The recirculation fan system circulates air from the passenger cabin around the
lining of the forward cargo compartment. When the overboard exhaust valve is
closed, exhaust air from the equipment cooling system is also diffused to the lining
of the forward cargo compartment for additional inflight heating.
and
The lower cargo compartments are designed and constructed to satisfy FAA
category Class C compartment requirements. This means the compartments are
designed to completely confine a fire without endangering the safety of the airplane
or its occupants. The compartments are sealed and pressurized but do not have fresh
air circulation and temperature control as do the upper passenger compartments.
which brings us to my next question concerning the reliability of cargo hold pressurization in relation to the survival of AVIHs.
if there is no airflow into those compartments and they have to rely on those often-patched and battered cargo doors to hold the pressure....