Final answer:
The strength of the permanent waving solution is primarily determined by the concentration of the reducing substance.
Step-by-step explanation:
The strength of the permanent waving solution is primarily determined by the concentration of the reducing substance. In the permanent wave process, a reducing substance (usually ammonium thioglycolate) is added to reduce and rupture the disulfide cross-links in the hair strands. This disruption and reformation of disulfide bonds in the hair strands result in the formation of temporary or permanent waves.