  In this paper the nonequilibrium correction to the distribution function
containing a time and space dependent mass is obtained. Given that, fully
consistent fluid dynamic equations are formulated. Then, the physics of the
bulk viscosity is elaborated for Boltzmann and Bose-Einstein gases within the
relaxation time approximation. It is found that the parametric form of the
ratio $\zeta/\tau_R$ for the quantum gas is affected by the infrared cut-off.
This may be an indication that the relaxation time approximation is too crude
to obtain a reliable form of bulk viscosity.
