  Recently, the production of ultrahigh-density (~10^{19}cm^{-3})
spin-polarized deuterium (SPD) atoms was demonstrated, from the
photodissociation of deuterium iodide, but the upper density limit was not
determined. Here, we present studies of spin-polarized hydrogen (SPH) densities
up to 10^{20} cm^{-3}, by photodissociating 5 bar of hydrogen chloride with a
focused 213 nm, 150 ps laser pulse. We extract the depolarization cross-section
of hydrogen and chlorine atom collisions, which is the main depolarization
mechanism at this high-density regime, to be {\sigma}_{HCl} = 7(2) x
10^{-17}cm^2. We discuss the conditions under which the ultrahigh SPH and SPD
densities can be reached, and the potential applications to ultrafast
magnetometry, laser-ion acceleration, and tests of polarized nuclear fusion.
