NEW DELHI (Reuters) -Delhi’s 20 million residents were effectively breathing smoke on Thursday as the air quality index (AQI) breached the “severe” and “hazardous” categories in nearly all monitoring stations of the Indian capital, spurring calls to close schools. The AQI exceeded 450 in many places early in the day, according to data from the Central Pollution Control Board. A reading over 400 affects healthy people, with serious impacts on those with existing diseases, the federal government says. The index exceeded 800 in some pockets of the city, according Delhi Pollution Control Committee…