In the world’s biggest economies Johannesburg rates as the 7th most polluted. The title of most polluted city goes not to Beijing, as most would imagine, but the city of Ludhiana in India.

The Economist has posted an article describing how air pollution is measured, after a deadly smog covered Bejing over the weekend that “was so bad that air-quality readings from a monitor on the roof of the American Embassy said simply: ‘Beyond Index’.”

The embassy uses the US Environmental Protection Agency’s air-quality index (AQI), which measures PM 2.5 (particulate matter of 2.5 microns across or less per cubic metre). Such particles are small enough to be inhaled and can damage lungs.

A reading of above 100 is “unhealthy for sensitive groups” and anything above 400 is rated “hazardous” for all. At one point in Bejing a reading of 886 was recorded. You are basically breathing death at that point.

Using a different but more widely used measure collated by the World Health Organisation of larger particulate matter called PM 10 (particles of 10 microns or less), which allows cross-country comparisons

Using this measure, Joburg is rated as the 7th most polluted city within the world’s biggest economies. The whole list is below.

[Source: The Economist]