  There is now strong evidence suggesting that the 12CO J = 1-0 transition,
widely used to trace H2 gas, significantly underestimates its mass in
metal-poor regions. In spiral disks such regions are found in large
galactocentric distances where we show that any unaccounted H2 gas phase is
likely to be diffuse (~5-20 cm^-3) and warmer (T(kin) ~ 50-100 K) than the cool
(T(kin) ~ 15-20 K) CO-luminous one. Moreover we find that a high value of the
H2 formation rate on grains, suggested by recent observational work, can
compensate for the reduction of the available grain surface in the metal-poor
part of typical galactic disks and thus enhance this CO-poor H2 component which
may be contributing significantly to the mass and pressure of spiral disks
beyond their optical radius.
