Which gas is commonly known for being toxic and is a refrigerant?

Ammonia is the correct answer in this context. It is commonly used as a refrigerant in various industrial applications due to its high efficiency. While ammonia is effective in refrigeration, it is also known to be toxic when inhaled in significant quantities. This duality makes it important for those working with it to adhere to safety practices to avoid exposure.

The other gases listed have distinct characteristics. Freon-22, for instance, is less toxic than ammonia and is primarily used in air conditioning systems, but it contributes to ozone depletion, which is a different concern. Carbon dioxide is non-toxic and is used as a refrigerant and in other applications, but it does not possess the toxic properties associated with ammonia. Hydrochlorofluorocarbon compounds, while they can be harmful to the ozone layer, also do not have the same level of toxicity that ammonia does. Thus, ammonia stands out as both a refrigerant and a gas known for its toxicity.