You are correct, propane is heavier than air and will tend to sink,
acetylene is approximately the same density as air and natural gas
is lighter than air and will tend to rise. Even though the above is
technically correct, in a small studio, whether a gas is lighter
than air or heavier than air will likely not make much difference.
All torch gases will mix somewhat due to natural air currents in the
room and if the leak lasts long enough, will create an explosive
mixture and eventually find an ignition source ie a light switch or a
The key is to make sure that your fuel source never leaks, or if it
does to limit the amount of fuel that can leak.
The way to limit the amount of fuel that can possibly leak is to
limit the size of tank you use.
I have an oxy propane set up in my basement studio. I use a little
torch. My propane supply is a 1 pound disposable cylinder. I keep
one cylinder in the basement and store the unused ones in my garage,
so that at any given time, there is only one pound of ignitable fuel
in my studio. One pound of propane lasts me a very long time, but may
not work for you if your fuel requirements are larger.
Note also, that it in most cities it is illegal to keep a bbq sized
propane tank in your house due to the way the pressure relief valve
works on BBQ tanks.
Calgary Alberta Canada