South Korea's capital city has issued its third ultrafine dust advisory this month on Saturday, advising people to stay indoors.

   The measure was taken after Seoul's atmospheric concentration levels of "particulate matter-2.5" pollutants stood at 69 micrograms per cubic meter as of 10 a.m. after hitting 62 micrograms in 8 a.m.

   Seoul issues an advisory if the pollutant levels stay above the 60 micrograms per cubic meter for more than two consecutive hours.

The alert is lifted after concentration falls below the 45-microgram mark.

   The latest measure is the third to be issued after the first and second alerts were issued on Dec. 4 and on Friday.

   "City authorities have taken the step because ultrafine dust is coming in from China, and this has been compounded by slower wind movements and higher humidity," said an official from the Seoul metropolitan government.

   He said such conditions can pose health problems for people with respiratory ailments, senior citizens and young people.

   The official said that actions taken by the city have been posted on various digital road signs, Internet portal sites and Twitter accounts.

   South Korea's weather service, meanwhile, said ultrafine dust levels in some Chinese cities hit 151 micrograms per cubic meter earlier in the day. (Yonhap News)