NH3 (ammonia) is not directly used in modern refrigerators. While it was used in the past, it has been largely replaced by other refrigerants due to its toxicity and flammability.

Here's a breakdown:

* Historical Use: Ammonia was one of the first refrigerants used because it's efficient at absorbing heat and has a high boiling point. It was commonly used in early refrigerators and industrial cooling systems.

* Safety Concerns: Ammonia is toxic, corrosive, and flammable. These properties make it a dangerous refrigerant, especially in residential settings. A leak could lead to serious health issues or even death.

* Modern Alternatives: Modern refrigerators use safer and more environmentally friendly refrigerants, such as:

* Hydrofluorocarbons (HFCs): While still potent greenhouse gases, they are significantly less harmful than ammonia.

* Hydrocarbons (HCs): These are natural, flammable refrigerants like propane (R-290) and isobutane (R-600a). They are more efficient and have a lower environmental impact than HFCs.

* Current Use: Ammonia is still used in some industrial refrigeration applications due to its high efficiency, but it is not used in typical household refrigerators.

In conclusion, while ammonia played a role in early refrigeration technology, its safety concerns led to its replacement with safer alternatives in modern refrigerators.