The walls of the Red Fort in Delhi are flaking. Intricate carvings are losing detail. Plastered vaults are blistering. Dampness and salt deposits are spreading inside arches. These are among the warnings raised in a new scientific study that links growing air pollution in the capital to visible and accelerating damage at the 17th-century monument.
The research was published in June this year on the MDPI platform, which hosts peer-reviewed, open-access journals. Titled Characterisation of Red Sandstone and Black Crust to Analyse Air Pollution Impacts on a Cultural Heritage Building: Red Fort, Delhi, India, it is the first comprehensive scientific investigation into how Delhi’s poor air quality is degrading the UNESCO World Heritage site.
“Given the alarming air quality situation in Delhi, studying important monuments like the Red Fort is crucial for promoting effective conservation policies and interventions. This is the first research article addressing the impact of air pollution on the monumental complex of the Red Fort. Hopefully, this case study will enhance our understanding of the degradation processes affecting this monument and help evaluate the conservation state in relation to the outdoor environment of other Indian cultural heritage buildings,” the paper said.
The project was carried out under a collaboration between India’s Department of Science and Technology and Italy’s Ministry of Foreign Affairs. Scientists from IIT Roorkee, IIT Kanpur, Ca’ Foscari University of Venice, and the Archaeological Survey of India (ASI) conducted the work.
Rising damage
The research focused on the fort’s sandstone walls and the black crusts forming on their surfaces. Black crusts are a known symptom of heritage decay in polluted cities. They darken stone, trap pollutants, and weaken structures.
Analysis showed the crusts were mainly made of gypsum, bassanite, and weddellite, along with trace heavy metals such as lead, zinc, chromium, and copper. These originated from vehicle exhaust, cement factories, and construction dust.
The thickness of the crusts varied depending on exposure. In sheltered corners, deposits were as thin as 0.05 millimetres. On walls facing heavy traffic zones, such as near Delhi Gate, layers reached up to 0.5 millimetres. The thick layers bonded strongly to the stone, raising the risk of flaking and permanent loss of carvings.
Samples were collected from Zafar Mahal, Moti Masjid, Delhi Gate, and the fortification wall near Mahatma Gandhi Road, one of the busiest stretches in the area.
The samples were examined through Scanning Electron Microscopy, X-ray Diffraction, Fourier Transform Infrared Spectroscopy, and mass spectrometry. These tests confirmed that the Red Fort’s sandstone is primarily quartz and microcline, while the black crusts were rich in gypsum and salts formed from chemical reactions with polluted air.
Pollution data
The study also drew on air quality monitoring records from 2021 to 2023. Data from the Central Pollution Control Board showed that fine particulate matter (PM10 and PM2.5) levels in Delhi were consistently above safe limits.
As per the study, “PM10 concentrations were often three times higher than the national standard, while PM2.5 levels were more than double. Nitrogen dioxide levels were also above the threshold. Ammonia and sulfur dioxide were below national limits, but even small amounts of sulfur dioxide were enough to trigger gypsum formation on the stone”.
The study noted that high concentrations of particulate matter act as carriers for heavy metals and other corrosive substances, which then lodge on monument surfaces.
Researchers documented several types of damage inside the fort complex. Red sandstone elements showed clear flaking, which has already led to the loss of intricate carvings. Plaster walls and vaulted ceilings displayed blistering, a result of salt crystallization taking hold in damp conditions.
“Inside the arches, white crusts have begun to appear, indicating water infiltration into the structure. Rising dampness was also found to be degrading entrance doors, where colored patinas suggest possible biological growth. Makrana marble, used in structures such as the Moti Masjid, was found to be less vulnerable. Its crystalline structure provided greater resistance to pollution and weathering, particularly in sections that remain sheltered”, the study read.
Long-term threats
The Red Fort was built by Mughal emperor Shah Jahan between 1639 and 1648 and later modified by both Mughal rulers and the British. It became a UNESCO World Heritage site in 2007 and remains one of India’s most visited monuments.
Originally, the fort was protected by the Yamuna River on its eastern side and a moat around the other three. Today, the river has shifted away, leaving the eastern walls exposed to busy roads and urban development.
Experts warned that the combined effects of salt crystallization, black crusts, and pollution could cause not just cosmetic discoloration but structural damage if left unaddressed.
The study recommended that early removal of black crusts be prioritised. Gentle cleaning methods and protective coatings could help slow further damage.
Conservationists stressed that the Red Fort is not alone. Other Mughal-era monuments in Delhi, such as Humayun’s Tomb and Safdarjung’s Tomb, also face similar risks from air pollution.
Delhi is regularly ranked among the world’s most polluted capitals. During winter, PM2.5 levels can climb to nearly ten times the safe limit, driven by traffic, industrial activity, and stubble burning in nearby states.
A call for vigilance
The researchers urged long-term monitoring of both pollution levels and monument surfaces. They also called for more such studies on other heritage sites across India, many of which face the same pressures but lack detailed data.
“Monuments like the Red Fort are irreplaceable,” said Dr. Maria of Ca’ Foscari University. “They carry history that belongs not just to India but to the world. Protecting them must be a shared responsibility.”
For now, the fort’s sandstone walls continue to weather Delhi’s toxic air, showing both the grandeur of the past and the urgency of the present.
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