It happened at a packing building on Road 136 north of Delano.
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All employees inside the building were evacuated, officials say. EMS crews were evaluating each person, and so far, four have been taken to a local hospital for treatment.
Teams from the Tulare County Fire Department, Visalia Fire, Tulare County Environmental Health, Tulare County OES and sheriff's office were called were working to clear the area of any hazardous materials.
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Authorities have not determined what the substance is at this time.