I had no idea, but apparently propane is heavier than air and even when left to sit in an open space, propane can still exist in concentrated amounts right in front of you and naked to the human eye. That was the unfortunate lesson Hannah Storm experienced when she went to re-ignite her propane grill after the flames had gone out. Unbeknownst to her at the time, the propane was still right in front of her and as she lit a flame the entire grill exploded, causing serious burn and damage to her body.