  We derive the collision term in the Boltzmann equation using the equation of
motion for the Wigner function of massive spin-1/2 particles. To next-to-lowest
order in $\hbar$ it contains a nonlocal contribution, which is responsible for
the conversion of orbital into spin angular momentum. In a proper choice of
pseudo-gauge the antisymmetric part of the energy-momentum tensor arises solely
from this nonlocal contribution. We show that the collision term vanishes only
in global equilibrium and that the spin potential is then equal to the thermal
vorticity. In the nonrelativistic limit, the equations of motion for the
energy-momentum and spin tensors reduce to the well-known form for
hydrodynamics for micropolar fluids.
