An argon injection experiment on the Frascati Tokamak Upgrade (FTU) plasma has been utilized to clarify the physical mechanism responsible for impurity production. A simple 0-D model for the scrape-off layer (SOL), coupled with sputtering by the main deuteron flux and self-sputtering, can reproduce the variation of impurity concentration, nickel in the case of an Inconel limiter in FTU, when argon is puffed into the plasma. The increase of radiation power losses leads to a decrease of the SOL temperature, and therefore to a decrease of the sputtering yield. Zeff remains essentially unchanged by argon. All the other plasma parameters (temperature and density profiles, ohmic power and energy confinement time) also show no substantial changes. Radiation losses increase, up to 80% of the input power, at relatively high densities, with a corresponding decrease of the heat load on the limiter surface