Residents woke up to heavy smog in New Delhi yesterday as the air quality in the world's most polluted capital city reached hazardous levels. The website of the United States Embassy in India said levels of fine pollutants known as PM2.5 - which are most harmful to the health - reached 703. That is more than double the threshold of 300, which the authorities class as hazardous. Delhi's air quality usually gets worse before the onset of winter as the cooler air traps pollutants near the ground, and prevents them from dispersing into the atmosphere. The Indian Medical Association, meanwhile, has called for the cancellation of the New Delhi half-marathon, slated to take place on Nov 19, because of the air pollution.