Gyeonggi Province issued an ultrafine dust advisory for eleven cities at 3 p.m. Monday as the level of microscopic particles in the air remained high.

The level of ultra fine dust hit an hourly average of 94 micrograms per cubic meter in the eleven cities, including Suwon, Ansan, Anyang and Bucheon.

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.

Gyeonggi Province advised citizens with heart and respiratory problems as well as children and the elderly to stay indoors.