The government of Chiba prefecture east of Tokyo issued a warning Monday morning to residents to avoid unnecessary outdoor activity and extended exercise because of unusually high levels of air pollution. It was the first time such a warning has been in issued in the Tokyo area under new guidelines introduced in February.

Smoke rises from steel plants in the Chinese city of Benxi, Liaoning province, on Nov. 3. Reuters

At around 9:00 a.m. (0000 GMT), the Chiba government said levels of PM 2.5 -- tiny particles of pollutants harmful to human health -- were expected to exceed an average of 70 micrograms per cubic meter through the day.

It was the first such warning in greater Tokyo since a new warning system was established in March under the new guidelines. The warning came as concerns persist over air contamination in China in recent months. A Chiba government official said the pollutants travel in the air and can be caused by cars or factories, including those located in Chiba itself, but that the source of the pollution wasn't clear. "We aren't sure what the origin is," said Motoko Yonemoto, an official with Chiba's air quality section. "Some might worry that it's coming from China, but we don't know for sure if pollutants from China can travel to the (Tokyo) region and have a significant impact." Ms. Yonemoto said past data suggest such pollutants have a greater tendency to accumulate in winter. Based on the provisional standard for PM2.5 set by the national government earlier this year, local governments warn residents when PM2.5 levels are expected to exceed 70 micrograms per cubic meter on a daily average. In March, the southern prefecture of Kumamoto issued the first warning in the country based on the new guidelines.

Copyright ©2020 Dow Jones & Company, Inc. All Rights Reserved. 87990cbe856818d5eddac44c7b1cdeb8