The UVB erythemal irradiance measurements obtained at Thessaloniki, Greece (40°N) are analysed. The variability of the erythemal irradiance is compared with that of the total columns of ozone and SO2. It is shown that these two factors explain about 50% and 26% respectively of the total variability of the erythemal irradiance. The influence of the reduced total ozone levels observed during the last 4 years on the increase in the solar UV irradiance at 305 nm is also presented. Finally, a comparison between stations with different air pollution levels under clear skies shows that air pollution can significantly reduce the UVB levels received at the ground.