Dr Karl: Dirt & dust

DUST IS inescapable. Travelling in Earth’s atmosphere, it’s simply solid stuff that’s been ground very finely. Rocks can be rubbed against each other by wind or water and, surprisingly, biology can also be involved when snails and bacteria break down rocks.

Not even oceans are safe. In 1846 Charles Darwin, while travelling on the British ship Beagle, recorded that “the quantity of dust which falls on ships in the open Atlantic is considerable and…the atmosphere is often rendered quite hazy”.

Each year about 2 billion tonnes of dust is lifted into the atmosphere, often after being stirred up by big storms. It comes mostly from the world’s arid regions, including deserts in Asia, Australia, North and South America and, of course,

North Africa’s Sahara. These and other drylands cover about a third of land on Earth. Once dust has been lifted skywards it journeys through the atmosphere in intercontinental airborne rivers. The most powerful of these are often fed by dust storms. As well as dust, these carry microorganisms and chemicals – from nutrients to toxins.

One major storm in April 2001 showed dust’s global reach. It began in the Gobi Desert in China and Mongolia. After crossing Korea, Japan and the Pacific Ocean in five days, it continued over North America and, after travelling across the Atlantic, finally arrived in Europe. While moving over the USA, the dust was so thick it dropped air-quality ratings to “poor”.

Dust can come from more local sources too. In the USA dust levels have increased by 500 per cent during the last two centuries, due to the massive expansion of livestock grazing and agriculture.

But where does the dust inside your house come from? Only about 60 per cent in the typical home comes from outside, either carried in by the air or tracked in on footwear.

The rest is organic matter generated within the house, such as organic fibres from paper, clothing, carpets and upholstery. Some is also sloughed-off dead skin cells from people or pets.

So next time you’re bemoaning having to continually remove your house dust, remember at least some of it was once part of you.

A young family looks across the outback town of Alice Springs as a dust storm approaches.