the secret life of dustI've been reading a brilliant book on, of all topics, house dust! Hannah Holmes "The Secret Life of Dust" raises some questions of importance to those whose allergy symptoms are triggered by allergens like house dust mite, mould and pet dander. We knew that house dust is a complex mixture of particles of both chemical and biological origin, but I had no idea just how mysterious this substance actually is.

Holmes describes an intriguing complex substance called 'The Personal Cloud' which came to light in house dust experiments carried out in the 1990s. A group of 178 participants based in California were wired up to personal dust monitors, which they wore for 12 hours at a time as they carried out their usual activities. The scientists, from the US Environmental Protection Agency, also monitored indoor and outdoor air for dust.  The results showed that the personal dust monitors registered much higher levels of dust than the external measurements – in other words, the individuals were actually emitting dust themselves. That's where the idea of the personal cloud comes from.  Lance Wallace, one of the EPA team, noted that the personal cloud accounts for a large amount of the dust in your home.

So what does this invisible cloud actually consist of? Skin flakes were the major known component. In 12 hours, the dust monitor collected 150,000 to 200,000 skin flakes per individual. In fact, that's just a small percentage of the skin we shed from the epidermis every day. The total amount is about 50 million scales which, as we know, is the prime diet of house dust mite, whose droppings contain the potent allergen dreaded by asthmatics everywhere. Most of this skin flake burden gets rinsed off when you bathe or shower. And, according to Wallace, you breathe in 700,000 skin flakes per day while the rest sinks into the carpet, your bed, or your furniture. These skin flakes amount to around 10% of your personal dust cloud.

Another component is lint – the tiny fibres emitted from your clothes. But, again, this amounts for only a minor proportion of the personal cloud. The rest of its components are still unknown. However, Wallace's team carried out further experiments which did shed some light. He wired up his own house with dust and gas monitors and found how they spiked each weekday morning with a cloud of car exhaust – even though the road was over a mile away. So the monitor network was very sensitive to any dust-generating activity.

One day Wallace accidentally waved his arm towards a monitor and noted how it recorded a sudden dust flurry. A colleague was unable to reproduce this effect and it turned out that this man had his shirts laundered and wrapped in plastic till he put them on. Wallace's shirts were home-laundered and hung in the wardrobe, collecting dust particles ready to be emitted when he moved his body (that is what the dust monitor picked up when he waved his arm). Even working quietly at a computer will multiply dust levels in a room.  It'll be interesting to learn what the other components of the personal dust cloud are and whether they are allergens. In the meantime, get a grip on the house dust by dusting, vacuuming with a leakage-free allergy vacuum cleaner and using a high-grade air purifier.

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