Residents in India's northern states woke up to another day of poor air quality on Tuesday, as a layer of dense fog shrouded most of the region, and pollution in the capital Delhi remained severe.

India's capital city of Delhi has virtually shut down as air pollution hit "severe plus" levels, according to Indian government officials in its pollution control authority.

Air pollution chokes New Delhi every winter, but this year's smog has already derailed everything from transport and road work to schools.