Residents of Kumamoto Prefecture on Japan's southwestern island of Kyushu were asked to voluntarily stay in their homes Tuesday, after air pollution levels surpassed government safety standards.

The request was the first since the environment ministry released new safety guidelines in February, and came as Japan grows increasingly worried about toxic air pollution spilling over from China and threatening Japanese coastal areas.

A spokesman for the prefecture said readings made at one of the prefecture's 18 checkpoints, Arao-city in northern Kumamoto, indicated that levels of "PM 2.5" were above 85 since early morning, "so we decided to issue the warning."

PM 2.5 is particulate matter with a diameter of 2.5 micrometers or less that can be inhaled causing asthma, bronchitis, lung cancer and other illnesses.