The Seoul Metropolitan Government has issued an ultrafine dust advisory as of 3 p.m. Monday.

According to the Seoul Research Institute of Public Health and Environment under the city government, ultrafine dust levels surged after air pollutants from outside the country coincided with fine dust particles that remained from the previous day.

The average ultrafine dust level for Seoul’s 25 districts was 78 micrograms per cubic meter as of 2 p.m. and 79 micrograms per cubic meter as of 3 p.m, staying above the advisory standard of 75 micrograms per cubic meter for two hours.

Ultrafine dust particles are two-point-five micrometers in diameter or less, whereas fine dust particles measure up to ten micrometers in diameter.

City officials advise seniors, children, and anyone with respiratory or cardiovascular disease to refrain from going out, and for everyone else to wear masks if they go outdoors.