“They were exposed to an unknown substance inside the building,” Jody Hagemann, a spokeswoman for Cal Fire/Riverside County Fire said, according to The Press-Enterprise in Riverside, east of Los Angeles.
She said the substance had not yet been isolated, the news outlet reported.
Around 150 employees at the Amazon Fulfillment Center in Eastvale, west of Riverside, were evacuated and at least six people were taken to the hospital for undisclosed mild to moderate symptoms, Cal Fire/Riverside County Fire Department said in a statement posted to Twitter.
"Cal Fire" refers to the California Department of Forestry and Fire Protection.
After a HAZMAT crew entered the building they were unable to find the substance. Cal Fire said the building would reopen shortly for normal operations.