Heat Hinders Penetration With Red Light Therapy: Cold Increases Transmittance

Photobiomodulation is the science behind Red Light Therapy. It is defined as a non-thermal application of light on biology. Most studies do not cause significant heating as reported in the literature.

Thus, non-thermal treatments are the evidence-based way of doing clinical grade PBM. As brands and influencers try to normalize heating with PBM, they can only make speculative claims based on extrapolating from the non-thermal studies. More importantly, they conveniently ignore the potential downsides of combining heating with PBM. 

One factor that is commonly overlooked is how heating affects penetration depth.  

How Heat Hinders Penetration:

Heating the skin before or during red light therapy has been consistently shown to reduce the transmittance and penetration depth. Cooling has been shown to increase penetration.

1. Heat stimulates increased blood flow to the skin as part of the thermoregulation response. Additional superficial blood will absorb more light and hinder deeper penetration.
2. Studies indicate that heat changes the optical properties of the tissues in a way that additionally hinders penetration. 
3. On the contrary, cooling the skin demonstrates an improvement in light penetration. Vasoconstriction causes reduced blood flow in the skin creating a more transparent path for photon penetration. [1]
   

Like most things in life, we can imagine the relationship between intensity and penetration as following an inverted U-shape like the biphasic dose response. The peak penetration occurs at the highest intensity that does not cause heat, and penetration decreases as more heating is produced. 

Practical Takeaways:

If we want to optimize penetration depth, then we need to prevent skin heating before and during Red Light Therapy treatments. We can even employ cooling methods before or during Red Light Therapy to enhance penetration. 

• Use non-thermal parameters for PBM. That may include low intensities, non-thermal wavelength selection, and limiting exposure time. These are typically the baseline parameters seen in PBM peer-reviewed studies. 
• Using higher intensities may require external cooling, pulsing, or scanning techniques to reduce heating.
• Schedule the Red Light Therapy treatments before activities that cause heating. For example, do not do Sauna or Exercise immediately before Red Light Therapy. It is better to do Red Light Therapy first and then Sauna or Exercise when trying to optimize deep tissue treatments. 
• Cooling techniques can be used before and during Red Light Therapy to optimize penetration such as cold plunges, cryotherapy, ice packs, or air conditioning. 
• Do not combine Red Light Therapy and Heat therapies, such as Full-Spectrum saunas, heating pads with LEDs in them, and red-coated incandescent heat lamps. These are beneficial as heat therapies, but would reduce the efficacy of the Red/NIR component. 

This means we do not need to tolerate uncomfortably hot LED panels just for a false promise of deeper penetration. Quite the opposite, remaining cool during Red Light Therapy treatments will help improve penetration, aside from being generally safer with less risks of burns. 

Summary of the Evidence:

• Low Powered Lasers Penetrate Deeper Than High Power

A recent 2025 article measured the penetration of different lasers through the Achilles tendon on living humans. The powers were 100mW, 450mW, and 9,000mW. [2]

They found that the high powered laser penetrated significantly less percentage than the lower powered lasers, contrary to the popular assumption. 

"We hypothesize that an increased thermal impact—defined as the amount of heat generated by the device at the skin surface—may negatively influence light transmission to deeper tissues. "

...

"A possible explanation is the high rate of light absorption by the skin, which promotes heating and reduces photon penetration into deeper tissues. From an optical perspective, this is consistent with the inverse relationship between absorption and transmission: greater surface absorption leaves fewer photons available to propagate through the tissue. " [2]

They believe it was the heating from the high powered laser that caused the reduction in transmission (penetration) through the skin. There was more superficial absorption triggered by the heating.

• High Intensity Penetration is Less Than Expected

One recent 2024 article made important observations about tissue optics during treatments with high intensities that cause heating. The normal assumption is that increasing intensity has a linear effect on penetration. But, they found that with higher intensity and heating then the penetration was much lower than expected.

"Under ideally consistent conditions, if tissue optical properties remained unchanged, doubling the power applied to the surface would result in a doubling of the irradiance measured at the target level. This response should hold for any scaling of the surface power. "

...

"Sustained thermal responses could have altered the tissue’s surface scattering or absorption properties [26,27] compared to baseline, significantly affecting the diffusive transmission of light in the skin and, consequently, the amount of light reaching the target level. It can be expected that the extent of optical property changes would increase as the laser power increases, due to the greater energy deposited as heat [12,24]. " [3]

In this case owing to the changes in optical properties in the tissues caused by heat. This led to an unexpected decrease in penetration with higher intensity.

• Contact Cooling used to Increase NIR Penetration

One 2013 article brings together all the practical points we mentioned earlier. They used a contact cooling freezer gel to enhance the penetration of light and allow the skin to tolerate higher intensities comfortably. 

As the gel cooled during treatment, the penetration gradually became more superficial. 

“Further, the contact-cooling and freezer-stored gel reduces the temperature, perspiration, and blood vessel dilation at the skin surface, enabling safe delivery of near-infrared energy into the deeper tissues, including muscle.”

..

“These specific wavelengths, the contact cooling, and the freezer-stored gel enabled near-infrared to be delivered to the deeper tissues without pain or epidermal burns”

..

“During treatment, as the gel and the superficial layers of the skin gradually became warm, the energy of near-infrared was absorbed in the superficial layers of the skin” [4]

Thus, they were able to use higher intensities and longer wavelengths more comfortably and with deeper penetration. 

We can see the intentionality when doctors understand these techniques. Allowing heat to target more superficial tissues, and using cooling to target deeper tissues. 

• Textbook Recommended Protocols for Heating and Cooling:

The 2014 Phototherapy textbook by Dr. Tuner and Dr. Hode also make their recommendation clear that heating decreases penetration and cooling increases light penetration. They explain that cooling can be done before light therapy treatments, and heating should be done after light therapy treatments. 

"If heat is used in combination with laser therapy, the laser treatment should come first. Heat will increase blood flow in the tissue, thus increasing the absorption of the light in blood. The opposite then applies to cryogenic therapy."

...

"If heat is used in combination with LPT, should the laser treatment come first? Heat will increase blood flow in the tissue, and by increased absorption of light in the blood, the penetration will be reduced. On the other hand, more blood will be irradiated which can lead to positive systemic effects." [5]

One positive point they make is that the increased blood absorption can still have beneficial systemic effects. So it isn't entirely a waste to have heat and can help explain why people still get some benefits with superficial absorption caused by non-contact treatments and heating.

• Cooling Skin Increases Penetration:
  

A 2017 study also measured the penetration through the Achilles tendon of living humans. This time they iced the skin for 20 minutes prior to measuring the penetration with 810nm and 904nm lasers. [6]

They found an increase in transmittance of over 26% for 904nm and about 25% for 810nm. An increase of penetration of 25% is quite substantial, especially for a technique so simple as icing the skin. [6]

• Computational Relationship between Skin Temperature and Penetration

This relationship has been documented as far back as this 2003 article. They modeled the relationship of temperature and blood flow affecting absorption coefficients in the skin that limits penetration.

"Changing the temperature of this cutaneous region by heating and cooling will change blood flow into the capillaries by opening and closing the arterio-venus shunts, thus affecting μ_a of the cutaneous tissue." [7]

And the opposite was shown by cooling the skin. 

"the calculated light penetration depth (δ) increased as temperature was lowered, indicating cooling-induced transparency of human skin." [7]

This was also confirmed in living human subjects, in addition to the mathematical modeling. 

• Cooling for Tissue Optical Clearing

Another 2013 article built off of the previous work to find an increase in penetration of up to 1.39x when cooling at 10-20C compared to 30-40C. [8]

This was proposed as part of a technique for many forms of phototherapy that require deeper tissue treatments. 

Heliotherapy is Cold, Sunbathing is Hot:

Heating from light therapy causes a snowball effect. Once the heating starts, it causes more superficial absorption. More superficial absorption leads to more heating. 

This is why the adverse effects of overdosing on high intensity LED panels are mostly superficial. The body is able to protect itself with this mechanism to reduce penetration when it senses high intensities and heating. From a positive perspective, we should be thankful that this mechanism is in place to protect us. 

This may make sense from an evolutionary perspective. Radiant heat from sunlight triggers the mechanism to reduce light penetration in the skin. This offers protection to the sensitive organs and muscles from absorbing too much light. This is one way of explaining how we don't get an inhibitory "overdose" by being out in the sunlight for prolonged periods of time. 

Sunlight therapy is called Heliotherapy. It was briefly popular over 100 years ago. The Heliotherapy clinics were historically located in cool climates in the mountains like Leysin, Switzerland; Perrysburg, New York: and Glenwood Springs, Colorado. 

Photograph from the J.N. Adam Memorial Hospital opened 1912 in Perrysburg, New York [Link]

Dr. Auguste Rollier was the original Sun Doctor that started and inspired many of these clinics around 1903. It was his recommendation to locate the heliotherapy clinics in cooler climates or high elevations. 

Dr. Rollier once stated:

"The intense heat of the sun is tempered and rendered wonderfully bracing by the action of the dry, cold air on the whole surface of the body."[9]

In a 2017 interview between Dr. Mercola and Dr. Hamblin they discuss this topic. They cover the differences between sunbathing and heliotherapy. According to the article:

"“He said if you just go and lie in the sun on the beach, all you’re getting is a sunbath. But if you go up in the mountains, you’re actually getting a medical therapy,” Hamblin says."

https://articles.mercola.com/sites/articles/archive/2017/02/26/photobiomodulation.aspx

Many health advocates promote the mornings and evenings around sunrise and sunset for optimal sunlight exposure. This is because the sun at a low angle allows the UV and Blue to be scattered by the atmosphere and what reaches the body has a higher ratio of Red/NIR compared to mid-day sun. 

The mornings and evenings are also quite cooler than mid-day. As one prominent neurosurgeon would say; nature doesn't make mistakes. The combination of Red and Near-Infrared wavelengths from sunlight always coincides with the cooler parts of the day.

It is not a coincidence that our bodies have adapted to optimize Red and NIR light penetration when it is cooler, and is more protective during the warm mid-day sun. Our upright posture also optimizes Red/NIR absorption at an angle, and minimizes light absorption from the mid-day at peak temperatures.

So when the top health influencers promote morning and evening sunlight, they are also inadvertently recommending to get Red and NIR light during the coolest hours of the day. 

Nature vs Therapy:

This is a common rhetorical argument made to promote the sales narrative of heating and overdosing. High intensity LED panels that produce heat are just mimicking warm sunlight. Certainly, sunbathing is generally healthy and that warmth is a natural aspect of sunlight exposure.

As we learned, there is a big difference between Heliotherapy vs Sunbathing. And it is a common fallacy that LLLT/PBM does not necessarily mimic sunlight exposure for it's therapeutic effects. We are not trying to replace sunlight with Red Light Therapy. We are trying to find the optimal parameters of light therapy that are often independent from sunlight. 

In other words, invoking the parallels to sunlight is the Naturalistic Fallacy, the appeal to nature. The assumption that natural conditions are healthier than artificial. While it is generally true that natural factors are healthy, it also must be supported by evidence and not left as an assumption based on a fallacy.

However, for many drugs and medical therapies, the artificial conditions make for a stronger effect. Our skin is already tailored to protect itself from non-contact continuous high intensity light and heating. To bypass those natural protections for deeper penetration then we need to use skin contact method, cooling, or pulsing. Techniques that are unnatural that our skin did not evolve ways to protect itself from. 

So, when we want true clinical grade effects from Red Light Therapy it is delivered non-thermally and with skin contact. If we just want general wellness from warm light then we use sunlight or high intensity non-contact LED panels. 

Conclusion:

The penetration depth is a popular marketing point for Red and NIR light therapy. It is well established that Red and Near-Infrared wavelengths penetrate the deepest into the body compared to the rest of the solar spectrum. 

We often want to improve cellular functions of vital organs, brain, heart, muscles, or joints located deep in the body. The consumer needs to be assured that devices and treatments can effectively treat the deeper tissues. 

Thus, many products will claim to have the best penetration based on some rhetorical arguments about wavelengths, high intensity, overdosing, tolerating heat, non-contact, pulsing, etc. Often these claims are poorly supported when we dive into the science. 

A common misconception is that feeling heating is a sign of deep penetration. It does tangibly confirm there is high intensity when one feels radiant heating. And it was falsely claimed that high intensity is key to deeper penetration, even though we broke down the science that intensity has an insignificant effect on penetration depth. 

As the perpetual bearers of bad news, we must inform you that heating instead confirms the opposite. It is a sign that there will be reduced penetration by increased blood flow and thermoregulation systems. There may even be innate changes to tissue optical properties that additionally reduce penetration when heated. 

Our heat sensing nerves are primarily located in the surface layers of the skin, so we literally cannot feel heat in the deeper tissues. When we feel heat, it is a superficial sensation.

Heating is typically a non-issue with Photobiomodulation, since it is by-definition a non-thermal therapy. When using clinically recommended parameters, there is negligible heating.

As mentioned in one of the recent articles, following WALT guidelines produces practically no heating effects. We can appreciate how many PBM studies go out of their way to confirm that there is minimal heating. It seems rather important.

"when using doses within the therapeutic window recommended by the World Association for Photobiomodulation Therapy (WALT) guidelines [22], the thermal effects of both devices were negligible [19]. " [2]

However, simply following the science is insufficient to convince the RLT influencers, since it is their job to sell high intensity products that cause significant heating. The influencers need to see overwhelming overt harm before they would relinquish such a profitable sales narrative. 

To their disappointment, the influencers have only been able to inflict superficial side effects to a small percentage of their followers. This is because the skin protects the deeper tissues by hindering penetration when it senses heat. If their recommendations actually penetrated deeply, then the side effects would be much more dramatic.

Thus, we are able to connect what the science is saying to the anecdotal experiences of consumers using heating LED panels. It is well established that properly administered heat therapy is good for managing pain, inflammation, and promote healing - so they can conflate the benefits from their LED Heat Lamps with the reported benefits of non-thermal PBM science. 

This blog only details the evidence behind one issue when combining PBM and Heat, the reduction of photon penetration depth. 

The irony being that influencers often falsely claim their products penetrate the deepest, when then in reality they are recommending some of the worst penetration products possible. Thus, the benefits can only be explained by superficial absorption triggering systemic mechanisms, or now via simple heat therapy mechanisms. 

As always, our information can be used in an unbiased way. If you want to do non-thermal Photobiomodulation with high intensities, then you can use cooling techniques and pulsing/scanning techniques to reduce heating and promote deeper penetration in a safer way. That simply means getting a box fan and slowly rotating like a rotisserie in front of a high intensity panel. 

Our goal is to reveal the real science of PBM to help people to get the best benefits in the safest possible way. Which is a lot more than many brands and influencers can truthfully claim at this point. 

References:

[1]

Institute of Medicine (US) Committee on Military Nutrition Research; Marriott BM, Carlson SJ, editors. Nutritional Needs In Cold And In High-Altitude Environments: Applications for Military Personnel in Field Operations. Washington (DC): National Academies Press (US); 1996. 7, Physiology of Cold Exposure. Available from: https://www.ncbi.nlm.nih.gov/books/NBK232852/

https://www.ncbi.nlm.nih.gov/books/NBK232852/

[2]

Leal-Junior ECP, Hess F, Dias LB, Lino MMA, Machado CDSM, Martins PHGN, Casalechi HL, Tomazoni SS. Light Transmission and Thermal Impact of Different Photobiomodulation Therapy Devices on the Achilles Tendon of Human Volunteers: A Comparative Study. Photodiagnosis Photodyn Ther. 2025 Sep 25:105234. doi: 10.1016/j.pdpdt.2025.105234. Epub ahead of print. PMID: 41015277.

[3]

Piao, D.; Sypniewski, L.A.; Bartels, K.E.; Burba, D.J.; De Taboada, L. Transcutaneous Transmission of Light of Photobiomodulation Therapy Wavelengths at 808 nm, 915 nm, 975 nm, and 1064 nm to the Spinal Canal of Cadaver Dogs. Photonics 2024, 11, 632. https://doi.org/10.3390/photonics11070632

[4]

Tanaka Y, Tsunemi Y, Kawashima M, Tatewaki N, Nishida H. Objective assessment of skin tightening in Asians using a water-filtered near-infrared (1,000-1,800 nm) device with contact-cooling and freezer-stored gel. Clin Cosmet Investig Dermatol. 2013 Jun 26;6:167-76. doi: 10.2147/CCID.S47299. PMID: 23837000; PMCID: PMC3699058.

[5]

Hode, Lars. Tuner, Jan. Laser Phototherapy Clinical Practice and Scientific Background. 2014 Prima Books AB

[6]

Haslerud S, Naterstad IF, Bjordal JM, Lopes-Martins RAB, Magnussen LH, Leonardo PS, Marques RH, Joensen J. Achilles Tendon Penetration for Continuous 810 nm and Superpulsed 904 nm Lasers Before and After Ice Application: An In Situ Study on Healthy Young Adults. Photomed Laser Surg. 2017 Oct;35(10):567-575. doi: 10.1089/pho.2017.4269. Epub 2017 Jun 30. PMID: 28677985.

[7]

Khalil O.S., Yeh S.-j., Lowery M.G., Wu X., Hanna C.F., Kantor S., Jeng T.-W., Kanger J.S., Bolt R.A., de Mul F.F. Temperature modulation of the visible and near infrared absorption and scattering coefficients of human skin. J. Biomed. Opt. 2003;8:191–205. doi: 10.1117/1.1559997. 

[8]

Changmin Yeo, Heesung Kang, Yunjin Bae, Jihoon Park, J. Stuart Nelson, Kyoung-Joung Lee, and Byungjo Jung, "Development of an Optical Tissue Clearing Laser Probe System," J. Opt. Soc. Korea 17, 289-295 (2013)

[9]

Loignon AE. Bringing Light to the World: John Harvey Kellogg and Transatlantic Light Therapy. J Transatl Stud. 2022;20(1):103–28. doi: 10.1057/s42738-022-00092-7. Epub 2022 Feb 7. PMCID: PMC8819196.