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[timelets]

Shanghai used to be a relatively clean high-tech office city. Now, it's as polluted as the rest of Chinese major cities.

On Tuesday, a curtain of grey smog fell over Shanghai, China's business capital with a population of over 20 million. It limited visibility and drove the city's air quality index (AQI) above 300, a level deemed "hazardous" on most scales and which can have a long-term impact on health.

The levels of PM 2.5, dangerous tiny pollutants, hit 281, the highest since mid-January, according to data compiled by the US Department of State. PM 2.5 particles are a major cause of asthma and respiratory diseases, experts say.