ROCKFORD ¡ª A plant at a northern Illinois industrial park has been evacuated because of a toxic chemical spill.

The spill of anhydrous ammonia occurred when a valve came off an outdoor tank that was being filled at Rockford Heat Treaters Inc. No one was injured.

The Rockford Register Star reports that a hazardous material team was on standby Friday as the chemical leaked from a quarter-inch pipe.

More than 800 gallons of the colorless chemical evaporated in the process.

Acting Rockford Fire District Chief Frank Fuca explains that authorities decided to let the chemical disperse, saying a good breeze allowed it to "vaporize harmlessly."

The plant carries out heat treating, a process in which metals are heated or cooled to alter their physical properties.