Fresno, CA- More than a dozen workers at a South Valley plant were taken to the hospital after carbon monoxide was discovered.

Just after 11:30 PM Tuesday night, firefighters say an employee at the Nichols Pistachio plant in Kings County started feeling ill in a break room. When emergency crews arrived, they discovered high levels of the poisonous gas.

13 other employees who felt sick were taken to University Medical Center and are expected to be okay.

Currently, firefighters do not know where the carbon monoxide came from.