What has to happen for gas to burn
A flame needs three things: fuel, oxygen and an ignition source. Methane and hydrogen can serve as fuel; nitrogen and carbon dioxide cannot. But having a little combustible gas in a mixture is not enough. Its concentration must fall within a range that can sustain combustion, and the gas must meet something hot enough to ignite it.
That is why the composition of intestinal gas matters. Microbes in the gut break down parts of food that the body has not digested, producing different gases as by-products. Which microbes are present—and what material they are processing—affects how much hydrogen or methane ends up in the mix. The rest may include gases that dilute the fuel rather than burn.
Why the same trick may not work twice
Gas production and composition vary among people and can change with diet and digestion. Even when combustible gas is present, the amount may be too low, or it may disperse into the surrounding air before it can burn. A visible flame therefore depends on several conditions lining up, not merely on the fact that gas has been released.
The chemistry is ordinary combustion, but the setting makes it hazardous: a flame close to the body can cause burns. A gas mixture’s ability to burn is not a reliable measure of whether it is safe to ignite.
