The Optimal Intensity is not the Highest Intensity: The Big Dosing Myth
A long time ago - researchers and doctors would discuss the optimal intensity as being a low intensity that has been proven to deliver consistent results. Many are still talking like this today.
In the commercial industry, the "the highest intensity" marketing is often hinged on claims that you (1) get faster treatment times and (2) get better penetration. To the extreme point that it seems people are scorching themselves with reckless levels of intensity for fears they would not get adequate penetration otherwise.
Yet the research and experts often contradict these points.
Exposure time is often cited as being the most important aspect of dosing itself, even if you reach the same "dose" of Joules/cm^2 in a short amount of time. Sacrificing exposure time due to having high intensity may lead to less beneficial results.
Penetration depth is mostly determined by wavelength, regardless of intensity. So as long as there is adequate intensity and exposure times, then PBM researchers will rarely promote high intensities in the pursuit of slightly deeper penetration. Excessively high intensities will eventually hinder penetration because they will require to be used in non-contact mode and induce heating.
Opening the Vault:
Dr. Mercola's original Photobiomodulation articles from 2017 are now accessible to read. These were hidden for several years as Dr. Mercola changed his website due to mainstream pressures.
Opening these blogs feels like peering into a Vault from a bygone era. With the current era being a wild west that has conveniently ignored and forgotten many of these insights.
From 2017 until now, we find that the science and perspectives of researchers have remained fairly consistent. Only the marketing rhetoric is divergent from these views.
Dr. Hamblin Photobiomodulation Interview:
From Dr. Mercola's 2017 interview with Dr. Hamblin, they discuss the importance of not only optimal dosing (J/cm^2), but the optimal intensity to deliver said dose (mW/cm^2).
"As a general rule, an ideal power density is around 10 milliwatts per square centimeter. Most photobiomodulation devices use between 10 and 20 milliwatts per square centimeter. "
https://articles.mercola.com/sites/articles/archive/2017/02/26/photobiomodulation.aspx
Reading this in the year 2024 is rather strange, most devices today are much more powerful than the 10-20mW/cm^2 that Dr. Hamblin stated. Presumably the true clinical grade devices that Dr. Hamblin has experience with are within 10-20mW/cm^2.
Dr. Hamblin's ideal intensity of 10mW/cm^2 is 10x less than the claims of >100mW/cm^2 that would sweep the industry soon after this blog was published.
Optimal Intensity ≠ Highest Intensity
Perhaps the most shocking aspect of this topic is the mere concept that there could possibly be an "optimal intensity" that is different than "the highest intensity".
Such discussions about intensity have been conspicuously absent over the past 6+ years. The default assumption is to favor higher intensities.
Dose Rate Importance - Interview with James Carroll
Dr. Mercola's 2017 interview with James Carroll expressed similar sentiments that the dose-rate (intensity) is more important than the dose (J/cm^2) alone.
"Many researchers also only talk about the total energy delivery, when in fact the rate of delivery is actually more important. "
https://articles.mercola.com/sites/articles/archive/2017/11/12/photobiomodulation-light-therapy.aspx
The key points here:
- More power is not always better.
- There is an effective range of intensities, meaning there is an upper limit where too much power becomes ineffective or counterproductive.
- The rate of delivery (time) is more important than just the total energy delivered (Joules, or J/cm^2).
They discuss how delivering 4 Joules per Centimeter Squared from a 4 Watt laser for 1 second would likely not produce as favorable results as a low milliWatt laser for several minutes. The context is that studies will not produce positive results with too much intensity, too short of exposure time, or incorrect dosing methodology.
They invoke the popular analogy that dosing Red Light Therapy is similar to cooking. Cooking a chicken at high temperature for a short time will lead to an unappetizing result.
Having the proper temperature and cooking time is more important than simply viewing cooking as an energy balance equation. Similarly, proper intensity and exposure time is often more important than simply the energy calculation.
Indeed, there is a Biphasic Curve for the effectiveness of increasing intensity, not just a biphasic response to Energy Density (J/cm^2) alone.
Optimal Exposure Time:
Having proper intensity (dose-rate) always leads to the importance of exposure time for Red Light Therapy treatments.
As James Carroll expresses based on his experience:
https://articles.mercola.com/sites/articles/archive/2017/11/12/photobiomodulation-light-therapy.aspx
A 2017 Mercola interview with Dr. Lew Lim from VieLight also reinforces this treatment range.
Certainly both of these experts have the resources to design their products with higher intensity and shorten the required exposure time. Yet they both seem to choose to design their products with having proper exposure time in mind.
The 2014 Laser Phototherapy textbook by Tuner and Hode also confirms this concept of dose-rate and the superiority of low intensities for longer exposure times:
"There is an increased awareness about the necessity to deliver fairly low doses over longer time to optimise anti-inflammatory results. This means that, at least for healing processes, low power over long time is more effective than high power over short time, for stimulation of cell proliferation."
[1, pg.73]
Confirming that relatively lower intensities with longer exposure times will generally produce more effective outcomes particularly for anti-inflammatory effects and cell proliferation that is normally desired from Photobiomodulation Therapy.
Lets jump forward to the present day.
A March 2024 interview on the Summus Laser "LaserLife Insights" podcast with Dr. Praveen Arany reinforces this concept of the importance of exposure times.
"And the most critical parameter is the treatment time."
https://www.spreaker.com/podcast/laserlife-insights--6011615
A recent May 2024 article also confirms the typical treatment range of time for transcranial photobiomodulation (targeting the brain through the scalp) is between 10 to 30 minutes.
"Transcranial PBM is the most studied PBM paradigm for the treatment of AD. This involves the non-invasive delivery of visible and/or NIR light, typically using light-emitting diodes (LED) applied to the head and brain regions for appropriate lengths of time (typically 10—30 min) " [11]
Another June 2024 systematic review article of PBM on the brain also notes a trend of using 20 minute exposure times:
"In regards to the irradiation time during a treatment session, many studies chose a treatment time of 20 min per LED placement" [12]
In other words, the marketing rhetoric of higher intensity and shorter treatment times may lead to less effective results. Although it seems counterintuitive against the Law of Reciprocity, the researchers that pay attention to successful parameters have empirically noticed the importance of adequate exposure time.
The Big Dosing Myth: The Law of Reciprocity is a Lie
Young scientists will learn many laws of science as the foundation of their studies. However, in real-world complex situations these laws may work in counterintuitive and unpredictable ways.
The Ideal Gas Law assumes the behavior of a gas follows the equation PV=nRT. However, we quickly learn that there are no ideal gasses, that this law can only approximate the behavior of some gasses within limited conditions. Real-world experimentation is often required to learn the behavior of many gasses.
Imagine the trauma of studying theories and laws of science for years just to realize most of them are irrelevant in real-world situations.
Similarly, it is not surprising that the Law of Reciprocity only provides an approximation of the expected outcomes within a limited scope. For complex systems like human biology, the LLLT and PBM experiments have shown that the intensity and exposure time are more important than obstinately adhering to this law.
For example, the Bunsen-Roscoe Law of Reciprocity is what allows us to calculate the so-called "dose" of Energy Density in units of J/cm^2 (Joules per centimeter squared) by multiplication of Intensity and Exposure Time.
- 5mW/cm^2 for 1000 seconds
- 50mW/cm^2 for 100 seconds
- 500mW/cm^2 for 10 seconds
The sales and marketing experts would want you believe that since these are all the same Energy Density of 5 J/cm^2, they would have identical beneficial results.
However, this has been specifically contradicted in many articles. Here is a 2011 article co-authored by Jenkins and Carroll:
"The belief may have arisen from the Bunsen–Roscoe law of reciprocity (also known as the third law of photobiology, and the reciprocity rule), which asserts that the effects of irradiation will be independent of irradiance and exposure time as long as radiant exposure is maintained.12,13 This rule has subsequently been disproven in general photobiology, photography, and LLLT/photomedicine."
[4]
The authors clearly understanding the Law of Reciprocity had been empirically debunked in LLLT studies all they way back in 2011. This section cautions that over-reliance on J/cm^2 as the sole parameter will lead to poorer outcomes.
A 2020 systematic review and meta-analysis of effective PBM parameters on diabetic foot ulcers agrees with this method:
"Even though when a device provides a high power, it means that the necessary “dose” is reached in less time. However, there is a “dose rate effect,” and when the dose is applied very quickly, the beneficial effects diminish. This is because the intensity (irradiance / power density) is very high."
[5]
Very clearly stating that the benefits of Red Light Therapy diminish when applying high intensities for a short amount of time, even at equivalent Energy Density (J/cm^2).
I notice a pattern of many peer-reviewed articles referring to Energy Density (J/cm^2) as the "dose" in quotation marks. This is a literary tactic to help highlight that the author understands that this is the improper usage of the term, and only complying with using this term as the vernacular or for brevity. Many authors are indirectly telling us through this punctuation that they know that the true dose is not the Energy Density.
https://en.wikipedia.org/wiki/Scare_quotes
As has been established in the Biphasic Dose Response peer-reviewed article back in 2009.
[8]
Clearly stating that the equation for "dose" of Energy does not necessarily follow the Law of Reciprocity.
A 2021 article also confirms that there is indeed no "dose" of red light therapy, that proper intensity and exposure time are key.
"In laser therapy, there is no such parameter as “dose” or “dosage”. There is “energy” = power × time [J] and “energy density” = energy/area [J/cm2]. The mathematical demonstrations are absolutely not necessary."
[9]
No need for "dosing" math based on the Law of Reciprocity. The effects depend on the Intensity and the Time.
In the March 2024 interview with Dr. Arany, he also expresses this sentiment.
https://www.spreaker.com/podcast/laserlife-insights--6011615
Even in 2024 this method holds true. The Law of Reciprocity only works within certain contexts and a zone of reasonable parameters. It is more important to find the "sweet spot" of intensity and exposure time, rather than focusing on the energy calculations.
Again the emphasis from leading researchers is to find the ideal, sweet spot, and/or goldilocks intensity that is different than merely finding the highest intensity.
He also invokes the same analogy of cooking a chicken properly.
It is always cooking chickens. Never turkey or pot roast.
Shortly after this blog was originally published, a new presentation by James Carroll was uploaded to YouTube on 5/13/2024 on the excellent brain-PBM channel. He notes his distain for only talking about "dose" as merely J/cm^2.
"So, Joules Per Centimeter Squared by itself is inadequate for determining whether or not something is going to work."
-James Carroll, YouTube
And naturally his preference is to discuss proper dosing via Intensity and Exposure Time, with Energy Density (J/cm^2) being optional. He even refers to J/cm^2 as the "so-called dose" with air-quotes with his hands at one point.
Another recent May 2024 article also confirmed the irrelevance of the Law of Reciprocity.
The study demonstrated that at low intensities there was no response, medium intensities there was a stimulatory effect, and high intensity had an inhibitory effect. All at the same "dose" of energy. Similar to several of the examples given by James Carroll's presentation, there appears to a biphasic response to intensity, not just the dose.
So while it is helpful to use a Dosing Calculator like what we provide in a previous blog as a good reference range, too much reliance on the dosing math will lead people to forget about getting proper intensity and exposure time as the experts often recommend.
Limitation of Intensity is Heat:
In the Lew Lim interview, Dr. Mercola stresses this advice for avoiding heat or warmth with proper usage of photobiomodulation:
"As a general guideline, as soon as you feel warmth, move it. You do not need, nor necessarily want, heat when doing photobiomodulation. "
Impressively, Dr. Mercola understood the basic definition of photobiomodulation back in 2017 more than most brands and influencers today. Heat is neither needed or wanted during true Photobiomodulation.
Jumping ahead, the 2024 interview with Dr. Arany mentions a similar perspective.
"when you experience this treatment or if are delivering this treatment, nothing happens, there is no change in the patient that people can appreciate. Yeah you feel a little warm, sometimes, but, and you see a bright light"
https://www.spreaker.com/podcast/laserlife-insights--6011615
Discussing how most of the time nothing is felt at all, leading to skepticism since there are no immediate observable changes in the tissues. Only sometimes is a little warmth felt during treatment.
The 2014 Laser Phototherapy textbook by Tuner and Hode has specific sections on "Sales tricks" often calling out the fallacies of using high intensities.
"while heat is fine for many conditions, the biostimulative effect does not depend on heat. There is one good side of giving more heat in the skin and that is the patient "feels" the treatment better which might give a better placebo effect"
[1, pg. 69]
The benefits and mechanisms of Photobiomodulation are photochemical and not photothermal. Which is why Mercola said that warmth is not needed for PBM, and reinforced by this textbook that the effects do not require heat.
Perhaps some subtle warmth is acceptable to improve the placebo effect and assuage the skepticism when people don't feel anything during normally non-thermal PBM treatments.
This advice has been echoed in my own blogs on getting proper distance from modern RLT devices. That you should increase your distance away from your device until you barely feel any radiant warmth for a true non-thermal PBM experience. Or at least reposition the device (or your body) when you feel heat building-up in an area.
Penetration is only Marginally Increased by Intensity
The favorite marketing line is to promise better penetration with higher intensity products, often paradoxically when promoting non-contact panels that have inherently bad penetration due to skin reflection losses and lack of skin contact compression.
Lew Lim is quoted in 2017 that you don't need a lot of power for penetration, especially when you have a deeply penetrating wavelength like 810nm.
Note again how the researchers rarely place emphasis on having excessively high power levels, even for deep penetration.
It is most commonly accepted that the wavelength primarily dictates the penetration depth in relation to the optics of human skin.
This perspective is repeated in the 2014 Laser Phototherapy textbook by Tuner and Hode:
"At around 808nm we actually have the best penetration into the tissue, and increasing power only increases the depth of penetration marginally."
[1, pg 69]
And in another section of the same book:
"And they do not penetrate much deeper due to the high output - in fact, the very act of making a high power laser 'safe' for long-duration exposures may make it less capable of penetrating as deeply as lower-powered laser that can e.g. be applied in contact and with slight pressure to the skin."
[1, 74]
High intensity devices are often required to be used in a non-contact method to avoid excessive heating, and they do not significantly increase penetration by virtue of high intensity alone. These authors clearly note that lower intensities pressed into the skin will penetrate deeper than higher powers that are used at a distance. As is just one example they provide for the detriment of high intensities on penetration.
The same textbook and several other sources note that too high intensities cause heating and will reduce penetration by inviting more blood flow to the area. [1][2][3] Since higher intensities don't significantly increase penetration anyway, then it quickly becomes counter-productive to use excessive intensities that will cause heating.
Red Light Therapy Dosing Guide in 2017
Red Light Therapy often takes a minute to learn, but clearly takes years to master.
From these 2017 blogs, a reasonable dosing protocol is to get 10-20 mW/cm^2 for 10-20 minutes.
While J/cm^2 isn't as important, for reference that would deliver:
- 6 J/cm^2 on the lowest end (10mW/cm^2 for 10 minutes)
- 24 J/cm^2 on the highest end (20mW/cm^2 for 20 minutes)
As long as at least 1 Red or NIR wavelengths are present, then we should expect good results for most people.
That is it. That is how simple red light therapy should be.
A January 2024 full-body red light therapy study for humans with fibromayalgia used 28mW/cm^2, 660nm+850nm, 20 minute sessions, 33.6 J/cm^2 per session, 3 times per week and got good results. [7] This recent data still confirms that this is an effective dosing strategy, not requiring excessive intensity and noting the importance of the 20 minute exposure time.
With most successful full-body red light therapy studies agree with the effective range of treatment times of 10 to 20 minutes. With none of them delivering intensities over 50mW/cm^2.
Conclusion
The perspective of leading researchers and their clinical research has been fairly consistent throughout the years. Dr. Mercola's old articles resurfacing help highlight many important aspects that have been seemingly forgotten in the commercial marketing today.
In Dr. Calderhead & Dr. Tanaka's excellent 2016 article on Phototherapy, they say this:
"If we use a pharmaceutical analogy, the PD is the medicine, and the ED is the dose. As any pharmacist will tell you, if the medicine is not correct, there is no use in playing around with the dose."
[6]
If the Power Density (PD) is not correct, then there isn't much point in figuring out the Energy Density (ED).
This is in agreement with the peer-reviewed article on Biphasic Dose Response:
b. The dose (time)"
[8]
The irradiation parameters (wavelength, intensity) is the medicine, and the true dose is the exposure time. Not to be oversimplified into an energy calculation.
When you change the Intensity, you change the "medicine" which will lead to different outcomes.
Emphasis on high intensity, short exposure times, and deeper penetration from high-intensity non-contact panels appears to be an unscientific narrative by salespeople. As the intensities of name-brand panels slowly increase each year, this is becoming a reckless and irresponsible narrative.
Appropriate (mostly non-thermal) intensity and adequate exposure time are core tenants of clinical grade Photobiomodulation therapy dosing. Recent interviews and studies seem to confirm these long-held principals.
All that said, we present to you our profoundly contrarian conclusion that the optimal intensity is likely not the highest intensity. It may even be much lower than many brands and their influencers would want you to believe.
References:
[1]
Low-Level Light Therapy: Photobiomodulation
Author(s): Michael R. Hamblin, Cleber Ferraresi, Ying-Ying Huang M.D., Lucas Freitas de Freitas, James D. Carroll
Published: 2018
https://doi.org/10.1117/3.2295638
PDF ISBN: 9781510614161 | Print ISBN: 9781510614154
[3]