Scientists use "hole" as a metaphor for the area in which ozone concentrations drop below the historical threshold of 220 Dobson Units.

科学家们用“空洞”一词表示臭氧浓度低于220个多布森单位（多布森单位是用来度量大气中臭氧柱尺度的单位。

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Scientists believe that ozone depletion in the atmosphere will continue until levels of ozone-destroying Cfc's drop to pre-1970 concentrations, around the middle of the next century.

BBC: News | Sci/Tech | Ozone hole reaches record size