Ultrafine dust particles in the air have increased nationwide amid rising temperatures.

The air quality forecast center of the National Institute of Environmental Research said Saturday that fine dust density was in the "bad" category in metropolitan Seoul, Gangwon, Chungcheong, Gwangju, North Jeolla and parts of Gyeongsang Province and the reading for the rest of the country was "medium."

Dust levels are forecast to become "very bad" briefly overnight in the capital region and South Chungcheong and North Jeolla Provinces.

The institute attributed the high concentration to fine dust remaining from the previous day in addition to new dust blowing in from overseas.

Fine dust levels are expected to subside from Sunday with better atmospheric circulation and readings ranging from "good" to "medium."