The 'cargo hold' on airliners is at the same pressure as the passenger cabin. The floor of the passenger deck is a 'floating' platform. The pressure skin is the outside, circular, cigar tube that is the fuselage. Cabin pressure only ever gets up to ( or down to depending on your view of physics ) about 8000'.
Although cargo holds are 'unheated' they are presurised by the same bleed air that is fed into the cabin. You could easily sit down there for the duration of a long haul flight.
Not that it makes the blindest bit of difference! Air pressure affecting chokes and barrels, what absolute bollox!