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It is an iconic landmark that attracts millions of visitors every year, but a particularly sticky form of air pollution is turning the gleaming marble domes of the Taj Mahal brown.

Scientists have found that tiny dust particles and soot from the burning of dung, wood and rubbish nearby are sticking to the surface of the famous building.

Once on the marble surface of the Taj Mahal, these particles absorb ultraviolet light, giving its normally white domes a dirty brown shade.

The Taj Mahal in Agra, India, needs regular cleaning as its white marble is being discoloured by air pollution

Researchers found that these particles are particularly troublesome as they are insoluble in water and so cannot be easily washed off. 

Instead workers currently cover the outside of the domes with a layer of clay every few years before peeling it off to restore the white colour of the building.

But now scientists hope their findings will help those caring for the 460 year old mausoleum in Agra, India, to develop new methods for cleaning the building.

Professor Mike Bergin, an environmental engineer at the Georgia Institute of Technology in Atlanta who led the study, said: 'Over the past several decades the outer marble surfaces of the Taj Mahal have begun to discolor with time and must be painstakingly cleaned every several years. 

'Although it has been generally believed that the discoloration is in some way linked with poor air quality in the Agra region, the specific components of air pollution responsible were yet to be identified.'

He first became interested in what was changing the colour of the Taj Mahal after seeing workers applying 'facial masks' of clay to the domes of the building.

Built between 1632 and 1653 by the Mughal emperor Shah Jahan in memory of his third wife Mumtaz Mahal, it was constructed from white marble excavated 230 miles away in Makrana, Rajasthan while some of the outlying buildings were built from red sandstone.

Acid rain, sulphurous gas in the air and oxidizing fog have all been blamed for the changing colour of the marble over the years.

In 1983 the Taj Mahal was named a UNESCO World Heritage Site and the Indian Government has set up a 4,000 square mile zone around the monument where strict restrictions on industrial emissions are in place.

However, the building has continued to discolour, suggesting something else was the blame. 

The Taj Mahal was built from thousands of blocks of marble carried to the site on wagons pulled by oxen

Dust and pollution from nearby industry was thought to be causing the Taj Mahal to change colour but the new research suggests local traffic along with the burning of wood, dung and rubbish is causing the damage

To discover what was causing the discolouration, Professor Bergin, together with scientists from the Indian Institute of Technology and the Archaelogical Survey of India, placed blocks of marble around the Taj Mahal site for a year.

They then analysed the particles that had built up on the surface and found high concentrations of light absorbing organic carbon, also known as brown carbon, dust and black carbon.

These were found to be insoluable in water, meaning they cannot be easily washed off.

By measuring the amount of light reflected by these particles, they were able to work out which were responsible for causing the change in colour as seen by the human eye.

Writing in the journal Environmental Science and Technology, the researchers said: 'A large fraction of the outer Taj Mahal surfaces are covered with particles that contain both carbonaceous components and dust.

'Our results indicate that deposited light absorbing dust and carbon from the combustion of fossil fuels and biomass are responsible for the surface discolouration of the Taj Mahal.'

Professor Bergin believes that cutting the amount of organic material and traffic levels around the Taj Mahal may help efforts to return the Taj Mahal to its original colour.

Professor Bergin started the study after seeing workers using clay to clean the domes at the Taj Mahal