NG is lighter than air. Propane is Heavier.
Which is why it is so dangerous when there is a leak in a commercial building.
I recall a story years ago about a propane tank leak at some warehouse overnight in Buffalo, NY. When the fire truck showed up it leveled the city block and blew the fire truck hundreds of feet away. Destroying the warehouse, the fire truck and killing the firemen.
look up Indianapolis coliseum explosion back in the ‘60’s
https://en.wikipedia.org/wiki/1963_Indiana_State_Fairgrounds_Coliseum_gas_explosion