One word. Chlorine. In the 1970s it was found that Chlorine damaged the O-Zone layer. In the 1980s it was found that the O-Zone was so damaged that a hole began to form over Antarctica. Chlorine was the culprit.
One of the leading contributing factors to Chlorine being released into the atmosphere were CFC/HCFC refrigerants. Either knowingly, unknowingly, technicians would vent excess refrigerant into the atmosphere when working on an air conditioning unit. Repeated venting across fifty years across the world caused the hole to form in the O-Zone layer.
In order to prevent the hole from getting any bigger there was an agreement that was signed by numerous countries. The agreement mandated the phase out of all CFC and HCFC refrigerants. Some of the refrigerants included were R-12, R-22, and R-502. This agreement came to be known as the Montreal Protocol.
In 1994 the first phase-out began in the United States. R-12 was discontinued from usage in automobiles and was replaced with the HFC refrigerant known as R-134a. In 1999 R-502 was phased out in accordance to the Montreal Protocol and was replaced with HFC refrigerant known as R-404A.
Lastly, in 2010 R-22 began it’s first phase out steps. In 2010 no new R-22machines could be manufactured or imported into the United States. In 2015 the production and importing of R-22 was reduced drastically. In 2020 R-22 will be completely phased out. The alternative to R-22 is the HFC R-410A.
Due to the Montreal Protocol it is predicted that the O-Zone’s hole will seal completely and the O-Zone will return to normal towards the end of the 21st century. The Montreal Protocol is widely regarded as one of the most successful treaties in the modern world.