The first ultrafine dust advisory in nine months was issued across Seoul on Thursday, the city government said.

The advisory was issued at midnight over the capital, the first since Feb. 11, according to the Research Institute of Public Health and Environment of the city government.

The advisory is issued when the hourly average concentration of PM 2.5 -- particles smaller than 2.5 micrometers in diameter -- stay above 75 micrograms per cubic meter for over two hours.

The hourly average concentration across Seoul's 25 districts stood at 76 micrograms per cubic at 11 p.m. on Wednesday and at midnight, the institute said.

It said the ultrafine dust level may have gone up as domestically generated fine dust piled up due to air stagnation. (Yonhap)