Fine dust has hit the nation amid warm spring weather on Sunday.

The National Institute of Environmental Research forecasts that fine dust concentrations will remain at “bad” levels on Sunday in many regions, including the Seoul metropolitan area, Gangwon, Chungcheong, Jeolla and Gyeongsang and Provinces and Jeju Island.

An ultra-fine dust advisory is in place in Seoul since 8 p.m. Saturday.

An advisory is issued if concentration levels remain above the 90-microgram-per-cubic-meter threshold for more than two hours.

Ultra-fine dust measures two-point-five micrometers or less in diameter, or one-thirtieth of a strand of hair, meaning it can infiltrate and do more harm to the respiratory system than fine dust.