Air pollution is accelerating the deterioration of Delhi UNESCO World Heritage Site Humayun’s Tomb by causing the formation of pollutant rich black crusts on its sandstone surfaces according to a joint study by IIT Kanpur and IIT Roorkee.
Published in the International Journal of Architectural Heritage, the study found that these black crusts are mainly composed of gypsum a sulphate mineral formed when calcium from road dust reacts with sulphur compounds present in polluted air. The deposits also contain carbon rich particles and trace heavy metals linked to urban pollution.
Researchers collected five black crust samples from rain sheltered rooftop sections of the monument under the supervision of the Archaeological Survey of India. To protect the heritage structure, only the thin surface deposits were removed, without disturbing the sandstone.
Laboratory analysis revealed that the crusts act as a “passive sampler” of Delhi’s air pollution, capturing pollutants such as sulphur dioxide (SO₂), nitrogen oxides (NOx), PM2.5, PM10, and trace metals transported by prevailing winds. The study linked these pollutants to emissions from vehicular traffic, construction and road dust, biomass burning, and industrial activities.
The research also found that black crust formation is more severe on wind exposed, rain sheltered sections of the monument, where pollutant accumulation is highest.
The researchers warned that Delhi’s persistently high pollution levels are speeding up the long term deterioration of heritage structures. They recommended periodic scientific cleaning of the black crusts, continuous air quality monitoring, testing of protective coatings, and stronger emission-control measures, including greater adoption of electric vehicles and cleaner fuels, to help safeguard historic monuments.