The Red Fort under threat as toxic air turns its walls black 

September 18, 2025

Delhi’s soaring pollution levels are now leaving their mark on one of India’s most iconic monuments. A new scientific study has found that the 17th-century Red Fort is developing thick “black crusts” on its red sandstone walls, a phenomenon directly linked to the capital’s hazardous air quality.

The study, published in June in the peer-reviewed journal Heritage, represents the first detailed analysis of how Delhi’s toxic air is physically altering the Mughal-era monument. The research was a joint effort of India’s Department of Science and Technology, Italy’s Ministry of Foreign Affairs, scientists from IIT Roorkee and IIT Kanpur, Ca’ Foscari University in Venice, and the Archaeological Survey of India (ASI). It was carried out between 2021 and 2023.

Researchers collected samples from the fort’s outer walls and from Zafar Mahal, a part of the complex, and examined them closely. What they discovered was troubling: black crusts measuring between 0.05 mm and 0.5 mm thick had formed on many of the fort’s surfaces. These layers were made up of materials such as gypsum, bassanite, weddellite, quartz and microcline, the study found. By contrast, the original red sandstone was primarily composed of quartz and microcline.

The scientists attributed the high presence of gypsum in the crusts to emissions from construction sites, road dust and cement factories. Vehicular traffic, industrial activity, biomass burning and soil dust in the vicinity of the monument were also identified as contributing factors.

Delhi consistently ranks among the world’s most polluted cities, with its air quality plunging sharply during winter months. The researchers used Central Pollution Control Board (CPCB) data covering the period 2021 to 2023. The findings revealed that the concentration of particulate matter—tiny, inhalable particles known as PM2.5 and PM10—was more than two and a half times higher than India’s national limits.

These particles, the study explained, are significant contributors to the soiling of surfaces exposed to open air. Over time, they settle and accumulate, forming visible layers that discolour and blacken stone. In the case of the Red Fort, the deposits have not only darkened the sandstone but also begun damaging delicate carvings and architectural details such as vaults and arches. Researchers observed blistering and flaking of the walls in several areas, an early warning sign of deeper deterioration.

The study has sounded an alarm for the future of the Red Fort, which is both a UNESCO World Heritage Site and a symbol of India’s history. The researchers warned that unless steps are taken to address air pollution and protect the monument, intricate Mughal-era carvings could suffer irreversible damage.

The impact of pollution on heritage structures is not unique to Delhi, but the Red Fort’s location makes it especially vulnerable. The monument stands in the heart of the capital, surrounded by heavy traffic, residential clusters and industries. During site visits, researchers noted constant vehicular movement, nearby factories and high particulate matter levels. They linked the black crusts directly to these conditions, describing them as the outcome of “atmospheric deposition linked to pollution.”

The findings underscore a broader issue: the toll of environmental degradation on cultural heritage. Delhi’s poor air quality, already a severe public health crisis, is also threatening its monuments. The Red Fort, built by Mughal emperor Shah Jahan in the 17th century, has stood for centuries as a symbol of power and tradition. Today, it is being slowly disfigured by soot and pollutants.

The researchers have not prescribed specific conservation measures, but the implications of their study are clear. Without stronger controls on emissions and urban pollution, the physical integrity of the Red Fort may be at risk. The study not only highlights the fort’s vulnerability but also serves as a reminder of the broader challenge India faces in preserving its heritage in the face of rapid urbanisation and worsening air quality.

For now, the blackening walls of the Red Fort stand as visible evidence of the capital’s toxic air—and a warning of what may be lost if action is delayed.