Lead from the bathtube.... doesn't surprise me, a huge plastic tube may be an option, but then one would the plastics inside the body from corosions by the salts and bakingsoda? Whats worse?

There are "safe" plastics around, but in that size?

Hmmm... everything seems poisonous to some degree...
The very best option would be an oldfashioned wooden tube to take our baths in!