This study also shows the hair swelling is a necessary phenomenon for cuticle cracks to form. To me this suggests that it may be better to allow hair to air dry to mostly dry before using a blow dryer*.
The study goes on to state that using certain ingredients that retard water evaporation before blow drying will prevent the cracks from forming. They named glycerin and propylene glycol as ingredients that worked in this regard. Products that did NOT prevent cracks are oils* such as triglycerides, mineral oil*, and petrolatum. Surprisingly silicone oils* were also listed as not helping prevent damage even though this other study shows that they do!
Ironing also leads to similar cracks in the cuticle but wet flat ironing will also produce the aforementioned bubbling/ blistering effect caused by steam escaping from the hair rapidly. Another study shows that treating hair with a low molecular weight conditioner prior to applying direct heat mitigated the damage. Low molecular weight conditioners are those that are able to penetrate the hair fiber so look for ingredients like hydrolyzed silk proteins, cetrimonium bromide and sometimes hydrolyzed keratin (but this depends on the molecular weight).
The above goes a long way to explaining why Cathy Howse gives the green light to heat usage on relaxed hair even up to two times per week but only on freshly deep conditioned hair!
Take Home Message
If you plan to put heat on your hair make sure you deep condition with a low molecular weight conditioner beforehand. Enjoy instant reconstructor conditioner or Paul Mitchell the conditioner are good examples of conditioners that contain cetrimonium bromide. When blow drying use a glycerin containing heat protectant* spray like Nexxus Heat Protexx or Tresemme Thermal Creations Heat Tamer. You can also apply some serum right before flat ironing for an extra smooth finish.
Heat has the potential to cause serious damage to your hair but I’m no hair Nazi and I realize that sometimes you just need to get your swang on and only silky straight hair will do. Just protect that mane, you hear?