The explosion occurred during repair work on a boiler. A technician had previously descaled the device using hydrochloric acid, after which he performed the neutralisation step with soda and drained the passivated acid. The significant scaling on the boiler required a second acid injection, followed by welding work, but this time the rest of the acid had not been drained from the boiler. Hydrogen emitted by a metal + acid reaction was the cause of this accident. An employee wound up in hospital in a coma. Labour inspection authorities noted that operating conditions were not respecting environmental protection requirements, i.e. failure to control discharge pH. They also noted the absence of written procedures.