Mention the Indian subcontinent and one of the first things that comes to mind is the Taj Mahal. Located in Agra, the mausoleum that is widely hailed as the "Jewel of Muslim art in India", was built in the 1600's by Mughal Emperor Shah Jahan in memory of his third wife, Mumtaz Mahal. Intricately crafted from pure white marble, the masterpiece that took about 22 years to complete, is considered one of the seven wonders of the modern world and visited by millions of people each year.

However in the late 1970's, the pristine white exterior of the Taj Mahal's famous dome and minarets started to sport a brown hue. Since the grit is not water soluble, every few years the structure now has to be cleaned by painstakingly applying and removing a layer of clay. Though air pollution has long been suspected no studies were done until recently, when a team of scientists led by Georgia Institute of Technology's Mike Bergin and Sacchichida Tripathi from the Indian Institute of Technology, Kanpur, decided to investigate the cause.

The researchers began by placing small pieces of pristine white marble at several locations around Taj Mahal's main dome for two months. They then used scanning electron microscopy to examine the particles deposited. In addition, they also took air samples from the area and analyzed them for pollutants. The results were consistent. Both the air filters and the marble contained particles of dust, brown organic carbon and black carbon.

The scientists who published their findings in the online journal Environmental Science & Technology on December 3rd, 2014, believe that the carbon particles come from a number of sources - fuel combustion, vehicle exhausts, trash burning and brickmaking. They suspect that the dust emanates from local agricultural activities, as well as the increasing number of vehicles that traverse the area.

Now that the pollution source possibilities have been identified, the next plan is to curtail the activities causing them. Though the government has already taken some measures to reduce vehicle and industrial emissions in the vicinity of the Taj Mahal, there is still much to be done. However, since some the pollution could be resulting from sources that are much further out, identifying and controlling them could prove to be a little tricky.

But the researchers believe that it is of utmost importance. That's because the pollution is not just browning the Taj Mahal, but also impacting the well-being of the residents of the area, since many of the particles are known to be harmful to humans. As Bergin succinctly puts it “The health of humans and the health of the Taj Mahal are intertwined.”

Resources: sciencemag.org,rsc.org