The spectacular army of some 7,000 pottery warriors is suffering from "nine different kinds of mould" according to environmental experts quoted in the China Daily on Tuesday.

Chinese monuments expert Susan Whitfield on why the figures are vulnerable

The damage is being caused by raised temperatures and humidity in the building which houses the soldiers. Tourists' breath is largely responsible.

The figures were buried in an underground mausoleum near the northern city of Xian in the Third Century BC to protect the tomb of China's first emperor Qin Shihuang.

The terrracotta army has had some famous visitors

Since they were discovered in 1974 by peasants digging a well, the figures have become one of China's most popular tourist attractions.

Huang Kezhong, the deputy head of China's Institute of Cultural Property, says the damage caused by the thousands of daily visitors just one example of the threat posed by pollution to historical artefacts and sites.

Tourism and pollution

"Few tourists are aware that they also have become a source of pollution," Mr Huang said.

He has called for urgent action to protect China's cultural heritage from further damage.

Mr Huang said acid rain and air pollution caused by burning coal has corroded bronze and marble artefacts and damaged frescoes and sculptures throughout the country.

He cited the corrosion of ancient bronze astronomical equipment in the eastern city of Nanjing and Song Dynasty (960-1279) tombs in Henan province.

Air pollution in China is particularly bad because the widespread burning of low-grade coal produces sulphur dioxide and other pollutants.