In a study published in the Journal of Cosmetics March 2018, researchers investigated the mechanism of hole development in the hair cuticle when exposed to UV radiation.
They concluded that In hair exposed to UV radiation in the wet state, the cuticle layers partially peeled off, and numerous cuticle holes were observed between the cuticle layers. It is thought that linoleic acid, which is the unsaturated fatty acid between the cuticle layers in hair is oxidized due to over-exposure to UV radiation.
Due to this, a radical chain reaction occurs, producing a highly reactive hydroxy radicals, which form cuticle holes and damage hair. Distilling this research paper further, one can hypothesise that oils with high linolenic acid could be damaging to the cuticle.
There are certain oils that people often use on the scalp in hope of for treating hair loss. ; Argan oil and Rosemary oil. Therefore, looking at the linolenic oil content, Rosemary oil is about 41% linolenic oil whereas Argan oil is approximately 33%.
Using the above-mentioned study, then one can question whether, using Argan and/or Rosemary oil on the hair shaft exposed in sunlight to be healthy for the cuticle or not. Rele and Mohile in 2003, studied the properties of mineral oil, coconut oil and sunflower oil on hair.
Coconut oil was the only oil found to reduce the protein loss for both undamaged and damaged hair when used as a prewash and post wash grooming product. Coconut oil has 2% linolenic oil.