Four people were taken to hospital following a gas leak in a building in Jurong yesterday.

The Singapore Civil Defence Force (SCDF) said in a Facebook post that it was alerted to an ammonia leak at 1, Fishery Port Road, at about 11.40am.

The building houses Ben Foods, a food distribution company that does food trading, packing and wholesale operations. About 100 workers had to be evacuated.

"The leak in a first-floor chiller room was shut off by the company and its staff were evacuated prior to SCDF's arrival," said the Facebook post.

SCDF personnel decontaminated three people by washing them down with water. The trio were taken to Ng Teng Fong General Hospital in a conscious and stable condition. Another worker was also taken to the hospital in the late afternoon after experiencing breathlessness.

Mr Salim Anwar, a 68-year-old finance and administration manager at NCS Cold Stores, the building adjacent to Ben Foods, told The Straits Times that he could already smell ammonia when he reached his workplace at 8.30am.

"At about 10.30am, the smell became unbearable. My eyes felt very painful, like they were getting pricked," he said. Mr Salim called his company's plant engineer, who said the problem was being looked into. He was also asked to get everyone to leave the building.

Mr Fauzan Tahir, 30, a delivery driver for Ben Foods, was on his way back to get a second batch of goods when his boss called to tell him not to enter the building. "My supervisor said there was an ammonia gas leak. I've been waiting outside since 1pm," he told The Straits Times more than two hours later.

SCDF said in an update at about 3.40pm that its hazardous materials (HazMat) specialists have detected low levels of ammonia in the immediate perimeter outside the affected building. The HazMat specialists, who were clad in protective suits, detected a leak within the ceiling of the chiller room and were working to access the concealed ammonia piping.

SCDF added later in the night that the source of the ammonia leak had been traced to a main distribution pipe. While the concentration of ammonia vapour within the affected area had diminished considerably, SCDF deployed water curtains to suppress any residual ammonia vapours which may escape beyond the affected area.

Exposure to high concentrations of ammonia in the air can cause burning sensation in the nose, throat and respiratory tract.

In July last year, 11 workers were taken to hospital following a chemical leak at a wafer fabrication plant in Kallang Way.