Tokyo and surrounding prefectures, baking in intense summer heat, have been subject to almost daily smog alerts since the beginning of August and observers expect the number of alerts will be the highest in a decade.
Smog tends to develop on hot, sunny days with little wind, conditions seen frequently this summer as temperatures in the capital climbed to 35.7 C on Thursday, the highest so far this year.
About 13 alerts have been issued for Tokyo and neighboring Chiba Prefecture every year since 1989, according to the Meteorological Agency. Saitama Prefecture sees an average of 17, the highest in the country.