Twelve people had to be decontaminated after a chemical explosion at a waste facility in Cannock.
More than 30 firefighters attended the Augean liquid treatment plant in the Bridgtown area of Cannock at 1200 GMT on Friday.
Two Augean staff were taken to the Queen Elizabeth Hospital in Birmingham.
Group manager Del Roberts said: "There has been a small explosion involving a mixture of acid gas residue from an industrial process on site."
The plant had been isolated with no further risk to the public or environment, he added.
Seven members the emergency response crews were decontaminated along with five Augean staff.
An Augean spokeswoman said: "We are in the process of working with the emergency services to investigate the cause and ensure area has been made safe."