Kurt Voigt, Associated Press
Nomaan Merchant, Associated Press

SPRINGDALE, Ark. (AP) — A solution of chlorine was poured into a drum of acid inside a Tyson Foods Inc. chicken processing plant in Arkansas this week, creating chlorine gas that sickened more than 170 people, authorities said in a report obtained Wednesday by The Associated Press.

About 300 workers were evacuated from the plant Monday morning in Springdale. Ten workers remained hospitalized Wednesday afternoon, none of them in intensive care, company spokesman Gary Mickelson said.

A Springdale Fire Department report obtained by The AP says the plant's safety director told authorities that a solution of chlorine had been poured into a drum of FreshFx LP, a mix of acids used for disinfecting.

Mickelson would not comment earlier Wednesday on what chemicals created the chlorine gas, which can cause respiratory problems up to sudden death. Company officials have said human error played a role in the accident.

"We believe we know what they were, but we don't want to speculate," Mickelson said.

Experts said Wednesday that many chicken plants use water mixed with chlorine to cool and clean chickens before they are processed. Acid is added separately to the water to make the chlorine more effective.

Adding chlorine directly to the acid can cause the mixture to expel gas, experts said.

"Any time you mix acid with chlorine, you can generate chlorine gas," said John Marcy, an extension food scientist at the University of Arkansas' Center of Excellence for Poultry Science.

The drum of acid was found inside a small mixing room in the plant when authorities arrived at the plant after 9 a.m. Monday, the report says. The gas had spread through an area with a "large number of employees," though all workers had been cleared, the report says. Authorities left at about 11:30 a.m., the report says.

The plant's work schedule was back to normal by Monday afternoon, Mickelson said.

The report doesn't say how the leaks occurred. Authorities do not believe the incident was criminal, and Springdale police do not have an open investigation, police spokesman Lt. Kevin Lewis said.

The Occupational Safety & Health Administration has opened an investigation that could take up to six months, an agency spokesman said.

Susan Watkins, an associate professor at the University of Arkansas, called such accidents rare. "To my knowledge, that's a very rare incident to occur," she said.

According to the U.S. Centers for Disease Control and Prevention, chlorine gas can cause a range of respiratory problems, from irritated tissue to sudden death from narrowing of the upper airway.

Sudden exposure to chlorine gas can bring on coughing and choking spasms, severe chest discomfort, vomiting and other symptoms, the CDC says.

The affected section of the Berry Street plant in Springdale is where chickens are slaughtered, Mickelson said. The two chemicals are used in the food safety process, and the plant doesn't use chlorine gas, Mickelson said.

A small number of chickens were discarded due to the incident, he said.

Tyson has opened a medical clinic at the plant, staffed with a doctor and nurses who are available to provide follow-up care to workers. The clinic will stay open for several days, he said.

Several Tyson workers declined comment when leaving the Berry Street plant Wednesday afternoon. Others, who declined to give their names, spoke of suffering from headaches during Monday's incident but offered praise for the quick and orderly evacuation.

One worker said he was told by his supervisor not to talk publicly about the incident and two Tyson security officials eventually told an AP reporter to leave the plant's employee parking lot.