How to build a 3D extinction model of the Galaxy

We show that anomalous extinction (deviations from the traditionally adopted RV = AV /E(B −V ) = 3.1 introduces large uncertainties in the distances of stars, for distances larger than 1-2 kpc. We argue that for such distances and for directions close to the galactic plane, the use of extinction models based on the gas distribution in the Galaxy is safer, for the moment, than the use of extinction maps.