A 50-litre standard hydrogen gas cylinder was temporarily placed in a laboratory to supply hydrogen gas to a flame ionization detector (FID) for use in gas chromatography (GC). On 20 January 2015, the safety relief valve on the pressure regulator failed and released about 0.34 kg of hydrogen into the laboratory. The gas cloud did not ignite so there was no injury or damage. The results of a full investigation with a complete course of action and reconstruction are presented that verify the cause of the leakage and estimate the gas concentration of the dispersion and gas cloud. A preliminary simulation of the likely explosion is provided. If the gas cloud had ignited, the explosion would most likely have caused significant structural damage to doors, windows, and possibly the walls. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.