Crews from the Tyne and Wear Fire and Rescue were called to Egger chipboard manufacturing plant in Hexham at 10.30 last night.
Around 60 fire-fighters were involved and they were on the scene until around 6am this morning.
A 200 metre cordon was put in place, but has now been lifted.
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Officers attended the scene where a man had suffered a head injury. He was taken to hospital where is condition is described as critical.