The new means to analyze risks associated with flammable gases is their “burning velocity,” and the new limit is 10 cm/s. So I wanted to provide some real-world examples of what this looks like. So I asked AI to compare 10 cm/s flame front with the speed of a person walking and a jackrabbit.
OSHA defines a CAT 1B Flammable gas as:
Gases which meet the flammability criteria for Category 1A, but which are not pyrophoric, nor chemically unstable, and which have at least either:
(a) a lower flammability limit of more than 6% by volume in air; or
(b) a fundamental burning velocity of less than 10 cm/s.
Here is the breakdown:
- At 10 cm/s, a flame front would take 10 seconds to travel just 1 meter (about 3.3 feet).
- In those same 10 seconds, a rabbit running at 30 mph would cover roughly 134 meters (about 440 feet) — the length of a football field or more.
- A rabbit could sprint across a large backyard or small field while the laminar flame front is still inching along like a very slow crawl.
To put the flame speed in perspective:
- It’s in the same ballpark as a snail’s pace or the speed at which some very viscous liquids flow
- In contrast, even a casual human walking speed (~5 km/h or 1.4 m/s) is already 14 times faster than 10 cm/s.
Why This Matters in Context
Fundamental burning velocity measures the laminar flame speed in a perfectly premixed, quiescent gas-air mixture under ideal lab conditions. Real fires or explosions are much faster because of turbulence, buoyancy, and confinement. The <10 cm/s threshold is deliberately low — it’s why certain mildly flammable gases (like some refrigerants) get the more lenient Category 1B classification. The flame simply doesn’t race through the mixture.
Bottom line:
A rabbit running full tilt is over 100 times faster than the upper limit of this “slow flame” criterion. The flame propagation at <10 cm/s is more of a leisurely creep than a sprint — which is exactly why it’s used as a dividing line for “mildly flammable” vs. highly flammable gases. If you’d like comparisons to other animals, human speeds, or even a visual analogy (e.g., how long it would take the flame vs. the rabbit to cross a room), just let me know!
Sources: Me and Gemini+