One appliance and crew from Beaconsfield, one from High Wycombe, one from Windsor Tinkers Lane, one from Gerrards Cross, and a support unit from Marlow attended, along with three officers.
Two chemicals had been mixed, causing a chemical reaction.
The site was evacuated. Thames Valley Police helped keep cordons in place.
The firefighters used two sets of breathing apparatus and gauntlets. They removed a 25-litre chemical container, and used positive pressure ventilation to help clear fumes.
Four people were assessed by South Central Ambulance Service.
A plant room has been closed off, and the incident left with the site owners to arrange for the mixed chemicals to be removed.