Staffordshire Fire and Rescue Service went to an incident at Queen’s Hospital and Hobbycraft, Burton-on-Trent yesterday.
Two fire engines from Longton and one from Burton attended, after four Hobbycraft employees went to the hospital's A&E department after coming into contact with the substance.
The powder was later identified as Napthalene, which is a low risk product, and used to fumigate products shipped from abroad.
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It comes as figures show there has been a rise in trespassing in the last three years across the West Midlands.
“He is thoroughly ashamed and was a very lonely man at the time he committed these offences"
The figures have been released by British Transport Police.