Seoul City's research institute on public health and environment issued the winter season's first ultrafine dust advisory at 4 p.m. Wednesday.

Such an advisory is issued when the density of ultrafine dust particles stays above an average 75 micrograms per cubic meter for more than two hours.

Under the advisory, grade 5 vehicles that are without emission reduction devices are not allowed to operate on roads from 6 a.m. to 9 p.m. Thursday. 

The institute says the air pollutants flew in from overseas via westerly winds due to a high pressure front moving eastward from the South Sea.

The institute's chairman Shin Yong-seung advised the elderly, children and people with respiratory or cardiovascular ailments to avoid going outdoors, and if they must, wear a mask.

Ultra-fine dust levels are expected to remain at bad levels through tomorrow.