A variety of challenging problems make it difficult to use such refrigerants in residential air-conditioners applications.
Although natural refrigerants such as carbon dioxide, propane, and ammonia have a minimal direct effect on global warming, . they require improvements to their stability, efficiency, and safety in order to render them suitable for use as a refrigerant. For example, ammonia is toxic, propane is combustible, and carbon dioxide has very low efficiency as refrigerant for air-conditioner. As a result, almost all ammonia applications have been used in large industrial refrigeration installations where full safety control can be provided. Carbon dioxide is used as refrigerant for water heaters, where it provides an efficiency equivalent to that of conventional refrigerants.