Hair permanents have been around for more than half a century. They gained a bad reputation in the past few decades for being damaging to hair, making it frizzy and brittle. Today, alkaline and acid perms are two popular forms of chemical waves.
Alkaline perms contain ammonium thioglycolate, a potent chemical that produces a noxious and undesirable odor in the process. Acid perms contain glyceryl monothioglycolate, a more "gentle" formula; however, it is known to cause allergic sensitivity over frequent exposure, according to Hairfinder.com
Alkaline perms contain a low acidity, between 9.0 and 9.6 pH. Acid perms contain between 4.5 and 7.0. They are much more acidic and require additional heat in order to process.
Alkaline perms process much more quickly than acid perms, taking only about 20 minutes to take effect. They are "cold" perms and don't require additional heat application. Acid perms take much longer because the acidic formula is more gentle on hair.
Alkaline perms are more popular than acid perms, according to Hairfinder, because they create a more firm curl that stays true to form longer. Acid perms, because they use a more gentle formulation and process, tend to create looser waves.
Those with strong, thick, and coarse hair are good candidates for alkaline perms. The potent formula works better on resistant hair types. Those with damaged hair, which is porous, will benefit more from the gentler acid perm or acid-balanced perm that processes faster than true acid waves.