A silent assault on heritage

The implications of air pollution on health, education and work productivity dominate headlines every winter, when air quality deteriorates to hazardous levels across northern India. New research now shows that these pollutants are also slowly corroding the symbols of India’s past, placing the country’s cultural heritage at risk.

A scientific study on the Red Fort has found that chemical interactions between air pollutants and the monument’s stone surface are destroying its distinctive red façade, gradually turning it black.

The findings echo earlier concerns about pollution-induced damage to historic monuments. Decades ago, pollution was identified as the cause behind the Taj Mahal’s changing appearance. Despite protective measures, the “wonder of the world” continues to make headlines for foul odours and marble surfaces turning green.

The Red Fort has long been a political symbol, serving as the backdrop for every Prime Minister’s Independence Day speech since 1947. Yet political will has fallen short in protecting it — and hundreds of other monuments — from the effects of air pollution. The Red Fort study is the first to link the formation of black crusts on its surface directly to emissions from cement factories, thermal power plants and vehicles.

“Indian monuments are already losing their settings because of rampant encroachment and unauthorised construction. It’s sad to see policymakers believe there is a Planet B. Otherwise, there would have been concrete measures to reduce air pollution,” says Abha Narain Lambah, a UNESCO award-winning conservation architect.

Not all is lost, however. Researchers say early interventions can prevent or slow airborne damage to heritage buildings.

A cocktail of chemical reactions

Many of the world’s ancient monuments are built from limestone and marble, materials favoured for their availability, durability and ease of extraction. Historic Mughal structures in Delhi — including the Red Fort, Qutub Minar and Humayun’s Tomb — use Vindhyan sandstone and Makrana marble.

These materials now face unprecedented stress amid rapid industrialisation. High levels of particulate matter and atmospheric nitrogen dioxide from traffic and industrial emissions have triggered a sulfation process on the Red Fort’s sandstone surface. This leads to the formation of a thin gypsum layer (calcium sulphate), which gradually develops into black crusts that trap dust and other pollutants, the study found. Beyond visible damage, erosion reduces the cultural and aesthetic value of heritage sites.

Early restoration efforts can mitigate this damage. “We always begin conservation using non-abrasive methods that don’t harm the stone,” Lambah explains. “Gentle water misting under controlled pressure, or techniques like multani mitti or paper poultices, are used. Chemical peeling is applied only when grime accumulation persists, and always under supervision.” Sanjay Kumar Manjul, Additional Director General at the Archaeological Survey of India (ASI) and an author of the study, says restoration has already begun at the Red Fort. “Every product contains chemical compounds. What matters is dosage — how much is used and how it reacts with the surface. Even something as safe as multani mitti has mineral components,” he says.

Protecting the Taj

For M C Mehta, the lawyer behind the landmark case to protect the Taj Mahal, restoration alone is insufficient without controlling emissions. “Even after 40 years since my case highlighted pollution-driven deterioration, and several hearings later, little has changed,” he says. His petition led to the creation of the Taj Trapezium Zone (TTZ) in 1996.

The TTZ restricts coal- and coke-based industries within a 10,400-square-kilometre area covering parts of Uttar Pradesh and Rajasthan. The aim was to limit the Taj’s exposure by relocating industries beyond the zone.

The TTZ Authority includes district magistrates, pollution control boards, ASI representatives and state ministries. It has powers to enforce fuel standards and halt utilities from polluting units. Monthly district-level meetings and quarterly reviews are mandated. Yet pollution persists. A recent RTI revealed that high suspended particulate matter levels have continued to yellow the Taj’s marble for nearly two decades.

“A major challenge is rapid development around the Taj,” says Amit Mishra, Regional Officer at the Uttar Pradesh Pollution Control Board. “PM2.5 levels have risen sharply due to vehicular emissions. While Agra Metro construction aims to reduce traffic, it has also increased dust.” In May, the National Green Tribunal flagged poor implementation of pollution-control measures in Agra, noting that initiatives like tree planting and electric buses remained largely on paper.

Controlling rising vehicular emissions

Vehicular emissions pose a growing threat in Delhi, where over 700,000 new cars were registered in 2024. SUVs, among the most polluting vehicle types, have become increasingly popular. Exhaust fumes deposit heavy metals — including lead, titanium and nickel — onto the black crusts forming on the Red Fort.

Sohail Hashmi, historian and heritage walk leader, emphasises public participation. “If we value our heritage, both government and citizens must act together. Affordable public transport and willingness to carpool are crucial,” he says. “Unless we know where we come from, how do we decide where we’re going? Without understanding our architectural tradition, evolved over thousands of years, how do we connect our present with our past?” Hashmi asks.

Mongabay India/TWF