Terracotta Warriors cry out for clean air

Earlier studies on the Terracotta Warriors on display show that humidity and dust are the most obvious factors affecting the ancient relics, Ma said.

Most of the more than 1,500 Terracotta Warriors that have been excavated have lost their color, and being exposed to the air will worsen the deterioration, he said.

The Emperor Qin Shihuang's Mausoleum Site Museum has been monitoring the environment in the pits since the 1980s, and has published successive measures for environmental control, including improving museum greening, adapting windows for proper lighting and banning vehicles from its gardens.

A team of scientists is also studying the influence of particulate matter on the Terracotta Warriors.

Zhou Tie, chief engineer of the Emperor Qin's Terracotta Warriors and Horses Museum, said environmental control at the museum must be further strengthened.

"Air curtain technology, which is a kind of modern science and technology, has found a good application in other fields. Applying this technology in the museum is a good idea, which can effectively isolate the outside contaminants from the heritage items inside," Zhou said.

However, Zhou said, as there is no successful precedent applying the system to a site museum - which involves water, earth and air in multiple fields with many environmental factors - the technology still needs long-term experimentation.

Air curtains have been used in museums outside China to fix the indoor environment, instead of preserving relics.

The Otago Museum in New Zealand announced a plan to use air curtain technology to resolve draughty conditions in the museum, Otago Daily Times reported in September. However, the air curtain only worked as an alternative solution to prevent cold air entering and causing visitor discomfort.

The reason behind the less-frequent use of air curtains to directly protect relics is, on the one hand, that air pollution in other countries is not so severe, and on the other hand there are fewer historic pit sites outside China that are especially suited for air curtains, Gu Zhaolin said.

Gu, as a prominent environmental expert, did not pay any attention to historic preservation projects before he visited the Han Yang Ling Mausoleum in 2006.

The Han Yang Ling Mausoleum is the largest underground archaeology museum in China. Most relics are kept in the primitive environment in the pit, separated from visitors by windows.