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Extreme air pollution in the nation's capital city of New Delhi has caused the development of sooty deposits on the exteriors of the historic monument, a new study finds.

Scientists observed that these crusts—created due to chemical interactions between particles and the red sandstone building—were ranging from 0.05mm and 0.5mm in thickness, and could deteriorate its elaborate artwork if conservation efforts are taken.

This study is the pioneering of its category to comprehensively examine the effects of polluted air on this ancient site.

Conservationists have repeatedly alerted about the detrimental impact of pollution on historical buildings in the city and multiple other states.

Previously, judicial authorities stated that the Taj Mahal—similarly iconic historic site—had yellowed due to environmental factors, calling for swift conservation steps.

The study on the monument, released in a academic publication in June, was performed between 2021 and 2023 by experts in multiple countries.

The Red Fort, established by ruler Shah Jahan, is one of the city's most prominent heritage monuments.

India's first Prime Minister Jawaharlal Nehru raised the flag from the ramparts on August 16 1947, a day after sovereignty from British rule was attained. Ever since, prime ministers have been delivering addresses on the national holiday from the walls of the structure.

Researchers analyzed the city's pollution data from recent years. They then scraped off the sooty layer found on multiple surfaces of the site and studied its content.

They found that airborne particles and other contaminants in the atmosphere had led to the black sediment to form on the structure's exteriors and also damaged other features such as ceilings, arches, and intricate designs.

The experts also identified signs of blistering and chipping on the walls.

"Fine particulate matter are commonly accepted as major causes to the darkening of materials in contact with the atmosphere. This occurrence takes place when particulate matter accumulate over time, leading to visible blackening of these surfaces," the paper states.

The analysis suggests the timely application of conservation strategies to preserve the monument.

"Appearance of a dark layer is a progressive process that usually begins with a thin dark film, which can be treated, particularly in the beginning," the study adds.

It also mentions that stone protectives could be used to highly-affected sections to slow down the formation of black crusts.