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All schools and colleges in the Indian capital city, Delhi, have now been closed after worsening levels of air pollution.

Some schools had already shut last week because of the situation and health officials have asked people to not do any outdoor activity, to avoid getting ill.

There may also be a lockdown put in place to improve air quality, as thick clouds of smog have covered the city since the festival of Diwali in early November.

A mix of factors like carbon dioxide emissions from cars and coal-powered factories, dust and weather patterns make Delhi the world's most polluted capital.

The levels of PM2.5 - tiny particles that can clog people's lungs - in Delhi are far higher than the World Health Organization's (WHO) safety guidelines.

Several parts of the city recorded figures close to, or higher than, 400 on Tuesday, which is categorised as "severe".

A figure between zero and 50 is considered "good", and between 51 and 100 is "satisfactory", according to the the air quality index or AQI.

The air turns especially toxic in winter months as farmers in neighbouring states burn leftover crops to make way for growing wheat.

Fireworks during the festival of Diwali, which happens at the same time, only worsen the air quality.

Low wind speed also plays a part as it traps the pollutants in the lower atmosphere.

This year, the pollution has become so severe that India's Supreme Court - the country's highest court - warned state and national governments to take "imminent and emergency" measures to tackle the problem.

India's pollution problem is not just a problem in the capital city, Delhi.

Indian cities regularly top global pollution rankings and bad air kills more than a million people every year, according to a report by US research group, the Energy Policy Institute at the University of Chicago.

It added that north India breathes "pollution levels that are 10 times worse than those found anywhere else in the world".