STERLING, Va. (AP) — An ammonia leak at a commercial food preparation facility in Virginia sent 33 people to the hospital, authorities said Thursday.

The leak at the Cuisine Solutions plant in Sterling on Wednesday evening was caused by a failed valve, the Loudoun County Combined Fire and Rescue System said in a written statement. A hazardous response team donned protective gear and shut off the valve, authorities said. The company said in a written statement Thursday that the leak happened outside the building and was contained within an hour.

Of the 33 who were hospitalized, 22 were initially in serious condition, while 11 more had minor symptoms, fire officials said. Cuisine Solutions said Thursday that all of the hospitalized employees had been released with the exception of four, who were still under observation.

The leak forced the plant’s 287 employees to evacuate, rescue officials said.

Ammonia can cause respiratory issues and skin burns.

Cuisine Solutions is known for a cooking method called sous vide, which places food in a vacuum seal and cooks it in water. The firm’s meals can be found in the prepared food sections of grocery stores, in delivered meal kits and on cruise ships.