On Thursday [Jan. 16], the city issued its first warning of the year for a wave of dangerous smog that exceeded the worst threshold in the warning system. Concentrations of PM2.5, a recently classified carcinogen, were 350 to 500 micrograms per cubic meter on Thursday morning. Later that afternoon, the air quality monitors at the U.S. Embassy in Beijing recorded a sickening 671. The World Health Organization (WHO) considers 25 or less micrograms per cubic meter ideal for human health. Above 300 is considered hazardous.