Staff were evacuated from an industrial laundry after a chemical spillage in Staffordshire.
Peroxides, acids and alkalines are thought to have been deliberately mixed together at Central Laundry in Burton-upon-Trent.
They spilled into the yard, a nearby road, footpaths and approximately 3,000 litres seeped into the drains.
Firefighters spent several hours clearing the scene before the all-clear was given at 0300 BST on Monday.
Seven crews from Burton, Barton-Under-Needwood, Chase Terrace, Lichfield and the Damage Limitation Unit from Hanley attended the scene.
Firefighters wore breathing apparatus and gas-tight suits to clean up the spillage.
Eight workers at Central Laundry, which provides linen services to the healthcare, hospitality and education sectors, were also evacuated from the site and received medical treatment at the scene.
Paul Cullen, Assistant Area Commander for East Staffordshire, said: "The chemicals were mixing together and releasing hydrogen gas, which is extremely flammable.
"We used gas monitors to ensure it had evaporated before leaving the scene, as our main priority was to make certain there was no risk to the public."
Mr Cullen said the mixed chemicals posed no threat to the public although some people may be able to smell perfume or a bleach-type of aroma from some drains.