When it rains in this city of big smelters, it pours sulfuric acid.

Chongqing, industrial behemoth of China's southwest, produces some of the world's most caustic acid rain. April showers here kill flowers and turn this valley city into a cauldron of falling vinegar.

The corrosive effects are manifested in the high rates of lung cancer, the scorched rice crops and the pocked stone buildings.

For China, however, pollution is the flip side of modernization, and Chongqing is paying the price of progress. Despite lofty rhetoric, the Communist government has taken few practical steps to sacrifice industrial growth at the altar of Mother Nature.

The problem is bound to intensify as the nation stretches to reach its goal of quadrupling economic production in the next 18 years.

The ecological hangover from the past 33 years of Communist planning only now is being acknowledged. Some nationwide casualty figures provided by official Chinese and foreign sources:

* Of every 10 rivers, seven are seriously polluted with such toxic wastes as mercury, cyanide and arsenic.

* Hundreds of square miles of land have been poisoned by the annual dumping of 70 billion tons of industrial refuse.

* Such major manufacturing centers as Peking, Shanghai and Wuhan register air pollution levels between five and 15 times higher than is considered safe in the United States.

Chongqing, Nationalist China's wartime capital known to Americans as Chungking, is a case study in the conflict between profits and pollution.

As a major steel producer, mining center and transportation hub on the upper reaches of the Chang (formerly Yangtze) River, Chongqing is a principal actor in China's modernization drive. Planners have designated it a "model economic zone" with far-reaching powers to launch an industrial takeoff.

Although city officials began three years ago to recognize the environmental costs, their work thus far has been longer on recommendations than remedies.

"There's a contradiction between pollution and development," said Lo Qiren, an environmental engineer here. "Equal attention should be paid to both problems."

So far, ecology is the loser. Chongqing's 6 million people, who live along the jagged mountains that surround the city, endure an almost perpetual fog of coal dust and petrochemical fumes.

Looking down into the center of this once picturesque city from a bridge over the Jialing River is like trying to see the bottom of a pot of boiling soup.

The pollution turns to acid through a simple chemical process activated by rain: water plus the sulfurous emissions of coal burning equals sulfuric acid.

Scientists measure acidity by what is known as the pH factor, on a numerical scale: 5.6 is neutral and 2.6 is sharp as vinegar. The average acid content of Chongqing's showers is said to be 4.5--about the same as industrial centers in the northeastern United States. But the index has dropped to as low as 3 here.

The impact is plain: Chongqing literally is being eaten away. Acid rain chews up metal lamp posts, bus roofs and bridges. It cuts through copper wiring and slices off cement hunks from buildings.

The faces of Song dynasty figurines that have survived for about 800 years lost their features in a few years of acid rain.

"Our new buildings look older than Manchu dynasty structures in Peking," city engineer Lo complained.

Farmers in Chongqing's outskirts do not pray for rain because it leaves their rice seedlings burned out. One peasant lost 10 percent of his crop after a single shower last June, said Lo.

If you get caught with your umbrella down, beware. Bronchitis is as common as the cold here, and Chongqing has one of the highest lung cancer rates in China, according to Lo.

Although acid rain falls in other cities, it is the worst in Chongqing because of its almost total reliance on high-sulfur coal, said Lo. The low chimneys of power plants and houses spew several hundred thousand tons of sulfur dust into the valley every year, he explained.

The problem is compounded by the city's rainy climate, which on the average dumps water on the sulfurous pall once every four days.

Lo said the problem has become serious enough for the central government to order all of Chongqing's factories to use only washed coal, or coal cleansed of some if its sulfur content.

But the city's two coal-washing facilities can handle just a fraction of the 6 million tons of the fuel burned annually here and no other facilities are being planned, he said.

Higher factory smokestacks could help disperse the sulfur emissions, he said, but the city government has no requirement for chimney heights.

There also are no requirements for smokestack filters or other environmental devices to trap the sulfur, the engineer said.

"We must balance our development needs with the need for a clean environment," he said.