Citizens of Seoul were put on highest alert Monday morning after the density of fine particles in the air hit a record level. Seoul city officials said the city has been under siege since 1 a.m. when the density of sand, dust and aerosols, or fine solid or liquid particles suspended in the air, measured 1,042 micrograms per cubic meter. Officials said...the figure hit a record 2,070 micrograms per cubic meter at 4 a.m. and is steadily decreasing. This is the first time ever the city has been placed under a condition of high alert due to the seasonal yellow dust phenomenon, which carries an enormous amount of sand and harmful aerosols from western parts of China to Korea, Japan and sometimes even California every spring. Under the alert, all kindergartens and primary schools are advised to cancel classes and all kinds of outdoor activities. Ordinary citizens, but in particular children, the elderly or weak, and those with respiratory problems are advised to remain indoors. People are advised to carry umbrellas, wash their face, hands and feet, gargle and brush their teeth after going outdoors and take other precautionary measures necessary as the fine particles in the air can irritate the eyes, skin and respiratory organs.