BEIJING - Beijing's air quality reached dangerous levels Wednesday as a large sandstorm swept across the city, turning the sky into a murky yellow and forcing many residents to stay indoors. The grey mix of sand, dust and pollution enveloped the capital's skyscrapers, and was exacerbated by a series of coal-fired power stations in the area. This is the second sandstorm to hit Beijing in three days, and it is a seasonal phenomenon caused by spring winds that are blowing from the nearby Gobi Desert, depositing fine dust over the sprawling Chinese capital of 21 million people. On Wednesday, the Beijing Air Quality Index rose to dangerous levels as sandstorms swept in from Inner Mongolia, smothering the dust up to 1,000 meters in a thick layer of thick, greyish-green dust. According to the Chinese Environmental Protection Agency , there is a possible source of this dust event in the region. Eastward - moving winds carry soil into the land, which regularly complains of dust clouds that filter sunlight and cover everything with dust. Northern China, including Beijing, is hit by violent sandstorms and dust storms in late winter and early spring. The dust is swallowing the northern region and causing severe air pollution, leading people to question the protection of the forests that are supposed to stop the sand. Li, the Greenpeace activist, said that sandstorms are so frequent and intense that we have to experience them every day to reduce the number of sandstorms reaching the capital. As Figure 1 shows, since the early 1990s, northern China has experienced an increase in sandstorms and dust storms in late winter and early spring. Dust storms in 2000 launched a new programme to address the worsening problem in the Beijing area caused by the dust storm in 2000. A report by the Chinese Meteorological Agency on the state of climate change in China's northern regions also shows that desertification is accelerating. China's own official measurement for Beijing, calculated using six different pollutants, including PM2.5, is 871 , which is classified as hazardous. Beijing has seen an increase in dust storms and sandstorms in late winter and early spring since the early 1990s, and average annual air quality is classified as hazardous by the World Health Organization and China's Environmental Protection Agency . China Seed The official EQI is forijing, which is calculated using four different levels with six different pollutants. Contains PM1.6, PM4.0 and PM5.1, 8,71 were classified as "hazardous." According to China's Ministry of Ecology and Environment, the off-chart figures are due to PM10 particles brought in by the sandstorm and other pollutants. Visibility in Beijing has dropped to about 1 km around the capital and is expected to deteriorate further. In an update at 0500 GMT on Wednesday, China's environment ministry , the country's top environmental protection agency, said dust and sand had covered more than 1,000 km of the city's road network, including the Beijing-Tianjin-Hebei-Hubei expressway, the world's second-largest road network, it said. Dust and sand covered less than a third of Beijing's road network, which includes the China-Guangdong-Henan-Shaanxi-Xinjiang-Gansu-Jiangsu-Sichuan-Hunan highway, "it said in a statement updated at about 0500 GMT on Wednesday. Saturday's sandstorm was even more violent as it deposited grit on roofs, sidewalks and trees, he said. He said it was also seeping into other parts of the city, with grit deposited on roofs, sidewalks and trees. Sunday's sandstorms seemed all the more serious because they left gravel on roofs, streets and sidewalks and in other areas, such as eastern Beijing, he said. Air dust is one of the main pollutants affecting air quality in China, according to the Chinese Environmental Protection Agency. In 1983, an estimated 1.6 atg of dust aerosols were deposited in Beijing alone, more than twice the amount in any other city. Managing air quality would affect the health and well-being of all Chinese citizens, not just the more than 700 million Chinese who live near a dust storm. On May 4, PM10 peaked at 1,000 mcg in Beijing and the sky turned murky yellow. In Beijing, five of the six largest pollutants measured in the Air Quality Index exceeded the 500 limit, the maximum limit set by the monitoring authorities. It should be noted that this A 2,3 level air quality is not related to the anthropogenic pollution caused by the static stability of weather conditions. Beijing is also a city that was badly hit by floods last year that hit large parts of normally dry northern China. In populous cities in northeastern China, including Beijing, huge plumes of dust stretched hundreds of miles, darkening the sun, reducing visibility and slowing traffic. China's state-run Forestry Administration said the dust storm had affected more than 1.5 million hectares of land in the north of the country.