Air pollution accelerating decay of Humayun’s Tomb

New Delhi: A black crust formed on the sandstone of Humayun’s Tomb in Delhi by years of exposure to air pollution is accelerating the deterioration of the UNESCO World Heritage Site, a joint study of IIT Roorkee and IIT Kanpur has found.

Published in the International Journal of Architectural Heritage, the study examined black crust samples collected from different parts of Humayun’s Tomb using laboratory techniques to analyse their chemical composition, mineral content and microscopic structure.

The researchers found that the black crust is mainly made up of gypsum — a mineral formed when sulphur dioxide in polluted air reacts with calcium-rich dust and moisture, the study showed. The gypsum layer traps soot, dust and tiny metal particles from vehicle emissions, construction activities, road dust, biomass burning and industrial pollution, gradually forming a dark coating on the sandstone, it mentioned.

The study found that the thickest black crust developed in rain-sheltered parts of the monument, where pollutants accumulate over long periods, while more exposed surfaces had comparatively thinner deposits because rain naturally washes away some of the pollutants.

According to the researchers, the black crust is more than a surface stain. The chemical reactions that form the crust gradually weaken the sandstone, making it more vulnerable to weathering and long-term deterioration.

They said the findings provide scientific evidence linking air pollution to the deterioration of Humayun’s Tomb and could help authorities plan conservation measures for the monument.