This is an interesting topic and I'd love to shed some light on it as well as my perspective. Short and sweet this week!
Some stylists will debate that ammonia is way more damaging to the hair and others will argue that it's the peroxide that does the major damage.
I have found them to be like a Batman and Robin situations. The peroxide def causes some serious damage but ammonia is its sidekick.
Let's break down ammonia, shall we.
Ammonia is a colorless gas that contains hydrogen and nitrogen. It's the most common alkalizer used in permanent hair color and is most often used with peroxide to lighten the hair by softening and swelling the cuticle layers. Ammonia is what allows oxidative hair color to decolorize AND help form the new dyes.
Remember, oxidative dyes that use ammonia as an alkalizer do both LIFTING of the natural hair color and DEPOSITING of the new, artificial color.
The developer is what allows the oxidation to happen, meaning, the lightening of the hair AND the development of the dyes. The dyes combine and couple with each other and increase in size so that they get trapped on the cortex and gradually fade over time.
Ammonia is the most common alkalizer used and upwards of 4% ammonia can be found in oxidative color. That 4% of ammonia gets reduced to almost half when mixed with developer and continues to dissipate as the hair processes. Remember, ammonia is a GAS.
Another reminder, Hydrogen peroxide has an acidic pH (below 3.5). When it is mixed with a color with a high pH (ammonia pH 9.0-11.0), the high pH of the ammonia causes the peroxide to decompose which causes the developer to release oxygen which in turn causes the natural hair to lighten by degrading the melanin (and the hair fiber in general).
The higher the developer, the higher the lift will be. (More oxygen working its way through the hair fiber)...The higher the lift, the more damage can be caused.
Remember, the higher the developer, the more oxygen is released, degrading more melanin. Now, during the coloring process you are lifting AND depositing..so degrading more melanin and degrading the hair fiber isn't as bad because the dyes you are depositing will couple and "fill the holes" that were created...but those dyes gradually fade leaving a GAP in the cortex until you deposit again.
Essentially, the cortical fibers have been permanently expanded and nothing you do will revert that process. Proteins and bond builders help restore some of the luster, but will not "fix" the problem.
One thing you need to keep in mind is the higher the developer, the more damage that's caused BUT also remember the higher the alkalinity of the color is, the more the peroxide decomposes and the more oxygen it releases and the more damage is caused.
So is one worse than the other? I think both ammonia and peroxide work together to cause damage but in my opinion, developer is more damaging because it degrades the hair fiber. The ammonia just helps it do that more.
Hope this was helpful!
Until next time,