Various forms of pollution have increased as China has industrialized, which has caused widespread environmental and health problems. According to the World Bank in 2007, 16 of the world's 20 most polluted cities are in China
The US embassy in Beijing regularly posts automated air quality measurements at @beijingair on Twitter. On 18 November 2010, the feed described the PM2.5 measurement as "crazy bad" after registering a reading in excess of 500 for the first time. This description was later changed to "beyond index",[21] a level which recurred in February, October, and December 2011.
In June 2012, following strongly divergent disclosures of particulate levels between the Observatory and the US Embassy, Chinese authorities asked foreign consulates to stop publishing "inaccurate and unlawful" data.[25] Controversy arose when U.S. Embassy declared Beijing air as “very unhealthy” on 5 June; underlying data showed 199 micrograms of particulate matter. In contrast, readings from the Beijing Municipal Environmental Protection Bureau declared Beijing air as “good”; its data showed levels between 51 and 79 micrograms for the corresponding period.
By January 2013 the pollution had worsened with official Beijing data showing an average figure over 300 and readings of up to 700 at individual recording stations while the US Embassy recorded over 755 on January 1 and 800 by January 12.
02-04-2013 01:00; PM2.5; 217.0; 267; Very Unhealthy (at 24-hour exposure at this level) 02-04-2013 02:00; PM2.5; 100.0; 171; Unhealthy (at 24-hour exposure at this level) 02-04-2013 03:00; PM2.5; 25.0; 74; Moderate (at 24-hour exposure at this level)
02-06-2013 10:00; PM2.5; 188.0; 238; Very Unhealthy (at 24-hour exposure at this level) 02-06-2013 11:00; PM2.5; 11.0; 36; Good (at 24-hour exposure at this level)