Hydrogen and oxygen gases can be combined to form water and a lot of heat. As with many exothermic reactions, the heat from this process can provide activation energy to the reaction of more H2 with more O2 in a runaway process commonly referred to as an explosion. I would like an atmosphere rich in both hydrogen and oxygen gases, but I don’t want it to explode. Well, I don’t want it to explode too much. Weather systems creating pockets of high pressure that spontaneously ignite, or lightning strikes resulting in exploding pockets of atmosphere sound awesome. What I need to avoid are planet-consuming runaway explosions that spectacularly scour the surface of the world of all life. A second consideration is that I would like this atmosphere to be breathable by humans.
How much hydrogen and oxygen can coexist in the atmosphere before apocalyptic runaway combustion becomes a possibility?
My current explanation for this world is that a biological or geological process constantly emits large amounts of hydrogen into the atmosphere but that constant small fires and explosions prevent it from accumulating to more dangerous levels. I’m trying to determine what level that is, where explosions are happening but there isn't a risk of extinction level events.