  Hadronic observables in the final stage of heavy ion collision can be
described well by fluid dynamics or blast wave parameterizations. We improve
existing blast wave models by adding shear viscous corrections to the particle
distributions in the Navier-Stokes approximation. The specific shear viscosity
$\eta/s$ of a hadron gas at the freeze-out temperature is a new parameter in
this model. We extract the blast wave parameters with viscous corrections from
experimental data which leads to constraints on the specific shear viscosity at
kinetic freeze-out. Preliminary results show $\eta/s$ is rather small.
