FUKUOKA, Japan — When the Chinese smog arrives, the medical masks come in fashion.

Every few months, this city of 1.5 million people in southern Japan, not far from mainland China, gets a dose of lung clogging courtesy of its neighbor.

Coal factories in the cities of Tianjin and Beijing, combined with the growing numbers of automobiles, pump out toxins that drift westward across the East China Sea. They hit Japan and, to a lesser extent, South Korea.

The most recent air pollution crisis came in February, when a whitish gray blanket of smog fell over Fukuoka. The city government put out an advisory on its early warning system — the first in Japan, started that month — urging everybody, and especially infants and the elderly, to stay indoors and wear face masks outside.

“There is concern among citizens over the health effects,” said Keiko Nabamuta, a city environment official. “Whenever this happens, we ask residents to stay indoors and avoid hanging their laundry outside,” a measure to prevent unsafe particulates from gathering on clothing.

The air pollution problem has become so pervasive that it has joined the list of diplomatic issues on the table between three fractious nations: China, which produces much of it, and Japan and South Korea, on its receiving end.

On May 7, top environmental officials of the three countries met in Kitakyushu, a city near Fukuoka, and agreed to set up a panel that will occasionally gather to explore solutions.

“Air pollution and climate change are common issues in the region," Japan's Environment Minister Nobuteru Ishihara told his counterparts in remarks carried by public broadcaster NHK. "Apart from domestic countermeasures, it is indispensable for China, South Korea and other countries to cooperate in solving them."

The promises of cooperation came at an odd time. This year, relations between the three countries have fallen to a nadir over a line-up of territorial disputes.

The nations have drummed up a war of words over two chains of islets: the Liancourt Rocks torn between South Korea and Japan, and the Senkaku Islands disputed between China and Japan.

Adding to the bad blood, the countries are also debating the history of Japanese atrocities during World War II.

South Korea and China expressed displeasure when, on Tuesday, the mayor of Osaka said that sex slaves, commonly referred to as “comfort women,” were a “necessary evil” to maintain discipline in the Japanese army during World War II.

China’s environmental protection minister, Zhou Shengxian, cancelled his attendance on claims he was preoccupied with the damage of the April earthquake in Sichuan, in south-central China, which killed some 200 people. Yet Japanese media speculated that his absence was related to rising tensions over the Senkaku Islands.

Early warning

Over the past year, the governments of Japan and South Korea have been stepping up their early warning capabilities for cities at risk from air pollution.

In Fukuoka, the municipal government raises the alarm on the municipal website when the average number of particulates with a diameter smaller than 2.5 micro-millimeters, known shorthand as PM2.5, reaches 35 or more micrograms per cubic meter in a day. That’s less than one-thirtieth the thickness of human hair.

It’s a mouthful, but a significant number. The label PM2.5 typically includes contaminants from coal plants and factories, and can lead to respiratory infections and cancer.

The fumes already pose a serious public health crisis for China. In April, a study published in leading medical journal The Lancet linked air pollution to 1.2 million deaths in the country in 2010. The number comprises more than a third of the world’s total air pollution-related deaths.

The conundrum is getting worse for China and its neighbors. In the first three months of this year, the levels of two airborne pollutants — nitrogen dioxide and particulates from the PM2.5 to the PM10 range — increased by almost one-third in Beijing over the same period last year.

South Korea, too, has set up an early warning system, although it’s dealing with a slightly different problem from the Beijing-born gasses.

Every spring, clouds of “yellow dust” move southward from the Gobi Desert — the mass that straddles Mongolia and northern China — and sweep the Korean peninsula and other parts of the Pacific.

The storms have been recorded for thousands of years, and have not always been a malignant force. But times are changing. China is industrializing and experiencing desertification, the process by which land becomes denuded and arid; the resulting winds are carrying heavy metals such as mercury, arsenic, and cancer-causing toxins into Seoul.

“The Korean government built six air quality concentration measuring stations throughout the nation,” said Kim Jong-choon, the director at the Korean Institute of Environmental Research, a government body, “and they analyze not only air pollutants but also harmful substances in them such as lead, cadmium, and arsenic.”

So far, South Korea has had mixed luck tackling the problem with China. In 2007, for instance, South Korea sent thousands of trees to China, in hopes they would be planted in the desert to halt the spread of yellow dust.

Instead, the Chinese government placed them next to a highway.

But Japan and South Korea haven’t experienced all gloom and doom dealing with their difficult neighbor. Korean and Chinese volunteers are helping plant some 4 million trees in China’s Gobi Desert, and the government says it’s making an effort to ensure trees are placed in the right zones.