Smog from China is drifting over parts of Japan, triggering health warnings for the young and sick.
Japanese media has been saturated with coverage of the thick, choking smog blanketing Beijing and other Chinese cities in recent days.
Now news programs have broadcast maps showing the pollution being pushed east towards southern Japan.
Japanese health experts are warning that people with respiratory problems and small children are susceptible to the smog, which is worst on the southern island of Kyushu.
The website of Japan's environment minister has been choked with users logging on to monitor the level of pollution heading their way.
"Access to our air-pollution monitoring system has been almost impossible since last week, and the telephone here has been constantly ringing because worried people keep asking us about the impact on health," an environment ministry official told the AFP news agency.
Relations between Tokyo and Beijing are already strained by a dispute over the sovereignty of a chain of islands in the East China Sea.
"China is our neighbour, and all sorts of problems happen between us all the time," Takaharu Abiko, 50, said in Tokyo.
"It is very worrying. This is dangerous pollution, like poison, and we can't protect ourselves. It's scary."
Officials have been wary of lumping all the blame on China, but Yasushi Nakajima of the environment ministry said "we can't deny there is an impact from pollution in China".
Air pollution over the west of Japan has exceeded government limits over the last few days, with tiny particulate matter a problem, Atsushi Shimizu of the National Institute for Environmental Studies (NIES) said.
Prevailing winds from the west bring airborne particles from the Asian mainland, he said.
Of specific concern is the concentration of a particle 2.5 micrometres or less in diameter, which has been as high as 50 micrograms per cubic metre of air over recent days in northern Kyushu.
The government safe limit is 35 micrograms.
Yellow sand from the deserts of Mongolia and China is a known source for these particles, as are exhausts from cars and smoke from factories.
"At this time of year they are definitely not yellow sands, so they're toxic particles," Mr Shimizu said, warning that "people with respiratory diseases should be careful".
Toshihiko Takemura, an associate professor of Kyushu University who runs another air pollution monitoring site, said "the impact of air pollution originating from China on Japan was scientifically discovered more than a decade ago".
"Especially in Kyushu, the level of air pollution has been detectable in everyday lives since a few years ago," he added.
"People in eastern and northern Japan are now belatedly noticing the cross-border air pollution."
Professor Takemura noted that pollution in Japan over the last few days has not been quite as bad as it was in February 2011, when "very hazy days continued for several days in western Japan".
He said people with respiratory diseases, as well as small children, should take extra care to avoid the problems.
Professor Takemura's website said an "extremely large" amount of air pollutants would arrive in part of Kyushu on Monday and Tuesday.
Mr Shimizu said information-sharing with China on air pollution has been difficult but "there are many things Japan can do, for instance encouraging China to use pollutant-filtering equipment in factories".