Which substance is recognized as having the greatest hazard due to its flammability range?

Hydrogen is recognized as having the greatest hazard due to its flammability range. The flammability range of a substance refers to the concentration limits within which a mixture of the substance and air can ignite and sustain combustion. Hydrogen has an exceptionally wide flammability range of approximately 4% to 75% in air, meaning it can ignite across a broad spectrum of concentrations.

This wide range significantly increases the risk of fire and explosion, as even small releases of hydrogen can create a hazardous atmosphere that can easily ignite under typical conditions. Additionally, hydrogen is lighter than air and can accumulate in enclosed spaces or areas with poor ventilation, further increasing the risk.

In contrast, while oxygen is essential for combustion, it is not flammable by itself; it supports combustion. Methane and propane also have flammability ranges, but they are narrower compared to hydrogen. Methane has a flammability range of about 5% to 15%, and propane about 2.1% to 9.5% in air. Thus, hydrogen's unique properties make it the substance with the greatest hazard due to its flammability range.