The size and shape of the Antarctic ozone hole on 17 September, 1979. Purple and dark blue areas are part of the ozone hole. As the images show, the word hole isn't literal; no place is empty of ozone. Scientists use the word hole as a metaphor for the area in which ozone concentrations drop below the historical threshold of 220 Dobson Units. Using this metaphor, they can describe the hole's size and depth. The map shows the state of the ozone hole on the day of maximum depth#the day the lowest ozone concentrations were measured. The measurements are made by NASA's Total Ozone Mapping Spectrometer (TOMS) instruments from and by the Royal Netherlands Meteorological Institute (KNMI) Ozone Monitoring Instrument (OMI) (from 2004-present).