For the past decade or so, ozone levels over Antarctica have fallen to abnormall low values between  August and late November. At the beginning of this period, ozone levels are already low, about 300 Dobson units (DU), but instead of slowly increasing as the light comes back in the spring, they drop to 150 DU and below. In the lower stratosphere, between 15 and 20 km, about 95% of the ozone is destroyed. Above 25 km the decreases are small and the net result is a thinning of the ozone layer by about 50%.  In the late spring ozone levels return to more normal values, as warm, ozone-rich air rushes in from lower latitudes. The precise duration varies considerably from year to year; in 1990 the hole lasted well into December.

In some of the popular newsmedia, as well as many books, the term "ozone hole" is being used far too loosely.  It seems that any episode of ozone depletion, no matter how minor, now gets called an ozone hole (e.g. 'ozone hole over Hamburg - but only for one day'). This sloppy language trivializes the problem and blurs the important scientific distinction between the massive ozone losses in polar regions and the much smaller, but nonetheless significant, ozone losses in middle latitudes. It is akin to using "gridlock" to describe a routine traffic jam.