TSUKUBA, Ibaraki Prefecture--Air pollution levels exceeded the environmental standard at nearly a third of all monitoring stations across Japan on one day in January, the National Institute for Environmental Studies said Feb. 21.
Airborne concentrations of PM 2.5—particulate matter of up to 2.5 micrometers in diameter—were higher in western Japan, indicating the pollution likely originated in China, NIES officials added.
PM 2.5, which can penetrate deep into human lungs and is believed to raise the risk of respiratory and circulatory disease, has been causing severe air pollution in Beijing and other Chinese cities.
The Japanese environmental standard for airborne PM 2.5 concentrations is a daily average of 35 micrograms per cubic meter.
The NIES summarized measurement data at air pollution monitoring stations across Japan. The readings exceeded the environmental standard at the largest number of monitoring stations on Jan. 13 and Jan. 31.
On Jan. 13, the concentrations exceeded the standard at 40, or 27 percent, of 148 stations across Japan where valid data was obtained. Many of those 40 stations are located in central Kyushu, on the Inland Sea, in the Kinki region and in northern Kanto.
On Jan. 31, the standard was exceeded at 48, or 31 percent, of 155 stations where valid data was obtained, with many of the excessive levels occurring in northern Kyushu and on the Inland Sea.
High PM 2.5 readings were registered on some days in the Kanto and Tokai regions, which are thought to be less susceptible to the cross-border spread of pollutants from China. Those cases may represent compound pollution, originating partly in Japanese cities and partly attributable to cross-border pollution from China, the NIES officials said.
Monitoring stations in the Kinki region and farther to the west saw 4 percent of valid measurements there exceed the environmental standard in January this year, largely unchanged from 3.5 percent in January last year.
NIES simulations also showed that PM 2.5 of Chinese origin would have drifted to Japan. But Toshimasa Ohara, director of the NIES Center for Regional Environmental Research, said the airborne PM 2.5 concentrations are diluted to one-10th to one-20th of the levels in China by the time the particulates reach Japan.
"People with respiratory diseases should watch out, but healthy people don't have to worry," Ohara said.
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