DATONG, China — The colossal Buddhist statues in the cliffside caves outside this northern Chinese city, carved from golden sandstone by Turkic-speaking nomad conquerors in the fifth and sixth centuries, were so covered in coal dust that when visitors blew on them, black clouds rose up.
Called the Yungang Grottoes, the relics had survived the rise and fall of dynasties, modern wars and the Cultural Revolution. But the scourge of a more prosperous China — industrial pollution — had been eating away at the sandstone.
Chinese officials and preservationists have embarked on an ambitious effort to protect them that could become a model for saving antiquities at other sites. They have not only cleaned the statues here and created a vast park, but also shut down nearby coal mines and removed or regulated other sources of air pollution.
“You don’t know how bad it was before,” said Huang Jizhong, the head engineer at the Shanxi Province Cultural Relics Bureau and former research director at the grottoes. “The contrast is very dramatic.”
Vast parts of China have some of the world’s worst outdoor air pollution, and ancient sites across the country are falling prey to its effects, officials and scholars say. Among the antiquities damaged by acid rain are the giant Buddha at Leshan in Sichuan Province and an 800-year-old thousand-armed statue of Guanyin, a revered Buddhist figure, at Dazu, according to Chinese news reports. A professor in Guangzhou, a provincial capital in the south, warns that acid rain is also eroding the red sandstone buildings there from the Ming dynasty.
Even the terra-cotta warriors of Xi’an, a symbol of Chinese civilization, may be under threat. Lee Shun-cheng, an engineering professor at Hong Kong Polytechnic University, has called for glass walls to be built around the warriors, now protected only by a roof.
Experts working on the Yungang Grottoes, in the heart of China’s coal country, are now advising officials seeking to preserve sites elsewhere. Mr. Huang visited the mountainside relics at Xiangtangshan and Yaowangshan, both in heavily polluted areas of northern China, and told officials there to shut down or move cement factories. In some cases, officials complied, and where they could not they built glass enclosures around the statues, Mr. Huang said.
At the Dazu rock carvings in Chongqing and the Longmen Grottoes in Henan Province, in central China, preservationists are using moisture-monitoring devices that were invented at Yungang.
“All these things share something in common: they suffer from the air and the water,” Mr. Huang said. “So these places can draw directly from the Yungang experience.”
A large part of the problem is coal. Burning coal emits sulfur dioxide, which further oxidizes in the atmosphere and then combines with water to produce sulfuric acid.
In the 1980s and 1990s, as the coal industry here in Shanxi Province powered growth throughout China, the impact of acid rain and air pollution on the statues in the Yungang Grottoes was “severe,” said Liu Xiaoquan, a senior manager at the Yungang Grottoes Research Institute. A national highway ran in front of the grottoes, where 51,000 statues stand in 254 niches and caves. Up to 20,000 coal trucks passed each day. Villagers burned coal for cooking and heating.
Restoration efforts, prompted by a bid for Unesco World Heritage List status, started in the late 1990s. Officials moved the highway in 1998 and barred coal trucks from using it. About 10 small coal mines were ordered shut, Mr. Huang said. Officials also moved six villages from the area, a total of 4,750 households, over the reported objections of some villagers, who said the compensation was too low.
Workers are also trying to make digital recordings of all the artwork. Those would help with construction of three-dimensional renderings of the caves that visitors and scholars can view.
A mine run by the Datong Coal Mining Group, a state-owned enterprise, is still operating within sight of the grottoes. But the company has closed some shafts, and “they do well with dust and pollution control,” Mr. Liu said.
On a recent morning, two photographers were setting up lighting equipment inside the first cave to take photos for the research institute. The cave has a stone pagoda in the center, typical of a sculpture in the middle period, said Ma Yexia, an official guide. A nearby twin cave also has a pagoda, though its base has been eroded by water that once filled the bottom of the cave. The water was pumped out sometime after 1949, when the Communists took over, Ms. Ma said.
Running water, which can damage sandstone carvings, has been a danger for centuries. During the Jurchen Jin dynasty, in the 12th and 13th centuries, a river that ran in front of the caves threatened the statues, and a great warrior ordered the river to be redirected, she said.
Cave numbers 16 through 20 have the oldest statues, built by the monk Tang Yao. The simple Buddhas here were carved from rock at the rear of the caves. Ms. Ma said one enemy was the rainwater, which makes the sandstone brittle and more vulnerable to the wind. The wall surrounding the entrance of cave number 20 has collapsed, leaving the sitting Buddha there exposed.
The oldest caves were used by the early Tuoba rulers of the Northern Wei to help spread the idea they had the divine right to rule over the conquered locals, Ms. Ma said. The Tuoba eventually moved the seat of their dynasty south to Luoyang, in present-day Henan Province. There, they built more grand Buddhist statues at the Longmen Grottoes. Officials at Longmen, which is also a Unesco site, have been carrying out preservation efforts similar to those at Yungang, following what is becoming standard practice for local governments that have sought international recognition.
What Chinese officials have been unable or unwilling to control are the stratospheric levels of air pollution. Xinhua, the state news agency, reported that acid rain hit 135 cities in early 2013. Of those, 23 were severely affected. Mr. Huang said, “The current air pollution still poses a threat.”