NEW DELHI, Nov 3 (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 world's most polluted capital is blanketed in smog every winter as cold, heavy air traps construction dust, vehicle emissions and smoke from the burning of crop stubble in neighbouring states to clear the fields for the next crop."I know children don’t vote for you, but still, requesting all the chief ministers of Delhi (capital region) to immediately SHUT DOWN all the schools," environmental activist Vimlendu Jha wrote on Twitter."It's not NORMAL to breathe 500+ AQI, not for our children, where every third child already has some pulmonary challenge."Delhi Chief Minister Arvind Kejriwal, whose party also rules Punjab state where crop burning is rampant, said on Twitter that the "people of Punjab and Delhi are taking all steps at their level" to tackle pollution."