The Seoul municipal government issued an ultrafine dust advisory Monday, urging people to stay indoors.

The measure was taken at around noon, after Seoul’s atmospheric concentration levels of “particulate matter-2.5” pollutants recorded 92 micrograms per cubic meter.

The city government issues the advisory if the average pollutant levels of 85 micrograms per cubic meter persist for at least two hours. A higher-level alert of warning is issued when the levels reach 120 micrograms.

“PM-2.5,” or particulate matter that is 2.5 micrometers or less in diameter, is largely made up of sulphate and nitrate particles.

Exposure to ultrafine particles can cause several serious health effects such as cardiac and respiratory problems, as they can penetrate the respiratory system further than larger particles, according to experts.

“While high-density fine dusts were created in China and moved westward toward the Korean Peninsula, haze and moderate wind here have caused the atmosphere to stand still, pushing the levels of the ultrafine dust higher,” a city official said.

China has been hit by record levels of the pollutant, with some 15 percent of the country being cloaked in thick smog for nearly a week.

“Elderly people, children and patients with respiratory and cardiovascular diseases should refrain from going out. When going out, citizens are advised to wear yellow-dust masks,” the official added. (Yonhap)