Do Women Need Less Intensity in Red Light Therapy? Understanding Skin Optics for Better Dosing.
However, this advise is generic and universal - and never takes into account important factors of bio-individuality. Which may be a massive reason why there are such wide ranges of effective doses seen in the literature.
Even basic factors like age and gender should be taken into account, which we can often see studies mentioning such problems.
One study notes:
"The structure, appearance, and biophysical properties of human skin differ between individuals according to their age, sex, and race." 
And with massive differences in skin characteristics, then we need to adjust the dosing strategy to not overdose or under-dose based on generic guidlines.
Women and Elderly Likely Need Lower Doses than Men:
Like most holistic health techniques, the proper dose is highly individual based on many factors. It has been clear from the beginning that the “one size fits all” dosing concept would be a failure for red light therapy. Many people will respond at different doses, even the clinical studies report extremely wide ranges of successful doses. Our observations show that perhaps some groups (like women or elderly) tend to be more responsive than others.
What happens when women or elderly are overdosed by following generic dosing guidance? Well it could lead to biphasic dose inhibitory response (lack of benefits), and often it could overheat skin tissue (collagen breakdown), or cause other symptoms of overheating like hyperpigmentation.
Indeed, resoundingly we hear the anecdotes for these issues primarily coming from women. Then when we take it that men are primarily running these red light therapy panel companies all vying for the "highest" intensity crown, and young beefy men like Ben Greenfield and Alex Fergus are endorsing high intensity exposure – it is easy to see how women are being forgotten by this industry.
These potentially adverse responses have perpetuated the myth that Near-Infrared LED light is deleterious, when the real culprit is that the devices were delivering too much intensity overall. So by turning off the NIR LEDs, they solve the problem by cutting the power in half.
All of which reinforces our premise for this article that women or elderly would do better with less power or intensity.
So lets hold true to the Scientific Method, we take our observations and try to see if the research, mechanisms, and studies can verify our assumptions about them.
"Optical Window" and the Optimal Wavelengths for Photobiomodulation:
Before we look at the differences between gender and age, lets review the basics of the optical properties of human skin.
Many studies will mention the “optical window” of ideal photobiomodulation wavelengths as being between 600nm to 905nm. But what does that really mean?
It means that wavelengths below 600nm tend to be absorbed primarily by melanin and hemoglobin (blood) in the surface layers of the skin. Thus why Blue, Green, and Yellow wavelengths are superficially absorbed and the color of blood is red!
Notice how only the Red lights transmits through the finger tip to make it glow, yet the Blue and Green light is blocked by the skin. You can do this experiment by holding your finger over your phone flashlight (or any flashlight) and notice that your finger only glows red (other colors are blocked/absorbed).
Above 905nm is where water absorption starts to take over. Generally, wavelengths above 905nm will have more limited penetration because they are superficially being absorbed by the water in our skin. Thus, why we immediately feel the warm sensation on our skin when exposed to direct sunlight or heat lamps.
Typically the "low" point of absorption curves line up around 810nm, but we see a dramatic drop in absorption starting in the early 600nm's. 
The absorption spectrum for melanin, hemoglobin, and water have been well studied and are charted together above. We see the "window" is between 600nm to 900nm where the absorption coefficients are at the lowest for all 4 factors. Notice that having the lowest absorption in the skin allows for higher penetration!
Less Absorption and More Reflection Leads to better Penetration:
This seems counterintuitive at first. The wavelengths that are LEAST absorbed by the skin are the most desirable for the “optical window”! Indeed, the inverse is true – the wavelengths with the MOST reflection from the skin are the same Red to NIR optical window range which we show in a previous blog.
In some studies, having high reflection in our skin composition is a good thing because allows for more “scattering” of the light once it passes through the skin. And the easy way to bypass the reflection losses is to use skin contact.
So while it seems counterintuitive that the wavelengths of Red to NIR have the MOST reflection and LEAST absorption of the entire light spectrum – it does make sense because we don’t want them absorbed too quickly and allow them to penetrate deeper than other wavelengths.
Now lets keep these important concepts of absorption, reflection, and scattering in mind for the next section.
Gender Differences in Skin Optics:
One study investigated the tissue optics of 198 Japanese participants in terms of absorption and scattering coefficients.  The purpose of the study was to directly aid the Light Therapy and Photobiomodulation industry in determining guidelines for safe and effective treatments.
Sure enough, the women consistently had lower absorption coefficients and higher scattering coefficient for most of the wavelengths analyzed. 
Based on our knowledge of tissue optics from the previous section, this would indicate that women would be more responsive to red and near-infrared light therapy than men.
Gender Differences in Light Penetration:
One study with 660nm light found in live human patients that the red light penetration was significantly greater in females than males! They noted that all of their male and female subjects were lean so adipose tissue was likely not a factor in this case. They theorize the differences could be explained by inherent differences in skin, muscle, bone, hair, and other connective tissues and blood vessels. 
Of course if light penetration is deeper for females then we would expect them to be more responsive to the therapy, thus needing less intensity or dose to get the same response as a male.
Gender and Age Differences in Skin Thickness:
Some studies indicate that skin thickness and echo density testing shows that male skin is thicker than female skin. 
Another study mentions that after menopause, female skin thickness decreases as estrogen levels decline. So we have both age and gender as important factors.
Now if there is less skin and blood as a barrier in the first place, that means Red Light Therapy can travel with less hindrance to the targeted tissue. Typically tissue below the skin will have higher mitochondria content and be more responsive to treatment – for example muscle or organs.
The average cell only has 100 mitochondria, but underlying organs, brain, and muscle tissue will be more responsive to red light therapy due to having a higher number of mitochondria.
Now this is a good thing for effectiveness for women, but they would again need to be more mindful of the biphasic dose response or even tissue heating.
And resoundingly the studies show that elderly skin is thinner than young skin.  Which means we may want to take extra precautions and start with lower intensity for elderly. This also is a good thing, because it means that elderly should be more responsive to treatment and may require less dosage overall.
Heat and Thermal Pain Sensations:
Because many red light panel companies are continually increasing their intensity output to be competitive (note: not for effectiveness, but for sales), this will eventually lead to heating of the skin and tissue.
Obviously if the skin is thinner and more responsive to Red Light Therapy in certain genders and age groups, this would also indicate less ability for thermoregulation to protect them from the heat.
In several studies found that females were more likely to report greater pain from heat than males. So there may be more of a correlation between heat sensitivity and female skin. 
Which as we have mentioned in earlier blogs, most of the potentially deleterious effects of Red Light Therapy come from overheating the tissue. Since females may be at greater risk of skin heating, this becomes an important issue.
Noted by one of the heat pain studies is that “hairy skin” plays a role in determining heat sensitivity.  Although rarely mentioned by studies, “hairy skin” could also pay a role in photobiomodulation.
Does too much hair block red light therapy from reaching the skin? Yes.
Some veterinary studies indicate that shaving the hair from horses plays a big role in light transmission and penetration.  The same theory could apply to why some males can tolerate higher doses and intensity than women.
“the area to be treated should be shaved beforehand. There was an increase in the depth of penetration of laser through all clipped (shaved) tendons, compared with the untreated tendons.” 
Hair could be simply blocking the light and helping with thermoregulation in males more than females.
Another interesting study looked at the penetration of different Red/NIR wavelengths and the correlation to skin tissue properties of mice. 
Surprisingly, they found that there was only a 10% difference in transmittance from 600nm to 905nm wavelengths, and even that difference wasn't significantly significant. That is a big revelation that maybe the difference between Red and NIR penetration is much less than we thought!
It is clear that skin thickness, composition, and subcutaneous adipose thickness all play a role in light penetration and transmission. Perhaps a much bigger role than people think.
They concluded that subcutaneous adipose tissue (fat cells below the skin) and connective tissue structure (affecting light scattering) were the most significant factors in light transmittance. 
Of course, in this day and age excess adipose tissue can affect everyone! Excess fat could also play a role in hindering light from reaching more active tissues.
Update 3/11/22: New Study on Women's Dosing
A new study titled " Evaluation of Gender Differences in Response to Photobiomodulation Therapy" has emerged which is specifically looked at the differences between men and women with regards to red light therapy dosing and response. 
Of course this new study not only validates our own standpoint in this blog, but shows that we have covered this important topic of gender differences before the leading researchers. They even note that they believe they are the first article on this topic.
The authors share the same concern as us, that women are underrepresented in clinical studies for photobiomodulation, leading to dosing recommendations which have been centered around a male anatomy. They specifically state:
"The majority of clinical studies on PBM had been conducted in males, resulting in often suboptimal safety and efficacy data for the equivalent effects in females." 
So this should be a very concerning statement from the authors, especially for women who are taking dosing recommendations from male-centric brands and influencers.
The article reviews several of the same differences that we discuss in this blog. A new topic they discuss is the role of estrogen and other intrinsic mechanisms and antioxidants that are different between genders, and how they affect the interaction with photobiomodulation.
Overall, this review article says there isn't conclusive evidence about the gender differences with photobiomodulation dosing, but only because there is a lack of research around this topic.
Wavelength Doesn't Matter For Light Transmission?
This is potentially an industry-changing conclusion that bears quotation:
"Thus, it can be suggested that light therapy dosimetry targeting subcutaneous tissues deserve better attention at light attenuation due to different skin features rather than what wavelength to use (at the observed spectral range)" 
In layman's terms, the proper dose and depth penetration depends more on the skin composition and much less on wavelength than most people currently believe (as long as you are between 630nm to 905nm range). This is a big deal.
Divergent Dosing Protocols for Men and Women?
Of course, the ideal scenario would be to have more studies directly measuring the benefits of various doses categorized by gender and age group.
We can only make so many observations and assumptions based on the science that we have available today.
In our previous blog at the only 5 studies on full-body red light therapy we noticed the that two of the studies used Male athletic teams and dosed them with only 6.9 J/cm^2 and 25 J/cm^2. Both of those studies failed to find a significant benefit.
There is one full-body study on a female athletic team that used 30 J/cm^2 did show an improvement in sleep and recovery.
So from this minimal data, we might be able to assume that the Male team used way too low of dose, and the female team used a sufficient amount. Could that be related to their natural differences in skin optics and composition? We think that is one of many factors we need to consider.
There is still a lot that needs to be learned about proper dosing of red light therapy, and it is often confused by manufacturers proclaiming they offer the “best science based” products and protocols.
Of course that is a marketing fallacy, since there is a highly bio-individual nature to treatments like Red Light Therapy. Even the scientific studies focus on treating all patients with the exact same dose, when they may need to consider additional factors of each individual.
Understanding more about differences in skin optics may be the first step towards narrowing down more specific treatment guidelines to different groups of people. We have identified key differences in skin optics like absorbance and scattering coefficients, skin thickness, hair, adiposity, and thermal sensitivity.
Some researchers have clearly noted that all of those skin optics factors are MORE important than wavelength when trying to determine light penetration and dosing. This can dramatically change and revolutionize the way we think about red light therapy dosing.
Even if companies offer generic dosing guidelines, everyone should consider their own unique situation and adjust dosages accordingly. Too often brands and influencers are pushing high-intensity products just to make easy sales. Eventually this mindset will lead to deleterious responses.
Mitochondria Diagram © GembaRed LLC - Icons made by Freepik, smalllikeart, turkkub, and dDara, from @flaticon
All other images are © GembaRed LLC or purchased and licensed through Shutterstock.
 Water Absorption Coefficient Spectrum:
 Hemoglobin Absorption Coefficient Spectrum:
 Melanin Absorption Coefficient Spectrum:
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