That's not necessarily unusual. Natural gas is heavier than air and pools in basements and things like that. There was a large payout having something to do with synthetic plastic piping that was deemed defective and proximate cause for a lot of building explosions in the 90's, etc.
Natural as is lighter than air, but can still get trapped in basements,etc.
Propane or LP gas is heavier than air.
Either one is dangerous but LP is much more so.
>> Natural gas is heavier than air and pools in basements and things like that.
I believe Natural Gas is generally lighter than air. It consists primarily of Methane, CH4, which has a molecular weight 16 g/mol. For reference, air has an average molecular weight of approximately 29. Propane, C3H8, has a molecular weight of 44 and is heavier than air.