Seoul, Nov 20 : An ultrafine dust advisory has been issued for all of Seoul for the first time in about six months, the metropolitan government said. The advisory was issued on Friday as the hourly average density of ultrafine dust particles soared to 96 micrograms per cubic meter in 25 districts across the capital, according to the city-run Research Institute of Public Health and Environment.(Yonhap) It was the first ultrafine dust advisory issued in Seoul since May 7. 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 authority issued another fine dust advisory at 9 p.m. (S. Korea time) as the city's concentration of the PM 10 -- particles smaller than 10 micrometers in diameter -- stayed above 150 micrograms per cubic meter for over two hours. The institute said the density of PM 2.5 and PM 10 have soared due to stagnant air flow under the influence of the high pressure located around the Korean Peninsula and the inflow of air pollutants. It advised citizens with respiratory or cardiovascular diseases, the elderly and the weak and children to stay indoors if possible and wear a dust mask when they have to go outside. /IANS