# Inhalation concentration and LFL / UFL

## Safe inhalation concentrations

The safe concentration for inhaled molecular hydrogen depends on what is being measured. The classical handbook value applies to closed-system pre-mixed conditions; the value relevant to inhalation devices is the empirically measured one.

### Where the 4% classical LFL applies

The chemistry handbook value for the **lower flammability limit (LFL) of hydrogen — approximately 4%** — was measured in **closed pre-mixed mixtures at rest**, for example inside a pipeline or sealed vessel. This is the figure used to set explosion thresholds in process engineering.

### The 10% empirical inhalation-environment value

Inhaler operating conditions are different: atmospheric pressure, open system, continuous dilution, and a non-zero gas flow. The closed-system assumptions of the classical LFL no longer hold. Empirical measurements under realistic inhalation conditions place the **safe operating ceiling at approximately 10%**, supported by multiple experimental and case-based lines of evidence.

### The UFL 75% paradox

Theoretically, hydrogen has an **upper flammability limit (UFL) of approximately 75%**: above that concentration, lack of oxidizer would prevent combustion. This argument is sometimes used to claim that 100%-pure-hydrogen output is intrinsically safe.

In practice:

1. **Boundary-layer concentration gradient**: even at 100% output, a continuous concentration gradient forms between the outlet and ambient air, necessarily passing through the 4–75% flammable range.
2. **Mixing within the airway**: nasal, tracheal, and pulmonary mixing with exhaled gas can transiently produce flammable compositions at local scale.
3. **Ubiquitous ignition sources**: static electricity in the airflow and mucosal friction can act as low-energy ignition sources.
4. **UFL measurement conditions**: the 75% figure assumes closed, pre-mixed, static gas. It does not apply directly to dynamic open systems.

The argument that "100% pure hydrogen output is intrinsically safe" therefore does not hold.

### Operational recommendation

- For inhalation applications, treat **10% as the operational upper limit**.
- High-concentration devices (66% / 100%) have documented accident cases and are not recommended (→ [accident cases](/en/safety-notes/accident-cases)).
- The rationale is supported by the four-paper inhalation safety threshold lineage (→ [paper lineage](/en/safety-notes/lineage)).

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> **Cite as**: H2 Papers — https://h2-papers.org/en/safety-notes/inhalation-concentration