Hair straightening is accomplished by three types of procedures:
Mechanical - heavy pomades to hold the hair in a minimally kinked form. While this method causes least hair damage, it is also the least effective in straightening hair.
Heat - hot-combing and hair "ironing" to alter the chemical structure of hair so it can be combed out straight. The application of high heat and hard combing can cause hair loss in some individuals.
Chemical - uses alkaline chemicals to alter the structure of hair-shaft proteins (keratins) and render the hair easier to be molded into a straighter form. The most potent chemical hair straightening agent is a lye solution, which is also dangerous to use and is little used today. Ammonium bisulfite creams are milder than lye solutions, but also less effective. Overall, chemical hair straightening is most likely to cause permanent damage to hair.