“To all the ones that have been asking about the damage the chemical relaxers can do to your hair, here is my opinion”, says the Dermatology Consultant, Dr. Nermeen Bedair: “First we have to start with a brief background on the difference between the old relaxers and the modern new treatments. The old ones were made of sodium hydroxide and potassium hydroxide, which means that they didn’t need to be heat activated. Those old treatments mainly broke the bonds between the hair’s amino acids; existing in the hair shaft, in order to straighten it. Therefore, this process was extremely damaging. Recently, new treatments are applied to the hair and then activated by heat, which forms a coat that keeps the hair in the straight form. Doesn’t sound too harmful, right? Well, it is not right. The heat itself used in the process can jeopardize your hair. Normally, dry hair can withstand temperature up to 300°C, while wet hair (which is normally the state of the hair when applying these treatments) will be damaged by a much lower temperature (115-120°C). Why? Because heat forms air bubbles inside the hair shaft and when heat is applied, steam fills those bubbles forcing them to expand. Those bubbles eventually burst on the surface and destroy the hair shafts.
On top of the heat hazard damaging the hair shaft, those new treatments may contain formaldehyde or ethylene glycol whose fumes can be extremely hazardous to your lungs. 

So, based on the above, my recommendations are: 
1-Heat can be very damaging for your hair especially when wet.
2-If you absolutely have to straighten your hair, 
A- Don’t use any treatment containing sodium/potassium hydroxide, formalin, formaldehyde nor ethylene glycol. 
B- The iron used must show the maximum temperature (230°C), and never use an iron without a thermostat.
3-Hair must be completely dry, not even damp.”