An extraordinarily strong yellow dust storm originating from the inland deserts in northern China and Mongolia blanketed all of South Korea on Monday, prompting authorities to issue a yellow dust warning for Seoul and almost all parts of the country.

The density of fine dust particles smaller than 10 micrometers in diameter, known as PM 10, surged to the "very bad" level in the greater Seoul area and all other regions, authorities said.

As of 10 a.m., the hourly average concentration of PM 10 reached 1,115 micrograms per cubic meter in Daegu, 842 micrograms in the southwestern city of Gwangju, 508 micrograms in Seoul and 749 micrograms in the central city of Daejeon, according to the state-run National Institute of Environmental Research's Air Quality Forecasting Center.

Notably, the hourly PM 10 average skyrocketed to 1,348 micrograms in parts of Daegu, about 300 kilometers southeast of Seoul.

The weather authorities here categorize concentrations of PM 10 between zero and 30 micrograms as "good," between 31 and 80 as "normal," between 81 and 150 as "bad" and more than 151 as "very bad."

The PM 10 level reached a peak of 545 micrograms in Seoul on Monday morning, the center said, noting Busan and the southern resort island of Jeju also registered very bad levels of PM 10 reaching 235 and 267 micrograms, respectively.

The center explained that the entire nation has come under the influence of a massive dust storm that originated in the Inner Mongolian region in northern China and the Gobi Desert in Mongolia on Friday and moved south by riding northwesterly winds.

It forecast the PM 10 reading will remain at the very bad level across the nation throughout Monday and at the bad level Tuesday due to residual yellow dust and atmospheric stagnation.

The Korea Meteorological Administration issued a yellow dust warning for Seoul as of 7:10 a.m., asking citizens with respiratory or cardiovascular problems, children and other vulnerable people to refrain from going outdoors.

The yellow dust warning was also issued for almost all parts of the country, excluding Jeju Island. A yellow dust warning is issued when the PM 10 figure is forecast to stay above 800 micrograms per cubic meter for longer than two hours.

According to the air quality center, the density of ultrafine dust particles smaller than 2.5 micrometers in diameter, known as PM 2.5, also soared to the very bad level in almost all parts of the country, including the greater Seoul area

As of 10 a.m., the hourly concentration of PM 2.5 rose to 169 micrograms per cubic meter in Daegu, 134 micrograms in North Gyeongsang Province, 130 micrograms in Gwangju, 126 micrograms in South Gyeongsang Province and 124 micrograms in North Jeolla Province.

South Korea's authorities have classified levels of PM 2.5 above 35 micrograms per cubic meter as "bad" and above 75 micrograms as "very bad," while the World Health Organization recommends keeping levels below 25 micrograms. (Yonhap)