In this study we focus on measurements and modeled UV index at the region of Athens, Greece, during a period of a low ozone event. During the period of 12 to 19 of May 2020, total column ozone (TOC) showed extremely low values 35-55 DU (up to 15%) decrease from the climatic mean (being lower than the -2σ). This condition favors the increase of UV erythemal irradiance, since stratospheric ozone is the most important attenuator at the UVB spectral region. A parallel intrusion of Saharan Dust aerosols in the region has masked a large part of the low ozone effect on UV irradiance. In order to investigate the event, we have used spectral solar irradiance measurements from the Precision Solar Radiometer (PSR), total column ozone from the BREWER spectrophotometer and Radiative Transfer Model (RTM) calculations. Model calculations of UV Index (UVI) showed an increase of ~30% compared to the long term normal UVI due to the low TOC while at the same time and for particular days, aerosols masked this effect by ~20%. The study points out the importance of accurate measurements or forecasts of both ozone and aerosols when deriving UVI under unusual low ozone-high aerosol conditions.