In her book, The Black Woman's Guide to Beautiful Hair, Lisa Akbari states the need to set aside discussing "grades" of hair and instead be attentive to "real information that will empower your hair."
This focus is the hair strand, which tells us the texture and type of hair. She notes that textures come in fine, medium and coarse. As texture relates to heat damage she writes:
"Fine strands cannot tolerate excessive pressing or heat from a curling iron, blow-dryer, flat iron, pressing comb or heat rollers. Fine hair will damage easily because it has fewer protective layers than medium and coarse hair strands. The smaller strands requires the least amount of maintenance products in order to manage.
Medium hair strands are of average size, have a medium quantity of layers and can tolerate heat and chemicals better than fine hair. Although this is true, all hair strands burn if the temperature of a heated tool is greater than 150 degrees. So you must be careful not to overuse heated styling tools or chemicals. I recommend that you avoid pressured heat, instead try a lower more indirect heat.
Coarse hair strands are the largest of the three are thicker and are difficult to damage...."
You can listen to Lisa's radio show on this site: http://www.blogtalkradio.com/Lisa-Akbari