Porosity refers to how "open" the hair's cuticle is. Porous hair has extremely open cuticles and can therefore have great difficulty retaining moisture and nutrients. What flows in flows right back out. While some hair types have a natural leaning toward porosity, most porosity issues are a result of damage. Excessive heat use and chemical abuse are the top contenders when it comes to excessive porosity. These practices virtually eat away at the cuticle. Avoiding frequent heat styling and overuse of relaxers and dyes is your only line of defense in fighting porosity. Products aimed at reversing porosity are TEMPORARY fixes at best - they may lay the damaged cuticle down for a bit but do absolutely nothing to restore the cuticle's natural position or grow back missing areas. As soon as the product is washed away the porous areas are still there. One good preventative measure against porosity is using either olive or coconut oil to seal the hair. These oils are clinically proven to penetrate the hair strand and help halt protein loss. Aside from that, stay away from extreme heat tools, overlapped/overprocessed relaxers and permanent hair dye! You can test your hair's porosity by taking some shed hair and placing it into a glass of water. Wait 4-6 minutes. If the hair sinks, your hair is porous. If it floats at the top your hair has the proper density. If you do have excessive porosity, start treating all new growth with extreme protective TLC and gradually trim away the porous ends.