"According to the data we have collected, the air quality is still in the category of state standards and will not affect people's health," Wu said.

However, Ma Yong'an, a researcher with the National Marine Environmental Monitoring Center, said oil spill cleanups are a challenge for any country, and if not handled properly, destruction could follow.

COOLING DOWN: On July 17, firefighters put out the blaze where the blast took place in Dalian, Liaoning Province (LI GANG)

Zhao Zhangyuan, a researcher with the Chinese Research Academy of Environmental Sciences, said the oil spill will pose a hazard to the area's ecosystem for the next 10 years.

Even after the estimated 15-day cleanup, all the pollutants will not be gone, Zhao said. He added the decomposed oil poses a cancer risk.

"Fish, algae and other creatures in the contaminated water will not survive and birds will fly away," he said.

Wu dispelled Zhao's claims, saying the impact would not be that severe. "The amount of spilled oil is not large and 10 years is an incorrect estimate," he said. The investigation on environmental damage will start soon, he said.

Meanwhile, vessel traffic at Dalian had been limited to allow the cleanup operations to proceed.

Wang Ning, an engineer with the provincial maritime authorities, told Xinhua that port operations had been limited but not completely suspended. Ships were allowed to enter and exit ports with permission, he said.

UNDER SURVEILLANCE: This photo, taken on July 19, shows the sea area affected by oil off Dalian's coast (LI GANG)

According to the Liaoning Maritime Safety Administration, the ports in Dalian had fully re-opened to traffic as of July 20, as waterways affected by the oil slick have been basically cleared for the resumption of shipping.

The blasts also affected the supplies of refined oil in south China, but industry analysts insist oil prices will not be affected.

Chinese petroleum companies have reduced oil shipments from Dalian to southern provinces as the port has been partially closed in the wake of the accident, according to Xinhua.

The Dalian oil reserve is at the heart of northeast China's crude oil production base and is one of the country's largest oil industry bases, while Dalian is China's second largest port for crude oil imports.

Analysts said the oil reserves in southern provinces are ample and the impact of reduced shipments has so far been limited.

City of Dalian

Lying on the east coast of Eurasia and the southern tip of the Liaodong Peninsular in northeast China, Dalian stretches from 120°58' to 123°31' east longitude and 38°43' to 40°10' north latitude, covering an area of 12,574 square km. With the Yellow Sea on the east, Bohai Sea on the west, Dalian is the marine gateway of north China. It is also an important port, and a trade, industry and tourism city. The permanent population of Dalian at the end of 2009 totaled 6.17 million.

(Source: www.dl.gov.cn)