A woman rides a bike in the heavy smog with a mask on a street in Haozhou, central China's Anhui province on Jan. 30, 2013. (AFP/GETTY IMAGES)

TOKYO — Even as it struggles to keep menacing Chinese ships from entering its territorial waters, Japan is bracing itself for an altogether different kind of danger from its larger neighbor – toxic smog.

Under guidelines issued on Wednesday, Japanese authorities will urge residents to stay indoors if the level of toxic smog spreading to Japan from China exceeds twice the allowable limit set by the central government.

The guidelines come in response to mounting concerns in Japan about the potential harmful effects of the toxic smog wafting across the ocean and after talks between the two countries on ways to curb pollution levels.

In recent weeks, levels of air pollution in Beijing have hit all-time highs – the concentration of fine particulates reaching levels far greater than that considered healthy under the latest U.S. standards – and triggered health concerns beyond China.

“There are increasing inquiries from the general public on the situation,” especially in western Japan, said Hitoshi Yoshizaki, deputy director of the ministry of environment’s air quality division. “Many people would like to know how to protect themselves.”

The new provisional guidelines, compiled by the environment ministry, recommend that people stay indoors if the average amount of air pollutant, PM 2.5, is projected to exceed 70 micrograms per cubic meter – or twice the ministry’s maximum permissible level of 35 micrograms a day. Beijing’s pollution regularly exceeds 10 times that level – on Wednesday evening Beijing’s PM 2.5 levels were 273 micrograms per cubic meter.

The air pollutant PM 2.5 is considered most hazardous because the particulates are small enough to enter the bloodstream and damage lung tissue.

Since April 2012, levels over 70 micrograms per cubic meter have been recorded at six monitoring stations in Japan. In future, Japanese local authorities will recommend that those most vulnerable – people with heart or lung disease, the elderly and children – stay indoors if the PM 2.5 level exceeds that level.

In China, where PM 2.5 has enveloped cities including Beijing, the government considers anything above 300 as “severely polluted” and “hazardous.” The main source of the pollution is believed to be intensive burning of coal in power plants and people’s homes in China, particularly in the cold winter months.

Levels of PM 2.5 are being monitored at more than 500 stations across Japan but the government aims to increase that to 1,300.

The municipal government of Fukuoka, the biggest city on Japan’s southern island of Kyushu and one of the closest to the Chinese mainland, has developed its own system for warning residents when levels of PM 2.5 exceed certain limits.

However, with just a few weeks of data since concerns mounted in January, it is hard to assess the real risks to health, Yoshizaki said, noting that overall air quality standards had not dropped dramatically from a year earlier.

Last week, officials from Japan’s Ministry of the Environment met counterparts at China’s Ministry of Environment Protection to discuss “potential collaboration” on projects to curb pollution, according to Yoshizaki.

— Financial Times

Ben McLannahan in Tokyo and Leslie Hook in Beijing also contributed to this story.