BEIJING -- As world leaders tussled over the fate of the climate in Paris , Beijing residents struggled to make out the silhouettes of buildings . The multi-day pollution emergency -- dubbed Airpocalypse by English-language media -- signaled the start of smog season in one of the most polluted capitals in the world.

On Tuesday, indices of PM2.5 particulates -- the small cancer-causing pollutants -- soared past the 500 mark and sent them shooting off traditional scales and into territory the U.S. Embassy once described as crazy bad . The U.S. Environmental Protection Agency has said readings above 300 are rare in the United States and often associated with major events such as forest fires .

With many outdoor activities canceled, some residents used the free time on a new hobby: drawing the outlines of buildings on photos where the landmarks have disappeared into the smog.