"Pete C." wrote

You're stuck on old paranoia based on information that is decades out of
date. All of the O2 purity grades specify no more than 0.05% impurities,
and the most lax of the standards is the medical / aviator grade. The
reality is that all the grades are filled from the same cryo O2 source
and all are better than 99.99% pure O2.

I know how a medical tank was handled. I don't know anything abut a welding
tank. The oxygen may be pure going in, but I don't know what was in the
tank beforehand.

I'm not stuck on decades information, my son owns a medical supply company
that supplies oxygen. I worked part time for him filling tanks and
delivering LOX. We followed the regulations on medical oxygen. You are
free to breath whatever you like though.