The A & E department of Queen's Hospital in Burton-on Trent is back open after being closed to the public this afternoon as a precaution.
NHS bosses locked the unit down after four workers from a local factory turned up at the hospital, having come into contact with an 'unknown white powder'.
The substance was sent to a laboratory for analysis. According to fire officers who attended the hospital, it turned out to contain napthalene – an organic compound found in mothballs.
The four workers apparently got some of the white powder on their skin as they unloaded a trailer. The fire service said they had attended A&E with coughing fits.