For an ozone layer to become an ozone hole, the concentration of ozone should be:

A region is classed as an ozone hole when the total column ozone falls below 220 Dobson units.

A value below 300 units is thinner than normal but does not meet the accepted hole threshold.

Below 320 units is likewise still within the range of ordinary healthy ozone cover.

Below 400 units is close to typical values and far from the hole definition.

Pre depletion Antarctic ozone historically measured around 300 or more Dobson units.

The 220 unit mark was chosen because such low values had never been seen before large scale ozone loss began.