They may have guarded the tomb of China's first emperor for thousands of years, but the terracotta warriors are facing their toughest battle yet.

The life-sized clay figures unearthed three decades ago in Shaanxi province are starting to fall apart and Chinese and US scientists have launched a two-year research project to study the impact that indoor air pollutants are having on the Emperor Qin Terracotta Warriors and Horses Museum in Xi'an.

Cao Junji, executive director of the aerosol and environment division at the Chinese Academy of Sciences' Institute of Earth Environment and head of the research team, said it was time to take action to save the relics.

If nothing is done now, in 100 years the warriors may have corroded to such an extent that the pits will look just like a coal mine and not have any aesthetic value, he said.

If a leg or a shoulder falls off, the whole figure is damaged. There are only a few hundred of them -- how many more can we afford to have damaged?

The richly coloured terracotta figures with individual facial expressions lay underground for about 2,000 years, but began to lose their luster and turn an oxidized grey once they were exposed to air.

Acidic particles in the air have eaten into the surfaces of the statues, leaving a fine powder. These particles are also weakening the gypsum that holds the joints together.

Dr Cao said the damage caused by corrosion was often minor at first, but larger features of the statues -- their noses, for instance -- could shrink as the surface was worn away.

He said the individual features of the warriors -- such as a moustache or certain hairstyle which indicate age or rank -- might become less noticeable over time, eroding the figures' cultural value.

Dr Cao, a 34-year-old native of Hubei Province, first visited the museum in 1990 when he was a student at Changan University.

The more visits he made, the more concerned he became about the future of the warriors.

The museum has technicians and various instruments to monitor the environment, but its expertise is not very mature.

Besides, from a museum's point of view, it is doing a good job as long as the antiquities are not lost, he said.

Dr Cao considered becoming involved in some sort of research after visiting the museum in 2002 with his friend Lee Shun-cheng, an expert in indoor air pollution from the Hong Kong Polytechnic University.

In May last year he started his search for funding and partners, and this March saw the establishment of a project to save the figures through a collaborative effort between the Nevada-based Desert Research Institute, the Chinese Academy of Sciences, the museum and the Polytechnic University.

Using instruments flown in from Hong Kong and the US, the team will monitor air pollution inside and outside the museum starting in September.

Air samples will be sent to laboratories in Hong Kong, Nevada and Xian to identify the key components, and fragments of the warriors will also be sent to the Polytechnic University for analysis.

Temperature, humidity, pollutants and solar radiation all pose threats to the statues.

Dr Cao said the team could only delay the ageing process by targeting the main pollutants and finding ways to reduce them.

Even then, he could not say how many more years the warriors would last.

Air pollution is a disease affecting cultural relics. [We cannot] wipe out air pollution, but we can alleviate and minimize it.

It's very hard to restore the original colour of the statues that have already been exposed to air, Dr Cao said.

But as the museum is still excavating new ones, we need to conduct more research to avoid a repeat of old problems.

It is estimated that the museum's three underground pits house 8,000 life-size pottery warriors and horses.

About 2,000 have been unearthed so far, of which 1,172 are on display, said Rong Bo, the museum's conservation scientist.

Asked if the research would come too late to save the relics, Dr Cao and Professor Lee said the necessary techniques had never been developed before.

Don't forget that this is the first study of indoor air quality in a Chinese museum. There are hundreds of museums in China that have similar problems, Professor Lee said.

He said the team wanted to set up workshops to teach conservation skills to local engineers and hoped their work would encourage other museums to set up their own indoor air-monitoring and management systems.

One of the team's main challenges will be to identify solutions if the source of the contaminants is, as expected, motor vehicles, power plants or the household use of coal.

It will be difficult to totally eradicate the pollutants. Most pollution comes from outdoors, which the museum cannot control, Professor Lee said.

Dr Cao also feared that funding for the conservation work might run out when the project ends and it might take up to five years to put the necessary rescue measures in place.

But the scientist still describes his rescue mission as an exciting journey. Protecting the warriors is not only the responsibility of the museum itself, but of everyone.

They are not only the treasures of the Chinese, but of the world.

(SCMP.com July 6, 2005)