You’ll recall the requirement isn’t just for two openings, but one low and one high. Given those two, natural ventilation will help ensure this. remove enough of those gases to prevent an explosive accumulation (note that even complying with the size requirements can’t always ensure this).

Another engineer thought he had me on this when he told me that my wood pile wouldn’t break down into explosive gases. He was surprised that I could immediately respond to that one with the story about the farmer who put his wood in an enclosed shed he’d built with no openings in it. To this day, that farmer swears someone must have set his shed on fire. However, I know what happens when you put green wood in an enclosed space. If you don’t, then that helps explain why you can’t understand the requirement for two air openings.
Ken Heselton, P.E., C.E.M.
KEH Energy Engineering
Joppa, MD