BEIJING is frequently shrouded in dense, yellowish smog so thick that the other side of the road is obscured. But the deadly smog that enveloped the city over the past weekend 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 the concentration of PM 2.5 (total mass of particulate matter of 2.5-micron diameter or smaller per cubic metre). Such particles are small enough to be inhaled and can damage lungs. The AQI range ends at 500; at one point a reading of 886 was recorded. A reading above 100 is deemed “unhealthy for sensitive groups” and anything above 400 is rated “hazardous” for all. These independent readings have put pressure on the authorities to release more detailed data of their own. A year ago Beijing's municipal officials bowed to public pressure and started reporting data on PM 2.5 for the first time. But Beijing is not even the most polluted city in China. Using a different but more widely used measure collated by the World Health Organisation of larger particulate matter called PM 10 (total mass of particles of 10-micron diameter or smaller per cubic metre), which allows cross-country comparisons, that dubious honour is bestowed on Lanzhou in the north-western province of Gansu (and the city of Ludhiana in India is more polluted still). Better data may soon be available. This month China’s Ministry of Environmental Protection announced that 74 cities were to begin monitoring and reporting the levels of multiple pollutants, including PM 2.5, sulphur dioxide, carbon monoxide and ozone.
Correction: The original version of this post wrongly suggested that the chart showed the number of particles per cubic metre, rather than their total mass. This was corrected on January 17th.