Halogenated refrigerants are known to damage the ozone layer. Some halogens are carcinogens or otherwise toxic to humans, so they are subject to regulatory controls and phaseouts mandated by the Environmental Protection Agency. The Clean Air Act of 1990 has led manufacturers to create environmentally-friendly products, like non-halogenated refrigerants.
Non-halogenated refrigerants include natural substances like air, carbon dioxide, water, nitrogen and helium. They require higher pressures for use as refrigerants and may be more expensive than halogenated refrigerants. In addition, existing systems will likely require changes to use alternative refrigerants, because their chemical and physical properties will differ.
In addition to removing heat, a good refrigerant should be chemically stable, non-toxic, non-flammable and generally non-hazardous.