Fire crews were called to the industrial estate in Newman Lane, Alton in the early hours
More than 30 firefighters have tackled a blaze at an industrial park in Hampshire.
The fire in Newman Lane, Alton, started just before 05:00 BST.
Thick acrid smoke pouring from the building led to police instructing workers and residents to keep doors and windows shut.
Hampshire Fire and Rescue Service said the fire at a pest control service involved toxic chemicals, believed to have been pesticides.
The fire service said officers had stopped the blaze spreading to premises on either side of the building including a South Central Ambulance Service station.
The fire service said it was currently "damping down" before a joint investigation with Hampshire Constabulary into its cause begins.
Hampshire Fire and Rescue said crews remained on scene damping down
