A toxic haze enveloped India's national capital on Wednesday morning as temperatures dropped and pollution surged, reducing visibility in some parts and prompting a warning from airport authorities that flights may be affected.

Delhi overtook Lahore on Wednesday to become the world's most polluted city in Swiss group IQAir's live rankings. India's pollution control authority said the national capital's 24-hour air quality index (AQI) score had touched 418.

Toxic smog in India and Pakistan has caused air quality in the major metropolitans of Lahore, Multan, Delhi and Chandigarh to deteriorate to toxic levels.

The government in Lahore has closed schools and public spaces and shut down factories. Environmental lawyer Ahmad Rafay Alam shares his perspective: "frustrated" but still fighting.

The satellite images show an extremely heavy grey smog over Pakistan. It stretches into northern parts of India, albeit is lighter than the one over its neighbouring country.