Students attend a sports class inside an inflatable clean-air dome at a school of Beijing on December 1, 2015. The dome is equipped with hospital-grade air-filtration systems, according to teachers of the school. Thick smog covered Beijing and much of north China on Monday as the worst period of air pollution this year stretches into a fourth day. Smartphone air quality apps showed an Air Quality Index (AQI) reading of 500, the maximum, for almost all monitoring stations across the city, advising people to avoid going outdoors. Concentrations of PM2.5, tiny airborne particles that embed deeply in the lungs, peaked at 900 micrograms per cubic meter in southern Beijing, the municipal environment monitoring center said. The World Health Organization's recommended maximum is 25 microgram per cubic meter. (Photo: China News Service/Liu Guanguan)
