Why is there ammonia in the permanent hair color?

In order for hair color to be permanent, it must have ammonia or an ammonia substitute. Ammonia (NH3) is an alkaline compound of hydrogen and nitrogen that is both naturally occurring and man-made. We chose ammonia because it is more efficient than MEA (Mono-ethanolamine). The MEA molecule has a molecular weight of 61, the molecular weight of ammonia is only 17. Therefore, MEA is 3.5 times larger than ammonia. When permanent hair color and peroxide meet, ammonia acts as a catalyst to start the oxidation reaction. This reaction lightens natural pigment in the cortex and allows the hair dyes to deposit inside the cortex. Then the ammonia flashes off of the hair and is gone. Because of the large-sized molecule, MEA-based permanent hair color products may not lighten efficiently and the MEA stays in the hair during processing, so there is a tendency for the color to developer darker than desired. MEA can also have a higher irritation potential.