REUTERS/Anushree Fadnavis Acquire Licensing RightsNEW DELHI, Nov 20 (Reuters) - India's capital Delhi re-opened schools and some building sites on Monday amid signs of receding air pollution, although it remained classified as hazardous, while a toxic foam besmirched stretches of the Yamuna river flowing through the city. The world's most polluted capital resumed its annual battle on pollution this month, despite government pledges to improve. Monday's air quality index (AQI) of 336 was down from Thursday's level of 509, but still "hazardous", Swiss group IQAir said. Delhi's air pollution gets worse in winter, when wind speeds drop and cooling air traps pollutants spewed by vehicles, industry and farmers burning agricultural waste in surrounding states to prepare for new planting. PM2.5 levels remained above 128 micrograms per cubic meter of air since Sunday in the National Capital Region, according to the federal pollution control board.

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Persons: Anushree, IQAir, Ankit Srivastava, Gopal Rai, Kanjyik Ghosh, Shivam Patel, Clarence Fernandez, Raju Gopalakrishnan Organizations: REUTERS, Delhi's, Vehicles, National Capital, World Health Organization, Thomson Locations: New Delhi, India, DELHI, Delhi, Mumbai