Mexico's rich archaeological heritage is at risk, thanks to a rise in air pollution. A recent report claims that acid rain could destroy the country's various Mayan monuments and temple structures, literally dissolving the stones which reflect the ancient civilization.

Dr Pablo Sanchez, a biologist from the Centre of Atmospheric Studies at the National Autonomous University of Mexico in Mexico City, has warned that chemicals in the rainwater could erode the buildings over time.

"We could lose all of the inscriptions and writing on stelas and columns within 100 years," he told the Latin American Herald Tribune. Sanchez pointed out that most of the historic sites — some dating back more than 4,000 years — are made of limestone, which is composed primarily of calcium carbonate, which gets eroded when exposed to acid rain.

"[Limestone] cannot be covered by a protective layer since these rocks need to breathe, absorbing humidity and water, so a sealant would only accelerate their deterioration," he said.

Typically, rain turns acidic when pollutants like sulphur oxide and nitrogen oxide get mixed in it, reducing its pH level to less than 5.6.

Conservationists and scientists are now looking at alternative options to protect the buildings while still allowing the stones to breathe.

Damage to historic structures due to pollution is an issue at various sites around the world. The Taj Mahal in India is facing similar threats from acid rain. The ivory-white marble mausoleum is situated in Agra, near the country's capital and the most polluted city and in close proximity to the Mathura refinery. The low-sulphur crude processing plant has been blamed for causing air pollution that made the Taj's white façade to get discoloured.

"We found that black carbon gives a greyish colour to the [Taj Mahal's] surface while... brown carbon and dust results in yellowish-brown hues," Dr S N Tripathi of the Indian Institute of Technology told The Diplomat.

Taj Mahal
The Taj Mahal in India has become discoloured due to air pollution and acid rain Getty/Daniel Berehulak