The Skinny on Intensity: Effective Intensity versus High Intensity
"Low levels of light are good for you, while high levels are bad for you." (2)
There is perhaps a favoritism in the PBM community that more intensity is better. In our Western mindset, bigger is better, hence high-intensity products (>=100mW/cm2) imply more value and "faster" treatments.
The law of reciprocity (Bunsen-Roscoe law) would tell us that we can achieve the same dose and effect regardless of intensity or administered timeframe. For example:
To reach a 6 J/cm^2 dose would require about:
- 1 minute at 100mW/cm^2
- 14 minutes at 7mW/cm^2
Is it logical to assume that both of these scenarios would provide the "same" biological response? Certainly not. So there are clear limitations in applying the Bunsen-Roscoe law to PBM.
An article on the ThorLaser website echos this same scenario and concern. They state that administering the same "dosage" in J/cm^2 at very different intensities will certainly illicit a very different clinical result. (11)
In experimental trials looking at Biphasic Dose, too high of intensity shows a lack of improvement. Indeed if we hold dose (J/cm^2) constant and only change the irradiance (mW/cm^2), researchers find a biphasic response curve! (2) Further, too high of an intensity >100mW/cm^2 would also risk thermal degradation effects. (1)
Why?
You cannot trick your body to absorb photons faster than nature intended. If you try, you can end up with a counterproductive result. (2) The sun offers the red and near-infrared light of about 23.6 mW/cm^2 (depending on environmental factors). (3) It might be best to emulate nature when possible.
"Photobiomodulation exposure to visible and IR-A light which emulates the conditions of natural sunlight in wavelength, intensity, and dosage can be beneficial to the skin." (1)
Have you ever taken high doses of Vitamin C all at one time? Might lead to a quick trip to the bathroom - your body cannot absorb all of it at once! When you want to raise blood levels of Vitamin C comfortably, you space out the dose over a longer period of time.
"Another guiding principle is that repeating the treatment daily (or even more or less often) until the wound is healed or the disease remission is observed is better than only one single application of LLLT. LLLT can be compared to a nutrient food for the human body; an adequate daily intake is best." (8)
So what does the science say?
In "Infrared and Skin: Friend or Foe" the author offers that lower irradiance <50 mW/cm^2 is less likely to induce detrimental effects. (1)
Irradiance from 5 to 50 mW/cm^2 is generally used for stimulation and healing, while higher irradiances can be more for nerve inhibition and pain relief. (7)
Rodent Studies:
For treating arthritis in Rats: A dose of 3 J/cm2 was significantly more effective when delivered at 5mW/cm^2 than 50mW/cm2. It performed the same as 30 J/cm2 delivered by 50mW/cm^2. Indicating that the duration of treatment also plays a role. (2)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2935792/
Another study reviewed the optimal parameters for ulcers in rats and used a range of powers from 0.7 to 40mW. They found the optimal power was 8mW. (2)
https://pubmed.ncbi.nlm.nih.gov/17659591/
While is may be easy to hand-wave these studies as being on rodents and not potentially relevant to humans, these are clearly significant findings that researchers are using when considering dosing protocols in human studies.
The first rodent article has been cited 47 times by subsequent studies, and is still being cited in 2024.
The second rodent article has been cited 45 times by subsequent studies, and was also cited once even in 2024.
So clearly most doctors and researchers have gotten the memo about this valuable dosing finding, and are still using it today to help design subsequent studies. They have not thrown away this data as an outlier. Only brands selling high intensity panels would encourage you to throw away this inconvenient data.
It would be expensive and require a lot of people to conduct studies with split intensity groups. Which is why we must first learn many dosing principals and mechanisms in small animal models. These principals are clearly being applied in human clinical trials, where influencers selling high intensity products would lead you to believe otherwise.
Human Studies:
Systematic reviews can take multiple human studies and document trends in the data. For example, analyzing the variables present during studies that have positive or negative results and finding a correlation.
One review article on many human studies for chronic joint disorders found that the studies that failed tended to have irradiance that was either too high or too low. (2)
"There have been several systematic reviews and meta analyses of RCTs and these have revealed some irradiance dependant effects: Bjordal published a review of LLLT for chronic joint disorders and identified 14 RCTs of suitable methodological quality, 4 of which failed to report a significant effect because the irradiance was either too high or too low, and/or delivered insufficient energy, the remaining eight studies all produced positive effects (Bjordal et al. 2003)."(2)
Another human systemic review concluded that failures were likely attributed to using intensities in excess of the WALT recommendations:
"Tumilty reviewed 25 LLLT RCTs of tendinopathies,13 of which (55%) failed to produce a positive outcome, all of these negative/inconclusive studies that recorded irradiance (or could subsequently be established) had delivered an irradiance in excess of the guidelines set by the World Association for Laser Therapy (www.walt.nu) (Tumilty et al. 2009)."(2)
A recent June 2024 systematic review article for human transcranial (through the scalp/skull targeting the brain) NIR light therapy. Their conclusion was that the lower intensity range tended to have better results than the higher intensity range.
is a more effective intervention than that with high-power density (40–90 mW/cm2)." [12]
So the lower intensity range tended to be more effective than the higher intensity range for brain health.
Even if it hasn't been studied directly, the mechanisms established by rodent studies are appearing as trends in human systematic reviews.
More Analysis:
When treatments are administered too closely together, the cumulative dose can reach an inhibitory range. (6) This could even mean that in the first days of treatment could have a beneficial effect and then cross over into a biphasic region. (7)
A 12-week study, understanding that the effects of PBM in muscle tissue lasts up to 48 hours, chose to use a low intensity of 8.1 mW/cm^2 for 15 seconds per treatment, 3 times a week. They cautiously designed a low dose to avoid the cumulative biphasic dose. And they were right, this course of treatment improved overall muscle fitness versus the placebo! (9)
The application of 5 J/cm2 with 4 mW/cm^2 had a significant effect on wound tensile strength, while the same dose delivered at 15 mW/cm^2 had no effect. (7)
The dose required to promote a therapeutic response is <10mW/cm^2, a dose 100 times this amount was found to have detrimental neuronal effect in mice. (4)
Hart et al. were so concerned about high dose biphasic response for brain PBM, that they were considering the additional exposure from Sunlight on patients adversely affecting clinical trials. (3)
Human stem cells had a peak response at 3 J/cm^2 with 16 mW/cm^2, where higher doses had diminished benefits. (5)
One study on collagen production found that medium irradiances and exposure durations were effective - 1.2 mW/cm^2 for 145 sec. (1)
What does this all mean?
Intensity does matter, but more isn't always better! Do you cook your turkey low and slow, or set the oven to broil and let it burn on the outside and be frozen on the inside?
There are certainly applications for higher irradiances. We would like to see a distinguishment in Red Light Therapy for a "Recommended Daily Allowance" as opposed to a "Therapeutic Dose". An RDA for light!
If you own your own PBM device and plan to use it daily, you do not need to chase therapeutic doses all the time. By using PBM consistently and responsibly, your body is getting the photo-prevention effects. Similar to taking a consistent dose of Vitamin C for proactive reasons. As you know too many antioxidants can have negative effects, the same mentality should be applied to red light therapy.
If you already own a high-intensity product, the solution is quite simple. Increase the distance away or use some pulsing or dimming mode to reduce the intensity. Ask your manufacturer to provide accurate data on irradiance data at various distances in various modes.
What do you think? Does the law of reciprocity strictly apply to PBM, or do the benefits change based on intensity and time-frame administered?
"High doses of laser energy can focus on the postoperative pain, but to obtain a good healing process, lower power and longer time are more advantageous." (6)
This quote is reinforced by the Arndt-Schulz curve describing the biphasic curve as "stimulatory" and "inhibition" regions. Stimulatory doses can promote growth, while inhibitory doses can be used to manage pain. However for the long-term benefits of Photobiomodulation, we want to stay in the Stimulatory zone.
Definitions:
PBM: Photobiomodulation is the biological effects of light.
Irradiance or Intensity: Power Density of light, typically in units of mW/cm^2 (milli-Watts per Centimeter Squared)
Dose: Energy Density when a light is applied to an organism, typically in units of J/cm^2 (Joules per Centimeter Squared)
Biphasic Dose: Too low of a dose is not effective, a too high dose is also not effective. A peak response may lie somewhere in between.
Disclaimer: All information in this article and website are intended for educational purposes only. It is not intended to treat, diagnose, or cure any ailment. Please consult with your doctor or trusted wellness practitioner before starting any new health activity including Red Light Therapy.
Update Notes;
10/14/2024:
- Expanded on Rodent and Human studies considering irradiance.
Sources:
(1)
Barolet D, Christiaens F, Hamblin MR. Infrared and Skin: Friend or Foe. J Photochem Photobiol B. 2016.
(2)
Ying-Ying Huang, Aaron C.-H. Chen, [...], and Michael R. Hamblin. Biphasic Dose Response in Low-Level Light Therapy
Dose Response. 2009; 7(4): 358–383.
(3)
Hart, Nathan S. Fitzgerald, Melinda. A New Perspective On Delivery of Red-Near-Infrared light therapy for disorders of the brain
September 19th, 2016 http://www.discoverymedicine.com/Nathan-S-Hart/2016/09/a-new-perspective-on-delivery-of-red-near-infrared-light-therapy-for-disorders-of-the-brain/
(4)
Johnstone, Daniel M et al.
Turning On Lights to Stop Neurodegeneration: The Potential of Near-Infrared Light Therapy in Alzheimer's and Parkinson's Disease
Front. Neurosci., 11 January 2016 | https://doi.org/10.3389/fnins.2015.00500
https://www.frontiersin.org/articles/10.3389/fnins.2015.00500/full
(5)
Yuguang Wang, Ying-Ying Huang, [...], and Michael R. Hamblin
Red (660 nm) or near-infrared (810 nm) photobiomodulation stimulates, while blue (415 nm), green (540 nm) light inhibits proliferation in human adipose-derived stem cells.
Sci Rep. 2017; 7: 7781.
Published online 2017 Aug 10. doi: 10.1038/s41598-017-07525-w
PMCID: PMC5552860
PMID: 28798481
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(6)
Pocket Dentistry. Jan 5, 2015
https://pocketdentistry.com/15-low-level-lasers-in-dentistry/
(7)
Ying-Ying Huang, Sulbha K Sharma, [...], and Michael R Hamblin
Biphasic Dose Response in Low-Level Light Therapy – An Update
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3315174/#!po=31.5385
(8)
Shang-Ru Tsai, Ph.D. and Michael R Hamblin, PhD
Biological effects and medical applications of infrared radiation
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Published in final edited form as:
J Photochem Photobiol B. 2017 May; 170: 197–207.
Published online 2017 Apr 13. doi: 10.1016/j.jphotobiol.2017.04.014
(9)
Cleber Ferraresi, PhD, PT, Danilo Bertucci, MSc, Josiane Schiavinato, MSc, Rodrigo Reiff, PhD, Amélia Araújo, MSc, Rodrigo Panepucci, PhD, Euclides Matheucci, Jr, PhD, Anderson Ferreira Cunha, PhD, Vivian Maria Arakelian, PhD, PT, Michael R. Hamblin, PhD, Nivaldo Parizotto, PhD, PT, and Vanderlei Bagnato, PhD
Effects of Light-Emitting Diode Therapy on Muscle Hypertrophy, Gene Expression, Performance, Damage, and Delayed-Onset Muscle Soreness
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(10)
Biostimulatory windows in low-intensity laser activation: lasers, scanners, and NASA's light-emitting diode array system.
Sommer AP1, Pinheiro AL, Mester AR, Franke RP, Whelan HT.
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(11)
"Dosage" is a difficult subject. Why ?
4 things you should know about PBM laser beam measurement and dosage
James Carroll - THOR lasers
https://www.thorlaser.com/Dosage.htm
(12)
Kai Su, Chunliang Wang, Jianbang Xiang,
Exploring the key parameters for indoor light intervention in treating neurodegenerative diseases: A systematic review,Building and Environment,Volume 258,2024,111587,ISSN 0360-1323,
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