Northern India, including the country's capital New Delhi, and neighbouring Pakistan are facing a major air pollution crisis that has disrupted the daily lives of many people and threatened their health significantly.
New Delhi has temporarily closed 4,000 schools and will start to ban cars on certain days in a bid to combat the problem.
A recent study found that pollution was linked to 2.5 million deaths in India in 2015 and New Delhi is its most polluted city. The air quality in the Indian capital is sitting at 40 times the World Health Organization's safe limit. Doctors say it is the same as smoking 50 cigarettes a day.
How is smog formed?
Smog results when a photochemical reaction takes place between air pollutants from crop burning, vehicles and others sources, and sunlight and heat. The resulting thick haze - also known as ground-level ozone - gets trapped near the earth due to warm air in the atmosphere.
The following graphic reflects the process of smog formation.