An explosion occurred involving a Microbiological Anaerobic Chamber of approximately 2 m3 capacity, containing an explosive mixture of hydrogen and air. A fire followed the explosion, but was rapidly extinguished by staff using fire extinguishers, prior to the arrival of fire service personnel. The pressure wave from the explosion blew windows out of the laboratory, with glass hitting a passer-by on a path outside, and glass shards landing up to 30 m away. Ceiling panels were dislodged in the laboratory and adjacent rooms, and a worker using the apparatus at the time was taken to hospital by ambulance to have burns treated. They have subsequently fully recovered from their injuries. Another worker in the lab at the time required medical observation but was otherwise unharmed. The local operating procedures used in the lab allowed high concentrations of hydrogen to be introduced into the chamber. A worker inadvertently admitted air to the chamber whilst undertaking maintenance, allowing the hydrogen enriched atmosphere in the chamber to mix with air, and subsequently ignite, most probably on contact with the oxidation catalyst in the chamber, resulting in the explosion and subsequent fire.An unknown concentration of hydrogen was present in the chamber, presenting a significant fire and explosion risk.