Authorities believe hydrogen was present at an experimental workshop, where it may have leaked and accumulated before igniting.

Five employees were working at the Seveso-classified chemical site when the explosion occurred at around 14:30pm on 22 December.

Two of the injured workers are reportedly in critical condition, while a third sustained serious injuries.

Around 100 firefighters and 30 emergency vehicles were deployed to the 600-sq-metre building, bringing the incident under control by early evening.

The Orsec emergency plan was activated, with a 1km exclusion zone imposed, residents ordered to shelter and transport links temporarily shut. Authorities said there was no toxic risk.

The Elkem Silicones Saint-Fons plant, located in France’s Chemical Valley south of Lyon, manufactures specialist silicone materials used across construction, automotive, electronics and healthcare markets.

Production involves complex chemical processes that require the handling of reactive and flammable substances under tightly controlled conditions.

While hydrogen is not an end-product in Lyon, it could be used within experimental and industrial processes.