India, Aug. 2 -- A black, sooty layer has been quietly spreading across the sandstone walls of Humayun's Tomb, one of Delhi's iconic Unesco World Heritage sites, and a new scientific study says the cause is unambiguous. It's Delhi's air.

The study, conducted jointly by IITs at Roorkee and Kanpur and published in the International Journal of Architectural Heritage, found that years of exposure to polluted air have formed patches of black crust on the tomb's red sandstone, and that this crust is now accelerating the monument's decay, according to news agency PTI on Sunday.

The researchers collected black crust samples from different parts of the tomb and analysed them for chemical composition, mineral content and microscopic structure usi...