Seoul City's Health and Environment Research Institute has lowered its ultrafine dust warning to an advisory as of Saturday noon.

Ultrafine dust particles are smaller than two-point-five micrometers in diameter, commonly known as PM 2.5.

The warning was issued when the hourly average density of ultra fine dust surpassed 162 micrograms per cubic meter. As the concentration dropped to 70 micrograms in 12 hours, the warning has been replaced with an advisory.

However a warning alert is maintained concerning fine dust measuring under ten micrometers in diameter, known as PM 10.

The Seoul city government has urged the elderly, children and people with respiratory or cardiovascular conditions to remain indoors and wear masks if they must head outside.

Ultrafine dust particles are more harmful to humans as they are one 30th of the thickness of hair and can reach our lungs unfiltered.