雅思阅读精选：北京空气污染—最黑暗的一天_雅思英语阅读

雅思阅读精选：北京空气污染—最黑暗的一天

　　Like many Beijing residents, your correspondent has mobile-phone apps that keep up with the pollution readings. At an otherwise pleasant Saturday-evening meal with friends, he joined his companions in compulsively checking for updates.

　　Those previously unseen numbers were hard to believe, but they did seem to match up well enough with the noxious soup we could see, smell and taste outside. We are all far more familiar with the specifics of air-quality measurement than we would like to be. Apart from the AQI readings above 700, we were quite struck to see the readings for the smallest and most dangerous sort of particulate matter, called PM 2.5, which can enter deep into the respiratory system. These are named for the size, in microns, of the particles. A reading at a controversial monitoring station run by the American embassy showed a PM 2.5 level of 886 micrograms per cubic metre; Beijings own municipal monitoring centre acknowledged readings in excess of 700 micrograms.

　　For perspective on that set of figures, consider that the guideline values set by the World Health Organisation regard any air with more than 25 micrograms of PM 2.5 per cubic metre as being of unacceptable quality.

　　Chinese authorities have complained about the American embassys insistence on independently monitoringand publicly reportingBeijings air quality. And sometimes much is made of the vast differences between those readings and Chinas own official ones, which are often less dire. Indeed, a key feature of one of those mobile-phone apps is the side-by-side comparison of those competing data-sets.