A pair of artificial lungs put up in New Delhi to demonstrate the lethal effects of smog have turned a sickly dark brown within ten days of their installation, underscoring the city's pollution crisis.

Delhi is the world's most polluted major city.

It has been covered in a toxic grey haze since the onset of winter last month, with pollution levels several times higher than the World Health Organisation's safe limits.

White at first, the oversized lungs were put up on the premises of the Sir Ganga Ram hospital on 3 November.

They had been fitted with high-powered particle-trapping filters to mimic the way a human body functions.

"The most striking bit is the rapidity with which the lungs have turned black. It is absolutely frightening," said Arvind Kumar, a lung surgeon who has been campaigning to raise awareness about the dangers of air pollution.

The top image shows the lungs on 3 November. The bottom image shows them ten days later

As cooler air traps pollutants close to the ground, levels of PM2.5 - particles so tiny they can enter the lungs and bloodstream - have soared dangerously.

"There is no reason to believe that the same material is not getting deposited in our lungs as well," Dr Kumar told AFP.

"The health consequences of this is going to be disastrous."

Today, PM2.5 levels in the city touched 263, more than ten times the recommended average of 25, according to the US embassy in Delhi which independently monitors air pollution.

It had hit a peak of 369 at 11am local time before a brief spell of rain brought some respite.