Hair: an obstacle for Photobiomodulation
Infrared radiation can pass through the various tissues of the skull
In transcranial photobiomodulation therapy , the aim is to illuminate the brain using very specific infrared radiation.
Your Photobiomodulation device does not have direct access to the brain, so the radiation must pass through the various human tissues before it can reach the brain.
So, only a very small percentage of this radiation will reach its destination... the rest will be lost! Because part of it will either be absorbed by the skin and transformed into heat, or reflected, or refracted. However, the few small percentages that reach the brain will be enough to stimulate the activity of your cells.
However, there is one thing that infrared radiation cannot penetrate: hair !
Infrared radiation blocked by hair
Indeed, in a very interesting study , Theodore Henderson and Larry D. Morries point out an aberration: most marketing departments of manufacturers of Photobiomodulation devices publish photos of patients who have placed a device on their hair.
However, the two researchers demonstrate that hair constitutes a barrier to the penetration of infrared rays into the brain. They illustrate this using a very telling example. They place a 2mm thickness of blond hair between an infrared laser and a laser sensor. Result: 98% of the laser power is absorbed or reflected by the 2mm of hair! Thus depriving the brain of infrared radiation!
Other researchers working on Photobiomodulation are of the same opinion. For example, in this study , it is possible to read:
The irradiation areas were chosen on various parts of the head, but the forehead was most often used due to the absence of hair which can serve as a formidable barrier to light penetration depending on its color and density.
The same goes for Professor Michael Hamblin , author of numerous scientific publications on the subject of Photobiomodulation:
In this transcranial PBM (tPBM) application, near-infrared (NIR) light is often applied to the forehead due to better penetration (no hair, longer wavelength).
Michael Hamblin, source
Conclusion : In view of the studies cited above, and in the absence of studies that will prove the contrary, hair is an obstacle to Transcranial Photobiomodulation. Patients wishing to illuminate their scalp will have to shave their hair beforehand (at least the illuminated areas) or choose clearer areas (such as the forehead).