Health authorities in south China's Shenzhen city have identified sodium nitrite as the cause of a mass food poisoning that left two people dead and 61 people in hospital on Saturday.

The health bureau of the city's Longgang District said the investigation had focused on a restaurant in which food and water samples were found to be contaminated with the chemical.

The news was announced as 42 people were discharged from hospital on Sunday, leaving 19 -- all described as stable -- still receiving treatment.

Sixty-three workers of BYD Company Ltd. suffered stomachaches, vomiting and other symptoms around 11:00 a.m. Saturday after having lunch at a restaurant near their workplace in Longgang District.

Two died after treatment failed. The other 61 had been removed from critical care, 42 of whom left hospital at around 3:00 p.m. Sunday after recovering, according to the Health Bureau of Longgang District.

"We knew that the restaurant had poor hygiene, but its food was cheap, so we liked to have lunch there," said Wei Tangwei, one of the 19 people who were still hospitalized.

Sodium nitrite, similar in appearance to salt, is used as an industrial color fixative, but is also used in small amounts as a meat preservative.

But police have ruled out the possibility that the chemical was mistaken for salt or sugar by the chefs.

Health experts warned that 0.2 grams of sodium nitrite could poison an adult and three grams were enough to kill.

Local police and health officers have begun questioning the manager of the restaurant, patients and employees of BYD Company.

(Xinhua News Agency February 25, 2008)