Not true. The lower explosive limit for natural gas (methane) is only 5.1% at ~68 degrees, a common thermostat setting. Methane is NON-toxic at those levels and that is also nowhere near a suffocating % of the atmosphere.
Natural gas is tainted with a mercaptan (the sulfur/skunk/cabbage smell) so you can detect a gas leak, AND CALL THE GAS COMPANY.
The trouble is the mercaptan tends to populate the smell receptors in your nose, and after a while, you can’t tell the leak has gotten worse/ more concentrated.
Without being an alarmist, when you smell gas (the mercaptan) CALL THE GAS COMPANY. They really don’t mind.
Sounds about right to me.
Moreover, I was once told (by a fireman or gas worker — don't remember which) that when the gas concentration gets extremely high, the danger of an explosion is lower than with a moderate concentration, because with so much gas there may not be enough oxygen in the house to allow combustion. He said that for this reason, it can be very dangerous to open the windows to let the house “air out” — unless the procedure is done by professionals who know exactly what steps are required.
In other words, if you think your house has a gas leak, don't operate any kind of electrical switch, don't leave any windows or doors open, and get out right away.