The lowest concentration of a flammable gas in air at which combustion can be sustained given an ignition source. Below LFL there is too little fuel for the flame front to propagate.
For hydrogen, the classical value is 4%, measured in closed, pre-mixed, static conditions. The value does not translate directly to open, dynamic conditions such as inhaler operation.
The highest concentration of a flammable gas in air at which combustion can be sustained. Above UFL there is too little oxidizer (oxygen).
For hydrogen this is approximately 75%. However, even at 100% pure-hydrogen output, the concentration gradient at the device–air boundary necessarily includes a layer in the 4–75% flammable range (→ UFL 75% paradox).
Combustion requires a flammable mixture, oxidizer, AND an ignition source. Hydrogen has an exceptionally low minimum ignition energy (≈ 0.017 mJ) — about one-tenth that of methane or propane. Mundane phenomena such as airflow static electricity, mucosal friction, and electrical sparks can therefore initiate ignition.
See Inhalation concentration for the operational implications.
https://h2-papers.org/en/safety-notes/lfl-ufl-explained