By Duncan Geere, Wired UK

Many estimates of air pollution in developing countries are innaccurate, as there's no network of surface-based sensors that can find the worst-polluted areas. Scientists regularly have to rely on a few dated observations of questionable veracity.

However, Nasa has just published the first long-term global map that shows density of particulate matter below 2.5 micrometres in diameter. This size is important, because it's small enough to get past the body's defences and accumulate in the lungs, making it dangerous to human health. Epidemologists believe that they cause millions of premature deaths each year.

Satellites can't easily scan the surface of the Earth – they instead scan a column of air in the atmosphere, and the difficulty comes in getting readings at a particular level out of that data. The team who produced the map, Aaron van Donkelaar and Randall Martin at Dalhousie University, in Halifax in Nova Scotia, Canada, blended total-column aerosol measurements from satellites with information about how aerosols are distributed vertically in the atmosphere to obtain the data.

The map, as you can see above, shows a wide band of very high concentrations of particulate matter across the Sahara, Middle East, Central Asia and China, only interrupted by the Himalayas. Central Europe also shows a spike, including the south-east corner of England, and urban areas in North and South America stand out too.

The World Health Organisation's recommended level is 10 micrograms per cubic metre, so anything on the map that's green or above is cause for concern. Once in the lungs, the particles can cause asthma, cardiovascular diseases and bronchitis. Some very fine particles can even get into the bloodstream.

Some of the particulate matter is man-made and some is natural, and scientists haven't quite worked out the relative quantities yet, but both are dangerous to human health. In the Arabian and Sahara deserts, its mostly natural mineral dust lifted by the wind, but in eastern China and Northern India, it's more likely to be soot particles emitted by power plants, factories and cars.

The next step is to try and verify some of these measurements by expanding the ground-based network of sensors, with the eventual goal of finding out how long-term exposure to these particles affects human health on large scales.

"We can see clearly that a tremendous number of people are exposed to high levels of particulates", said Martin. "So far, nobody has looked at what that means in terms of mortality and disease."

I**mage: NASA

Source Link: Wired.co.uk

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