I've never seen a switch spark when turning on a light switch, in sure like everything else they become faulty. I've even tried to make a switch spark but rapidly turning them on and off, I've used a thermal camera to check any energy released from a switch. Natural gas requires an ignition temperature of approx. 1200 F to ignite. On top of that the mixture has to be within the explosion range of the gas. Getting to that range by normal leak is next to impossible. I walked into a house on a call years ago and I could smell the gas at the front door, the leak was at a union in the basement, work performed by the owners brother six months previous, who was not licensed. They had AWH, CHS, clothes dryer and a range in the house and not even close to the LEL. Every year we issue thousands of red tags for leaks in the GTA, yet explosions are extremely rare. I don't know of any Nat gas exploration that was the result of a normal leak, like I said usually they are deliberate, or the result of pressure gas filling a building.