smog

"The day after a special 'Environment Forum' [in Beijing, China] dedicated to reassuring the athletics world that it had its air pollution under control, the city awoke to find itself engulfed by a thick pea-souper.

"Visibility was reduced to less than 50 yards, flights were delayed, major roads into the city closed and, most importantly, the smog was so serious that children and the elderly were told to stay indoors by the city's weather bureau.

"'Wear a face mask if you go out', its head, Sun Jisong, said. His office's report said the air was 'heavy with particulate matter'."