An ultrafine dust advisory has been issued for the city of Seoul for the first time in nine months as the concentration has risen to alarmingly high levels.

The Seoul Research Institute of Public Health and Environment issued the advisory at 12 a.m. on Thursday after the daily average density of ultrafine dust particles in Seoul’s 25 administrative regions stayed above 75 micrograms per cubic meter for more than two hours.

The capital’s daily average density of such dust reached 76 micrograms per cubic meter at 11 p.m. on Wednesday and 12 a.m. on Thursday.

The institute has urged people to remain indoors, calling for extra caution for the elderly, children and people with respiratory or cardiovascular conditions.