People wear masks on their way to work amid an ultrafine dust advisory, Seoul, Jan. 22. Yonhap

Korea was suffocated by thick ultrafine dust for the second day in a row on Wednesday, prompting authorities to regulate the operations of government-owned vehicles, coal power plants and other large-scale dust emission facilities.

Due to the continued influx of fine dust from abroad, the density of ultrafine dust particles smaller than 2.5 micrometers in diameter, known as PM 2.5, is forecast to remain at "bad" levels nationwide throughout the day, excluding the southern Jeju Island and east coastal regions, the Korea Meteorological Administration (KMA) said.

In particular, the PM 2.5 density is expected to soar to the "very bad" level in Seoul, southern Gyeonggi Province, western Gangwon Province, the central city of Sejong and the central province of North Chungcheong in the morning, the KMA noted.

As of 8 a.m., an ultrafine dust advisory has been issued nationwide, excluding the southern provinces of South Gyeongsang and South Jeolla.

Korea's authorities have classified levels of PM 2.5 above 35 micrograms per cubic meter as "bad" and above 75 micrograms as "very bad," while the World Health Organization recommends keeping levels below 25 micrograms.

As many as nine local governments, including ones in the Seoul metropolitan area, took emergency emission reduction measures, such as a mandatory alternate no-driving system for public vehicles, a road ban for old diesel cars and reduced operations of coal power plants and some factories.

The air quality levels are expected to be bad across the country until Thursday before tapering off after the onset of a northerly wind, the KMA said. (Yonhap)