A flow valve malfunction caused the ammonia spill Wednesday that resulted in 21 Birds Eye Foods employees being taken to area hospitals, company officials said.

The 8:30 p.m. rooftop spill at the Waseca vegetable-processing plant produced fumes that entered the building and sickened employees and four residents living near the plant. All were treated and released.

In a written response to questions, Birds Eye Senior Vice President Lois Warlick-Jarvie said although the flow valve malfunctioned, the relief valve functioned properly and relieved the system of pressure that could have resulted in a more severe incident.

The plant uses pure ammonia in its refrigeration systems, Waseca Fire Chief Gary Conrath said.

“The wind direction was just right that night to drive the fumes into the building,” he said.

Warlick-Jarvie said the company is still investigating to determine how the vapors entered the plant, which employs 94 full-time workers and reopened shortly after the incident.

Conrath said the valve spill was of short duration, and about 11⁄2 gallons of the fluid was cleaned up with little effort.

A Waseca hospital spokesperson said those affected were treated for respiratory symptoms.

Warlick-Jarvie said the plant routinely conducts intensive emergency response training in the event of such incidents, and Conrath commended plant personnel for their quick response Wednesday.

Conrath said Waseca’s worst ammonia spill occurred about 30 years ago, when an anhydrous ammonia leak at a grain elevator cast a vapor cloud over parts of the community.

He said no one was injured, but several neighborhoods had to be evacuated.