Chiba Prefecture urged residents Monday not to go outside because levels of the toxic air pollutant PM2.5 might exceed the provisional guidelines set by the government.
It was the first PM2.5 warning issued in the greater Tokyo region under the new alert system instituted in March.
The hourly average density of PM2.5 between 5 a.m. and 7 a.m. topped 85 micrograms per cubic meter, meaning it could exceed the daily average limit of 70 micrograms specified by the guidelines, the prefectural government said.
Densities of 88 micrograms to 127 micrograms were measured at three points in the city of Ichihara, it said, adding the cause of the rise is unknown.
The particles of hazardous matter measure below 2.5 microns (2.5 thousandths of a millimeter) and are believed to coming to Japan via the westerlies from China, where PM2.5 has been causing serious air pollution and health problems.
Chiba Prefectural Government said it is not clear whether PM2.5 from China was responsible for pushing up the density in Chiba this time.