Researchers from the United States and India reported that dust and carbon particles that are airborne are causing discoloration on the marble dome and minarets of the Taj Mahal, turning the white color of the Indian landmark to brown.

The Georgia Institute of Technology, the University of Wisconsin, the Indian Institute of Technology at Kanpur and the Archaeological Survey of India all collaborated in the study that analyzed the effects of pollution on the Taj Mahal.

"Our team was able to show that the pollutants discoloring the Taj Mahal are particulate matter: carbon from burning biomass and refuse, fossil fuels, and dust - possibly from agriculture and road traffic," said Georgia Tech School of Earth and Atmospheric Sciences professor Michael Bergin.

The landmark was erected in the 1600s as ordered by Mughal emperor Shah Jahan to honor the memory of Mumtaz Mahal, his wife. The Taj Mahal has a marble dome that is 115 feet high, with minarets that are 130 feet high.

The Taj Mahal has been a UNESCO World Heritage Site since 1983, attracting millions of visitors per year.

However, starting in the 1970s, there have been observations of a brownish tone on the white marble of the structure. To clean this up, routine cleaning in the form of mud-pack therapy is used.

The primary suspect for the discoloration of the marble is air pollution, but no studies have been previously done to confirm the claim until now.

Researchers utilized special equipment to take samples of the air surrounding the landmark from November 2011 to June 2012. In addition, small samples of fresh marble were placed on various points of the Taj Mahal.

An analysis of the filters of the air sampling equipment and the samples of marble indicated that the Taj Mahal was exposed to dust, black carbon and brown organic carbon.

The researchers believe that the dust came from agricultural activities in the region and from vehicular traffic, while the carbon came from several sources including vehicle exhaust and trash burning.

To confirm their findings, the researchers applied the same particles to a piece of marble. The result compared favorably to the observations of brown color on the Taj Mahal.

Unless the issue is resolved, the Archaeological Survey of India said that it might have to do the mud-pack therapy on the Taj Mahal yearly, when it was only previously done once every five to six years. While the therapy is currently effective, the process takes days to complete, wherein the Taj Mahal is closed to visitors. In addition, repeated usage of the process could permanently remove the original color of the Taj Mahal.