnants of aeons-old asteroids to traces of tens of thousands of modern-day chemicals: fragments of camel hair, ash from distant volcanoes, flakes of human skin, spores of forest fungi, crumbs of Saharan sand, flecks of sea salt, pesticides, bits of tyre rubber.
Dust - which by definition measures less than 63 microns, compared with an average human hair, at 100 microns thick - occurs in massive quantities. According to Hannah Holmes, author of The Secret Life Of Dust, you will have inhaled about 150,000 particles of it in the time it has taken you to read these couple of paragraphs. And that's if you live in a clean part of the planet; in dirtier climes you're likely to have inhaled more than a million.
About a third of your daily exposure to dust comes from your personal cloud - not a meteorological phenomenon but a description of the unique plume of dust that accompanies each of us. As Holmes explains, our personal clouds are what make our houses smell like home.
Although the majority of dusts in personal clouds are unknown, research based on hooking people up to personal dust monitors shows that about 10 per cent of identifiable personal-cloud dusts are flakes of skin. Every day an adult sheds about 50 million scales, many of which get washed down the drain, but estimates suggest you breathe in about 700,000 of your own flakes each day.
Indoors, human skin flakes play a key role in house-dust ecosystems. Along with shreddings from sweaters, sheets, pillows and wallpaper they provide food for moulds, which in turn produce spores. These become dust and, with the chemicals they produce, can cause allergies and increasingly common conditions such as chronic rhinosinusitis.
Grazing on the moulds are dust mites, little eight-legged creatures that live in matresses, clothes, carpets and curtains. Voracious eaters, female dust mites consume about half their body weight - and lay about two or three sticky eggs - each day.
They are most commonly found between "couch cushions, on the family-room floor and on the bedroom floor", according to Holmes. Although they are too big to float as air dust, their manure and decomposing body parts aren't. An enzyme in dust-mite manure is a common trigger for allergies and asthma - the website of the Asthma Society of Ireland explains how to reduce it in your home but points out that there is no guarantee the measures will make a difference to your asthma.
The evidence of the link between asthma and dust mites is contradictory: some suggests children in extra-dust-mitey homes are more likely to develop the disease; others say they are not. Holmes also points out that asthma is soaring in climates avoided by dust mites: arid states in the US and arctic Finland. Also, dust mites have been around for thousands of years, yet asthma is reaching epidemic proportions in the Western world only now: 274,280 Irish people are asthmatics, one in seven children and one in 20 adults.
The dust we breathe every day, however, is very different to the dust our bodies evolved with. "We evolved with outside air but not industrially polluted air," says Dr Paul Dowding, an air-quality specialist at Trinity College in Dublin.
Although toxic chemicals account for a very small percentage of typical house dust, explains Holmes, they are very potent. Formaldehyde from chipboard, perchlorethylene from dry-cleaned clothes, lead from ageing plastic mini blinds, paradichlorobenzene from mothballs and pesticides from flea powders and rose spotters all contribute to the toxic fraction of house dust.
Disturbing research from the US found that the carcinogenic insecticide DDT showed up in one in four Midwestern homes 20 years after it was banned. Given that house dust is not what it used to be, it accounts for a huge portion of children's total exposure to toxic chemicals. By the age of six, a child is likely to have consumed half a cup of dust.
Although it is difficult to control some of the toxic fraction of dust entering your home, one avoidable domestic air pollutant is cigarette smoke. As about a third of us smoke, Dr Dowding estimates that about half of Irish households have a smoker, making cigarette smoke "probably the number-one indoor air pollutant". Tobacco smoke contains 400 micro-sized chemicals; these are particularly lethal, as they penetrate deep into the lungs. In addition to causing lung cancer, they are thought to aggravate asthma and predispose children to becoming asthmatic.
Other house dust derives from cooking (sautéing creates the most), baking (Holmes reckons dust mites love a sloppy cook), talcum powder, candles and so-called air fresheners. But it's not just the pseudo-cleaning of drowning smells with stronger ones that can thicken the dust in a house: old-fashioned sweeping lifts dust into the air. Even vacuuming sends your carpet dust flying.
One high-tech approach to reducing the dustiness of house-cleaning is to install a centralised vacuum system. Paula Osborne of Beam, which supplies dust- and fume-extraction systems, says 25,000 of its centralised vacuum systems, costing an average of €1,200, have been installed in Irish homes.
They consist of a series of hidden pipes, accessible through an inlet in the hall or landing, that converge in a central power unit generally located in the garage or utility room. Automatically powered once the nine-metre hose is clicked into the inlet, it makes all areas of an average house easily accessible. Osborne says they are popular with people eager to relieve their asthma or allergies non-medically.
Timothy Ennis, a refrigeration engineer who suffers from asthma, installed one of Beam's systems in his new dormer bungalow two years ago. "I'm delighted. I find it great for my chest," he says. And although he admits he doesn't do a lot of the vacuuming, he says his wife thinks it's brilliant too. In the old house, he could tell when his wife had been vacuuming, because he would get a bit chesty. Now, he says, he very seldom gets asthma, unless it's a very warm day with a high pollen count.
But there may be more to it than clean carpets. Holmes quotes one asthma researcher, Thomas Platts-Mills of the University of Virginia, who believes the asthma epidemic can be partly blamed on "the way the better-off use their lungs".
In societies where walking remains the normal form of transport, asthma is rare even in very dusty conditions. But as exercise levels plummet throughout the Western world, two diseases are at epidemic levels: obesity and asthma. Platts-Mills says there is a growing correlation between the two and that the poorly exercised lung may be losing its ability to deal with the challenges posed by our dust.
Regardless, it seems wise to improve the quality of indoor air. Dr Dowding suggests minimising the amount of synthetic material you bring into the house, such as carpets, furniture made of fibreboard and paints (although you can use solvent-free paints). He also advocates stopping smoking and, assuming you do not live on a busy street, ventilating your home as much as possible.
And if you do live on a very busy street? "You should move," says Dr Dowding. "Good air is worth paying for."
The Secret Life Of Dust is published by Wiley, £9.99 in UK