Since the start of this year, 28 prefectures have issued photochemical smog warnings, the largest number ever and covering Oita and Niigata for the first time.
Alerts were also issued in Kumamoto and Nagasaki prefectures for the second straight year. And in May, athletic events at 85 elementary schools in Kitakyushu were canceled due to smog.
The warning is issued when the density of photochemical oxidants, which cause photochemical smog and include ozone, tops 120 parts per billion per hour. Despite worsening pollution, Japan has yet to finalize regulations on the emission of nitrogen oxides and volatile organic compounds, whose reactions create photochemical oxidants.