La poussière retourne à la poussière

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“IT GETS everywhere…in your eyes, up your nose, deep into your lungs. And given the right conditions, dust can hitch a ride on the wind and travel the globe. In April this year, skiers in the Swiss Alps got a dramatic reminder of just how mobile it can be when some 80,000 tonnes of fine sand fell over Geneva and the resorts of Zermatt and Verbier, turning the snow a reddish-brown colour. The sand hailed from the Sahara, thousands of kilometres away in North Africa.

This extraordinary mobility makes dust a formidable pollutant. Last month, the UN drew the world’s attention to the “brown cloud” hanging over Asia, a noxious mix of particles and gases from forest fires, vehicle exhausts and millions of small, inefficient cookers burning wood and cattle dung. The pollution is thought to kill thousands a year in the region and is probably disrupting its climate.

Dust and other particles suspended in the atmosphere – collectively known as particulates – come in a bewildering range of sizes and compositions, from minuscule particles of partially burnt fuel spewed from the exhaust pipes of cars, buses and lorries, to relatively massive grains of pollen, particles of rock dust blown from quarries, and flecks of soot or “fly ash” from coal-burning. Some particulates are simply a nuisance, like the films of red dust – also from the Sahara – that people living in London sometimes find on the windscreens of their cars. But some are downright dangerous.

Over the past decade, doctors have become increasingly concerned about how the tiniest particulates affect health. They may be responsible for up to 10,000 premature deaths in Britain each year. People with lung and heart disease are the most vulnerable. When breathed in, fine dust is carried deep into the lungs, where it can exacerbate inflammation in lung disease and even precipitate a heart attack in people with cardiovascular disease. During the notorious London smogs of the 1950s, poisonous mixtures of fog and smoke from coal-burning power stations and domestic fires are thought to have killed at least 4000 people. And as recently as 1991, the number of deaths in London from respiratory disease shot up by 22% during a bad four-day smog. We can’t choose the air we breathe: keeping it clean is vital.

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Increasingly sophisticated instruments have allowed finer and finer particles to be monitored. This has helped shift the focus of research away from coarser nuisance dust like that billowing around a building site – which might dirty washing or the windows of nearby houses but doesn’t pose a health risk – to the microscopic particulates that certainly are a threat. But particulates are complex beasts. They can be made of more than one type of material. One particularly nasty confection is diesel-coated pollen. And some are aerosols – tiny liquid droplets suspended in the air that originate in breaking waves and bursting bubbles at sea, in industrial smoke and fumes from hot surfaces and even from electric fires and cookers. Solid particulates include grit from quarries and building sites and soot in the fumes belched from industrial chimneys and vehicle exhausts. This diversity is a nightmare for scientists trying to understand the behaviour of particulates in the atmosphere and for those who assess air pollution, because each type behaves differently and each has a different effect on human

health.

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Then there are the vast quantities of secondary particulates created from gases in the air. Gases like SO2 are oxidised in the atmosphere to form compounds such as ammonium sulphate and sulphuric acid, while nitrogen oxides are converted to particulate ammonium salts and nitrates. Secondary particulates are usually less than 10 micrometres (thousandths of a millimetre) across. Some originate from combustion sources such as vehicles, but most are natural. An estimated 2 billion tonnes of natural secondary particulates form in the Earth’s atmosphere every year – a whopping amount compared with the 300 million tonnes we add. To put it simply, natural sources are the biggest contributor to dust in the air, releasing 10 times as much particulate matter into the atmosphere as humans do. This isn’t necessarily bad news: natural particulates such as mineral dusts tend to be inert and are thought to be less damaging to health. And the deadliest particulates are the finest, and we’re responsible for pretty much all of them. The bottom line is that we can exert some measure of control over how much we humans pollute, and try to mitigate the damage.

So what is it about the smallest particulates that makes them so dangerous? In general, particles greater than 10 micrometres across are deposited in the nose and throat, which are well protected with mucus. Those between 4 and 10 micrometres are trapped by the mucus coating the airways, which is continuously driven by billions of tiny hairs towards the mouth, where it is swallowed. Particles less than 4 micrometres across, however, can reach the naked gas-exchange surfaces of the tiny air sacs called alveoli. It’s not entirely clear why people with lung and heart disease are so susceptible. But in asthmatics, for example, dust may exacerbate inflammation in the lungs. After prolonged exposure, the immune system’s mast cells in the lining of bronchioles become sensitised to particulates. Further exposure then provokes these cells to release histamine and other inflammatory mediators. Histamine makes the smooth muscles that line the airways contract, narrowing the tubes, and increases mucus production. It also dilates capillaries and makes them more permeable, which causes tissue swelling or oedema.

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Dust to Dust

John MEREFIELD

September 21st 2002

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