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43sun%u2019s harmful ultraviolet radiation that reaches the earth%u2019s atmosphere. Without the ozone layer, the earth would be exposed to destructive levels of this ultraviolet radiation, making it uninhabitable for most life. In 1987, an international agreement was enacted to phase out the use of CFCs and other ozone-depleting substances. Since then, governments have enforced strict regulations, and the ozone hole has gradually been recovering.TAKE A DEEP BREATHHowever, ozone is not always desirable. Depending on the altitude at which it occurs, it can also be highly undesirable: ozone in the lower atmosphere and at ground level is an air pollutant that negatively impacts the health of humans, animals and plants. At these lower altitudes, it is sometimes called %u2018bad ozone%u2019. As it has a similar structure to oxygen, ozone displaces oxygen in the lungs of humans and animals, resulting in respiratory ailments. Ozone is also an irritant that causes eyes to become red and itchy. In vegetation and crops, it causes chlorosis and necrosis, undermining their ability to photosynthesise and thus reducing crop yields.Tracing the sourcesProf. Wang%u2019s interest in ozone pollution dates back to the early 1990s, when he established a background atmospheric monitoring station in Hong Kong %u2013 the first of its kind in south China. Using air current data, satellite data, and computer models, Prof. Wang and his team discovered that the increasing ozone pollution in Hong Kong was largely caused by long-range transport, initially from Mainland China and more recently from Southeast Asia.