A pool of low-ozone air resides in the Arctic stratosphere in summer. Its formation and maintenance arise from a combination of chemical ozone-destruction and transport processes. The summertime ozone destruction is induced by gas-phase chemistry dominated by nitrogen and hydrogen catalytic cycles, which are efficient due to long summertime insolation at high latitudes. It is shown that, during events referred to as low-ozone episodes (LOEs), column ozone can locally decrease to values comparable with the seasonal minimum. A combination is used of (i) assimilation of satellite ozone observations from the Global Ozone Monitoring Experiment, (ii) chemical trajectory modelling, and (iii) the gathering of new lidar and in situ ozone observations in the European Arctic in summer 2000. Hence it is shown that such LOEs involve the displacement of the pool of lowozone air in the middle stratosphere, which is more dynamic than previously thought and undergoes pronounced meridional excursions, in particular towards northern Europe. The thinner the ozone layer, the more ultraviolet (UV) radiation reaches the ground, where it can impact on human health and ecosystems. Erythemal UV dose enhancements of the order of 10-15% were observed in northern Norway during the LOEs in summer 2000.