A photochemical smog warning Sunday forced 85 elementary schools in Kitakyushu to postpone athletic meets.
The city issued the warning at around 8:45 a.m.
Municipal officials said the schools would reschedule their athletic meets for Tuesday or Wednesday.
The smog results when emissions from cars and factories react with ultraviolet rays, usually when the temperature rises to 25 degrees or higher. It can cause eye and throat irritation and people in affected areas are advised to restrict outdoor activities.