More than 40,000 marathon runners are preparing to compete in the Indian capital of New Delhi on Sunday, where air pollution has reached dangerous levels.

This week's city image shows that it is covered with thick yellow smoke, and some buildings barely see fog. Air quality indices ranged from 0 to 500, reaching 227 last Friday and classified as “very unhealthy”.

This score reflects the concentration of fine particulate matter per cubic meter called PM2.5. According to the World Health Organization, the acceptable air quality index for human normal breathing is 25.