Even by the city's standards, Beijing was very polluted on Sunday. The PM2.5 scale, which measures the number of micrograms of "particulate matter" per cubic meter, came up to a whopping 344. (To put that figure in perspective, the World Health Organization considers 25 micrograms to be healthy). The pollution was so bad that the Beijing Municipal Environmental Monitoring Center warned the elderly, the sick, and children to stay indoors, and everyone else to avoid outdoor activity.