(CNN)The Chinese capital was choking on sand and dust on Monday, as the biggest sandstorm in nearly a decade swept over the city turning the skies an eerie shade of orange.
Photos from Beijing, home to 21.7 million residents, show skyscrapers and cars shrouded in a thick haze, with air quality indexes recording a “hazardous” rating and authorities advising residents to stay indoors.
Many commuters continued to battle the elements, however, walking and biking through strong, sandy winds. Visibility was so bad in parts of the city that drivers had to turn on their headlights even in the middle of the day.
“In some places, there are strong sandstorms with visibility of less than 500 meters (1,640 feet),” said the China Meteorological Administration in a statement on Monday. “This is also the strongest dust and sand weather affecting China in almost 10 years.”
Air quality in Beijing was already poor, owing to high levels of pollution. When the sandstorm hit, the city’s air quality plummeted to dangerous levels, according to the World Air Quality Index.
The index measures the concentration of different pollutants in the air — the most important being PM 2.5. This harmful microscopic particulate matter is smaller than 2.5 micrometers in diameter, and is considered particularly dangerous as it can lodge deep into the lungs and pass into other organs and the bloodstream.
Beijing measured a maximum of 655 micrograms per cubic meter on Monday. The World Health Organization considers anything above 25 to be unsafe.
The sandstorm originated in Mongolia, where six people have died, and 81 are missing, according to Chinese state-run outlet The Paper.
From Mongolia, the sandstorm gradually moved southward. Beijing saw concentrations of the larger PM 10 particles exceed 8,100 micrograms per cubic meter according to the city’s environmental monitoring center, prompting the Central Meteorological Observatory to issue a yellow alert for sandstorms — the second level in a four-tier color-coded weather warning system.
Authorities advised the public to avoid going outside if possible, and the Beijing Municipal Commission of Education asked schools and educational committees on Monday to suspend outdoor activities.
Sandstorms used to be a regular occurrence in spring. In previous decades, each May saw at least two rounds of sandstorms, according to state-run Xinhua news agency. The frequency and severity of sandstorms was partly due to drought, growing population pressure and poor progress in revegetation, which caused rapid desertification of land in the north and northwest.
But sandstorms have since decreased dramatically; the annual number of sandstorm-impacted days in Beijing fell from a peak of 26 in the 1950s to just three days after 2010, Xinhua reported.
Since 2000, the Chinese government has invested billions of dollars toward sandstorm prevention. Authorities have launched various reforestation and ecological projects, and installed satellites to monitor sandstorms and alert weather agencies ahead of time.
Sandstorms have also hit northern Hebei and Shanxi provinces, western Gansu, and central and western Inner Mongoliaon Monday, Xinhua said. Other parts of the country, including northern Xinjiang, are seeing high levels of wind gusts. The sandstorms are expected to last through Tuesday.
Mongolia, which lies north of mainland China, is experiencing strong cyclones, said the meteorological administration. The sand and dust from Mongolia have moved eastward and southward over China’s northern regions, carried by the cold high pressure at the back of the cyclone.