Penetration Profiles of Visible and Near-Infrared Lasers and Light-Emitting Diode Light Through the Head Tissues in Animal and Human Species: A Review of Literature.
Level 5 - mechanism / opinion, no new human data
Narrative literature review of preclinical animal and human tissue penetration studies
PubMed 31553265 · doi:10.1089/photob.2019.4676
What was done
The authors reviewed published literature on the penetration profiles of visible and near-infrared (NIR) laser and light-emitting diode (LED) light (630–810 nm) through head tissues in animal and human species. Light penetration data were evaluated across factors including wavelength, coherence, beam characteristics, anatomical irradiation site, tissue layers examined, species, age, and sex.
What was found
Average transcranial red/NIR (630–810 nm) penetration rates across species were: - C57BL/6 mouse (skull only): 60–70% - BALB/c mouse (skull only): 1–10% (1.75% for 810 nm laser through intact skull) - Sprague-Dawley rats (scalp plus skull): 10–40% - Oryctolagus cuniculus rabbit (skull only): 20% - Pig (scalp plus skull): 0.11% (for 808 nm laser) - Humans (scalp plus skull): 0.2–10% The large variation in reported penetration was driven by differences in tissue thickness, anatomical site, wavelength, and light coherence.
Why it matters
Translatability of transcranial photobiomodulation from small animal models to humans is severely constrained by head tissue thickness, as human scalp and skull attenuate over 90% to 99.8% of applied red/NIR light.
Limits
The abstract does not state the number of included studies or systematic search methodology. Direct comparison between models is limited by heterogeneous experimental setups, differing tissue preparations (skull alone versus scalp plus skull), and varied laser/LED parameters.
Cited by
- contradicts Long-wavelength light from sunlight penetrates through the human body to charge cellular mitochondria.
- contradicts Long-wavelength red light passes all the way through the human body, even through light clothing, and is absorbed by the water in mitochondria to improve mitochondrial function in every cell with mitochondria.