The danger of leaking gas is that when it explodes in a confined space under the right conditions, it becomes a fuel-air explosive bomb. And we know how destructive a fuel-air explosive can be.
There are two times when natural gas is considered explosive is when it’s first leaking inside a building and the fuel/ air mixture is between 4 and 15% gas. The second time is when venting the structure after the leak is fixed and the air/ fuel mix passes through the 4-15% again.