Toxic fumes leaked from a plant in a chemical industry zone in Shanghai yesterday afternoon.

Around 3:40pm, nitric oxide gas escaped from a tower processing nitric acid in the Shanghai BASF Polyurethane Co plant in Jinshan District, said the company in a statement.

Yellow fumes formed around the tower, but the company said it quickly halted operations and took steps to stop the gas spreading.

Nitric oxide can irritate the lungs and affect resistance to respiratory illnesses, such as cold.

"No casualties were reported and the leak had no impact on the environment," said BASF.

Lei Guichun, an employee of the Shanghai Huntsman Polyurethane Co, located to the west of the BASF plant, said on his microblog that the wind was blowing toward his company at the time.

Staff took refuge in a safe room for an hour until the poison gas alarm was lifted, said Lei.

About 2 kilograms of nitric oxide leaked in the incident.

Fan Kangnian, a chemistry professor at Fudan University, told Shanghai Daily that the leak would affect the city's environment and people's health. "Workers and residents staying nearby will be affected," Fan said.

The risks were less for those further away, he added.

But the Shanghai Environmental Protection Bureau said the leak would have no influence upon city air quality and people's health.

The incident was tackled swiftly and no complaints were received from nearby residents and workers, said the bureau.

In March, a leak from Gaoqiao Petrochemical in the Pudong New Area caused pungent fumes to spread through the city. The company was fined 200,000 yuan (US$31,254). Last year, gas containing toxic ethanethiol also leaked from Gaoqiao Petrochemical.