[Lab studies] suggest that heat may be especially damaging to the already vulnerable hair of African Americans.So, what does this all mean? Natural hair is more susceptible to heat damage than Relaxed hair. Since Relaxed hair is chemically straightened, the use of heat is lessened compared to Natural hair.
Companion studies conducted by Lawrence Hu in Dr. Wickett's lab found that relaxer treatments also weakened African Americans' hair, but that use of relaxers on a regular basis actually led to a trend toward normalizing breakage strength.
And use of a relaxer did not further increase the vulnerability of African Americans' hair to heat damage.