Workers were evacuated from a Simi Valley business park, following a fire early Tuesday morning involving materials initially thought to be hazardous.
Some 650 gallons of nitric acid was discovered burning inside a 10,000 square foot building in an office and industrial park.
The call came in at 6:16 in the 100 block of West Cochran Street in Simi Valley.
The fire was brought under control and firefighters allowed it to burn itself out.
However, because of the nature of the burning substance as an irritant, emergency crews conducted downwind monitoring of smoke and fumes from the fire.
A shelter-in-place order was issued for businesses east of the hazmat fire.
That order extended from the 100 block of West Cochran to First Street in Simi Valley.
The cause of the fire remains under investigation.