Warwickshire Fire and Rescue Service was alerted to an incident at Parcelforce in Siskin Parkway, near Baginton, at 9.20am today 22 November.
What is believed to be a hazardous substance was found at the premises and chemical samples have been taken. Detection equipment is at the site to identify the chemical.
Two people are known to have been taken to the University Hospital in Coventry, where a decontamination area has been erected. Warwickshire County Council Emergency Plans officers are also assisting the Fire and Rescue Service at the scene with decontamination.
Warwickshire Police assisted with the evacuation of people from the building.