Ultrafine dust levels remained "bad" in Seoul and other western parts of the country on Monday amid an influx of particles from China, environmental authorities said.

As of noon, the PM 2.5 reading in Sejong, 120 kilometers south of Seoul, was 58 micrograms per cubic meter, followed by 55 micrograms per cubic meter in North Chungcheong Province, 50 micrograms per cubic meter in the northern Gyeonggi and Gangwon Provinces and 44 micrograms per cubic meter in Seoul, according to the National Institute of Environmental Research, which is affiliated with the Ministry of Environment.

PM 2.5 is a measure for fine dust particles smaller than 2.5 micrometers in diameter. By contrast, PM 10 is a measure for particles smaller than 10 micrometers in diameter.

The high concentration levels have largely been attributed to fine dust blowing in from China, where heating and the resumption of factories since the COVID-19 shutdown are believed to be increasing formation of smog.

Other parts of the country were forecast to see "average" levels of ultrafine dust, although the southern cities of Gwangju and Ulsan, as well as South Gyeongsang Province, were expected to temporarily see "bad" levels in the afternoon.

The four fine dust levels are "good," "average," "bad" and "very bad."

"In most areas the concentration will be high due to the accumulation of domestically produced fine dust, caused by stagnant air and the convergence of air currents," the institute said. "The eastern region will see high concentration levels in the afternoon due to the added fine dust arriving from overseas."

Seoul and other western parts of the country, including Incheon and North Jeolla Province, have been under ultrafine dust advisories since Sunday.

In Seoul, the advisory was issued for the first time since Feb. 22.

The "bad" levels are expected to continue through Tuesday morning in Seoul, Incheon, northern Gyeonggi Province, the Chungcheong Provinces and North Jeolla Province.

Southern Gyeonggi and the southeastern city of Daegu are expected to see "bad" levels throughout Tuesday, while the rest of the country shows "average" to "good." (Yonhap)