In order to transform Earth's atmosphere, the Zygons introduced modified CFCs into the air, through industry and the commercial release of toxic products. In so doing, they accelerated the process of global warming, poisoned the ozone layer, and made the atmosphere more amenable to Zygon life, by 1984. (AUDIO: The Zygon Who Fell to Earth)

Behind the scenes[]

Chlorofluorocarbons (CFCs) contain carbon (С), chlorine (Cl), and fluorine (F). They have been heavily regulated since the late 1970s. As the Eighth Doctor points out, hairsprays contributed to the problem: CFCs are used as propellants in aerosol cans.