Delhi's iconic Red Fort is silently turning black; the real reason may just shock you
Delhi’s Red Fort is turning black — and air pollution is to blame. A joint Indo-Italian study found that toxic pollutants in the capital’s air are forming “black crusts” on the monument’s red sandstone walls, threatening both its beauty and structure.
Delhi’s iconic 17th-century Red Fort, a UNESCO World Heritage Site and symbol of India’s rich Mughal heritage, is showing signs of distress, and the culprit is the city’s worsening air pollution. Scientists have found that the monument’s signature red sandstone is gradually turning black due to a chemical reaction triggered by pollutants in the air.
A joint study by Indian and Italian researchers has revealed that ‘black crusts', layers of pollution deposits containing compounds like gypsum, quartz, and heavy metals such as lead, copper, and zinc, are forming on the fort’s surface. These crusts not only darken the walls but also erode the stone, threatening the monument’s long-term stability.
What’s behind the Red Fort’s darkening walls?
The study, conducted between 2021 and 2023 and published in the Heritage journal in June 2025, is the first detailed examination of air pollution’s chemical impact on the Red Fort. Scientists collected wall samples from different sections of the monument and compared them with Delhi’s air quality data.
Their findings revealed that the fort’s red sandstone surfaces have developed crusts ranging from 55 to 500 micrometres thick, made up of gypsum, bassanite, and weddellite — minerals that point to external calcium sources, primarily from pollution.
According to the researchers, these crusts result from years of particulate matter accumulation, causing both discolouration and surface exfoliation. The pollutants responsible come from several sources:
- Construction and industrial activity: Emissions from factories, construction sites, and road dust contribute to the deposition of calcium and heavy metals.
- Vehicular emissions: Exhaust fumes and wear-and-tear from vehicles release harmful elements like titanium, vanadium, chromium, manganese, nickel, copper, zinc, and lead.
- Fossil fuel combustion: Continuous burning of coal and fuel oil further accelerates chemical corrosion of the monument’s surface.
If left unchecked, experts warn, the same phenomenon could soon affect other historic landmarks in Delhi — including Humayun’s Tomb and Safdarjung’s Tomb — which face similar environmental conditions.
Can the damage be reversed?
While reducing Delhi’s overall air pollution remains a long-term challenge, scientists suggest that early intervention can help protect the fort’s structure and appearance.
“The formation of a black crust is a progressive phenomenon that begins with a thin black layer, which can be removed without harming the stone if treated early,” the study notes.
The researchers recommend introducing a regular maintenance and cleaning programme for high-risk sections of the fort and using stone-protective coatings to prevent new crust formation.
Such conservation measures, they argue, could slow down the fort’s deterioration and preserve its distinctive red hue for future generations.