A local government in southwestern Japan on Tuesday advised residents to stay indoors or wear masks when they go outside in the nation's first official health warning over smog drifting in from China.
Officials in Kumamoto, on Kyushu island, said the quality of air was likely to be substantially below national standards, amid warnings of health risks for the young and the sick.
Of specific concern is the concentration of particulate matter 2.5 micrometres or less in diameter, which has been as high as 50 micrograms per cubic metre in several parts of Kyushu.
The government safe limit is 35 micrograms. Japan's environment ministry last week said local authorities should issue warnings when concentrations rise to an average of 70 micrograms per hour.
The government in Kumamoto prefecture, which lies hundreds of kilometres (miles) to the east of China, said readings were as high as 110 micrograms in some places on Tuesday morning.
It advised local residents to stay indoors and not to exercise outside, while encouraging them to wear a surgical mask when they ventured out.
A thick fug of pollution has blanketed Beijing and other Chinese cities a number of times over recent weeks.
Last month Japanese media reported a swirl of pollution was making its way to Japan from China, further complicating already-strained relations between Tokyo and Beijing, who are squabbling over disputed islands.
Kyodo News reported Tuesday that a meeting of environment ministers from Japan, China and South Korea set for May will discuss ways to combat pollution.
The toxic haze that blankets China has been blamed on emissions from coal burning in power stations but also on exhaust fumes from vehicles on the traffic-clogged streets of the world's largest largest auto market.
Scientists say prevailing winds carry airborne particles from the Asian mainland to Japan.
The phenomenon was first observed with the fine yellow sand from the deserts of Mongolia and China that gathers on cars and buildings in Japan at certain times of the year.