you speak of having the proper mixture, take a look at the Myth-busters episode when they were trying to make a porta-potti explode! The idea was that the methane gas that builds up within the potty could cause an explosion if one were to present an ignition source within.
Well, as stinky as those things are, there was not enough methane in one to ignite. So, they tried to artifically place enough methane into the potty to find out actually how much was actually needed to make the thing blow up! They had to cover all the vents in the enclosure, and then inject the methane into the potty. They then presented an ignition source...They did this several times, but only ended up with a few pieces of smoldering toilet paper. The problem? First, they were too lean, then they were too rich...they were having a VERY hard time trying to get the right mixture to blow it up!
So, the myth that a porta potty could explode was busted, but it also shows actually how HARD it is to create a gas explosion!
Yes, there have been a FEW instances where homes were blown right down to the foundation, but these are very rare, considering how many homes there are that are equipped with natural gas appliances. If you are really worried about dangerous things, I highly recommend not traveling in a vehicle that uses roads, since approx 37,000 people were killed on public roads last year. Your chances of loosing life, limb or property out on the road is significantly higher than having an accident with natural gas...still, I believe you are going to continue driving your automobile, and will never own a gas appliance! We can't avoid dangerous things in this world, so just enjoy what modern technology has graced us with, and just be careful with it!!!