Fine dust blanketed much of the country on Monday, the first day masks were not required on public transit.

The National Institute of Environmental Research forecast that levels of ultrafine dust, or particulate matter smaller than two-point-five micrometers in diameter, will be "bad" until Tuesday in the central region, including the Seoul metropolitan area, as well as western Gangwon Province and North Jeolla Province.

Levels are expected to be "very bad" in some parts of the capital area until Tuesday morning.

"Very bad" is the highest on the environment ministry's four-tier ultrafine dust warning.

Emergency fine dust reduction measures are being implemented from 6 a.m. until 9 p.m. in Seoul, Gyeonggi Province and Incheon, such as reduced hours at construction sites.