<P> A single chlorine atom is able to react with an average of 100,000 ozone molecules before it is removed from the catalytic cycle . This fact plus the amount of chlorine released into the atmosphere yearly by chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) demonstrates the danger of CFCs and HCFCs to the environment . </P> <P> The ozone hole is usually measured by reduction in the total column ozone above a point on the Earth's surface . This is normally expressed in Dobson units; abbreviated as "DU". The most prominent decrease in ozone has been in the lower stratosphere . Marked decreases in column ozone in the Antarctic spring and early summer compared to the early 1970s and before have been observed using instruments such as the Total Ozone Mapping Spectrometer (TOMS). </P> <P> Reductions of up to 70 percent in the ozone column observed in the austral (southern hemispheric) spring over Antarctica and first reported in 1985 (Farman et al .) are continuing . Antarctic total column ozone in September and October have continued to be 40--50 percent lower than pre-ozone - hole values since the 1990s . A gradual trend toward "healing" was reported in 2016 . In 2017, NASA announced that the ozone hole was the weakest since 1988 because of warm stratospheric conditions . It is expected to recover around 2070 . </P> <P> The amount lost is more variable year - to - year in the Arctic than in the Antarctic . The greatest Arctic declines are in the winter and spring, reaching up to 30 percent when the stratosphere is coldest . </P>

Where was it first discovered that the earth's ozone layer was being depleted