Participants run past the India Gate as they take part in the Airtel Delhi Half Marathon in New Delhi, India, October 20, 2019.

A runner wearing a face mask for protection from air pollution takes part in the Airtel Delhi Half Marathon in New Delhi, India, October 20, 2019.

A runner wearing a face mask for protection from air pollution takes part in the Airtel Delhi Half Marathon in New Delhi, India, October 20, 2019.

A man wearing a mask runs past the India Gate on a smoggy morning in New Delhi, India, October 28, 2019.

A man paddles a home-made boat across Yamuna river on a smoggy morning in New Delhi, October 28, 2019.

A woman wearing a mask walks inside a park on a smoggy morning in New Delhi, India, October 27, 2019.

A man paddles a home-made boat across Yamuna river on a smoggy morning in New Delhi, October 28, 2019.

Buildings are seen shrouded in smog in New Delhi, India, October 30, 2019.

Buildings are seen shrouded in smog in New Delhi, India, October 30, 2019.

A man rides a bicycle on a smoggy morning near India Gate in New Delhi, India, October 17, 2019.

A runner wearing a face mask for protection from air pollution takes part in the Airtel Delhi Half Marathon in New Delhi, India, October 20, 2019.

A man wearing a mask rides a cycle on a smoggy morning in New Delhi, India, October 28, 2019.

Birds fly next to electricity pylons on a smoggy afternoon in the old quarters of Delhi, India, October 30, 2019. Image for illustrative purposes.

Women walk in front of India's Presidential Palace on a smoggy morning in New Delhi, India, October 27, 2019.

Vehicles drive through smog in New Delhi, India, October 30, 2019.

A man paddles a home-made boat across Yamuna river on a smoggy afternoon in the old quarters of Delhi, India, October 30, 2019.

A man rides a cycle past the India Gate on a smoggy morning in New Delhi, India, October 28, 2019.