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The total ozone column (TOC) shows a strong annual cycle, so the threshold to define its lower values is better characterized as deviations from local climatological mean levels. A complete analysis of the effects of low total ozone on the UV radiation measured in Barcelona (Northeast of Spain) is described in the present work. The ozone “mini-holes” climatology in Barcelona, obtained from the daily ozone satellite data series (1978–2006), shows a seasonal behaviour with a maximum number of episodes in winter and early spring. Nevertheless there are a great number of events in the study area during September, when the UV irradiance reaching the Earth’s surface is still high and consequently the biological effects such as erythema could be more important. In fact, daily low ozone columnar contents in comparison with climatological means are detected throughout the year generally associated to weather systems passing. The deepest ozone “mini-hole” reported in the area was in 2004. It lasted three days with a minimum on 10 January, when daily TOC was 40% lower than the January climatological mean. During those days under cloud-free conditions the aerosol properties remained constant indicating that only low ozone levels were responsible of the increase in UV solar radiation. The erythemal irradiance at noon was 36% higher than the day before the extreme low ozone event. Although in absolute terms this increase is high, the erythemal effects are not so considerable because the UV levels in winter are low. On the other hand, the “mini-hole” event observed in this area at the end of spring had more harmful biological consequences although the ozone decrease was not so great: on 20 April 2004, low ozone levels and high solar elevation favoured the increase in UVB radiation detected by an operative spectroradiometer. At 300 nm, the irradiance was 80% higher than the day before, declining to 31% at 305 nm. In this occasion the UV Index at 12 UTC increased in one unit. [ABSTRACT FROM AUTHOR]