Unburned natural gas is not just a little stinky, it is profoundly nasty. Way, way, way before you could get enough concentration to hurt you, or blow somethin

Unburned natural gas is not just a little stinky, it is profoundly nasty. Way, way, way before you could get enough concentration to hurt you, or blow something up, you'd be vacating the premises. Mercaptan is really strong stuff. Like someone else mentioned, you probably had a malfunctioning furnace. Your symptoms match CO poisoning, and carbon monoxide sensors don't usually detect natural gas leaks -- it's a different molecule entirely.