Air pollution is accelerating the decay of Humayun’s Tomb in Delhi by driving the formation of pollutant-rich black crusts on its sandstone surfaces, according to a joint study by IIT Kanpur and IIT Roorkee of the UNESCO World Heritage Site.’The study, according to the researchers, “provides a multi-analytical evidence base to inform preventive conservation strategies and the prioritisation of cleaning and monitoring actions at this site and at comparable monuments in similarly polluted urban environments”.Published in the International Journal of Architectural Heritage, the study was carried out by Gaurav Kumar, Bhola Ram Gurjar and Chandra Shekhar Prasad Ojha from the Department of Civil Engineering at IIT Roorkee, along with Mukesh Sharma from IIT Kanpur.To understand how pollution affects the monument, the researchers collected five small black crust samples from rain-sheltered rooftop locations of Humayun’s Tomb under the supervision of a conservator from the Archaeological Survey of India.No sandstone was removed from the monument because of conservation restrictions. Instead, only the thin black deposits that had accumulated on the stone surface were carefully collected using a scalpel, avoiding mortar joints and repaired areas to ensure that no damage was caused to the structure. The samples were collected from different orientations of the rooftop to capture how prevailing winds and environmental exposure influence pollutant deposition.The sampling locations were chosen based on Delhi’s long-term weather patterns, explains the study. During the post-monsoon and winter months, prevailing northwesterly and westerly winds transport sulphur dioxide, nitrogen oxides, particulate matter and trace metals across the city.According to the authors, these winds carry “urban and regional air pollutants like SO₂, NOx, PM2.5, PM10, and trace metals eastward, which favors the accumulation and growth of BC (black crust).”Story continues below this adOnce collected, the samples underwent a comprehensive series of laboratory analyses. The analyses showed that the black crust is dominated by gypsum, a sulphate mineral formed through atmospheric pollution, along with carbon-rich particles embedded within the mineral layer. “Combined evidence from XRF, XRD, FTIR, SEM-EDX, and CHNS analysis identifies carbonaceous rich gypsum as the signature alteration phase, accompanied by trace amounts of heavy metals indicative of atmospheric inputs,” the study said.According to the researchers, the gypsum does not originate primarily from the sandstone itself. Instead, calcium supplied through road dust and airborne particles reacts with sulphur compounds present in polluted air to form gypsum on the monument’s surface.The study found that the crust also contains trace metals associated with urban pollution. “Trace-element profiles further suggest regional air pollution markers, consistent with Delhi’s major emission sources, including vehicular traffic, road/soil dust, construction dust, biomass burning, and industrial activities,” the authors wrote.The study also found that black crust formation varies across different parts of the monument depending on environmental exposure.Story continues below this adThe researchers reported that “gypsum was most abundant at the more wind-exposed, rain-sheltered rooftop locations and comparatively weaker at more protected areas, revealing a clear exposure and orientation related gradient in crust development.”One of the study’s principal findings is that the “black crust [is] identified as a passive air pollution sampler at the Tomb.” In the conclusion, they write that the results “support the interpretation of BC as a passive sampler of the Delhi ambient pollution scenario”,Calling for preventive conservation measures, the researchers state that “the formation of BC on heritage stone is a cumulative, long-term process that closely tracks regional air pollution levels. Delhi’s persistently elevated pollution burden has therefore accelerated BC accretion on local monuments.”They conclude that “this study highlights that the monuments in Delhi require targeted preventive conservation interventions, as the megacity is currently a polluted hotspot.”Story continues below this adThe study recommends periodic removal of black crusts using carefully selected and scientifically established cleaning techniques, evaluation of protective coatings through site-specific trials, continued air-quality monitoring, and the use of atmospheric modelling to assess future deterioration risks.It also suggests that reducing emissions through measures such as greater adoption of electric vehicles and lower-emission fuels could help reduce the pollutant load that contributes to black crust formation around vulnerable heritage monuments.