Therapeutic heating — “thermotherapy” for therapy geeks, the opposite of its trendier cousin, cryotherapy — is more useful than most people realize, mainly because muscle is a source of more and worse pain than most people realize, and muscle pain seems to respond well to heat. Muscle pain caused by over-exertion, muscle cramps and spams, and especially trigger points (muscle “knots”) are all common and sometimes severe, but often mistaken for other kinds of problems.
This therapeutic staple has mostly been ignored by science: its benefits are far from proven, and obviously it’s no miracle cure.1 Nevertheless, it’s probably doing more than “just” relieving symptoms. (Not that we should knock symptom relief.2) Everyone should understand heating the same way everyone knows how to put on a bandaid: it is a cheap, drugless way of taking the edge off an amazing array of common painful problems, especially neck and back pain, and maybe more.
What heat is for: mostly non-inflammatory body pain
Heat is primarily for relaxation, comfort, and reassurance, and taking the edge off of several kinds of body pain, mostly duller and persistent pains associated with stiffness, cramping, and/or sensitivity, which can be loosely categorized:
- Acute soreness from over-exertion: the pain you get after the first ski trip of the season. (Interestingly, not only is heat likely helpful for this kind of pain, it’s almost the only thing that is! More about this below.)
- Stiffness and pain in specific areas related to osteoarthritis, muscle “knots” or trigger points, and most kinds of cramping/spasm3 (menstrual, neuropathic, restless leg syndrome, for example, or even just stiffness from postural stress). But not, of course, cramps from heat exhaustion!
- “Hurts all over” pain and sensitivity. There are many kinds, but primarily: fibromyalgia, the rheumatic diseases, drug side effects,4 vitamin D deficiency, and sleep deprivation.
There are many other kinds of pain, of course, but these are the ones most likely to benefit from heat. No one with appendicitis or a 2nd degree burn wants a nice hot water bottle.
What heat is not for
Heat will make some conditions much worse. Never apply heat to an infection or fresh injury! Or any other acute inflammation, like a flare-up of arthritis. Just don’t do it! That’s what ice is for: soothing inflamed tissue.
And what’s a “fresh” injury? Any time tissue has been physically damaged, it will be inflamed for a few days, give or take. If superficial tissue is sensitive to touch, if the skin is hot and red, if there is swelling, these are all signs that your injury is still fresh, and should not be heated.5
If there’s no obvious/severe injury or infection, it’s okay to try a heat treatment. But it can be surprisingly unclear!
Sometimes it’s really tough to tell if pain is due to an injury. This is the puzzle at the centre of many chronic pain problems: the distinction between feeling damaged and being damaged. Several kinds of pain exist in a gray area between the two.
The best example is probably muscle strains versus muscle “knots.” A strain is an actual rip in the muscle — physical damage, while a “knot” or trigger point (TrP) is an irritable patch of tissue, probably a micro-spasm (more about this below). The difference between a minor strain and a bad trigger point can be subtle.6 And yet you want to heat the knot, not the strain! Fortunately, if the strain is mild enough that you can’t tell if it’s a strain, it’s probably not a big deal one way or another: just try some heat and see what happens.
But there are many situations where this kind of ambiguity is a challenge, like low back pain.7
If you suspect a muscle strain, but you’re not sure, here’s a good tutorial for helping you sort it out: Save Yourself from Muscle Strain! If back pain is your issue and you’re not sure what’s causing it, get thee to another tutorial: Save Yourself from Low Back Pain! And I have an article specifically devoted to helping people understand why heating back pain is usually better than icing it: (Almost) Never Use Ice on Low Back Pain!
The hot water bottle: The classic original source of therapeutic heat.
How does heat therapy work?
The next several sections explore different mechanisms and details of how heating might be helpful to people in pain:
- Heat is reassuring, and reassurance is analgesic. (This is applied neurology, not just a psychological effect.)
- Heat can penetrate a few centimetres into tissue, and cells and biochemistry speed up when the tissue temperature rises, which might have therapeutic implications. For instance …
- Heat may have an effect on the common painful phenomenon known as “trigger points.”
- Heat may help soreness after exercise.
- Tiger Balm and similar products are “spicy” not warm, but they might tinker usefully with sensation: a neurological distraction.
Heat is reassuring, and reassurance is analgesic
Our comfort zone is a warm place. And so, almost no matter what kind of pain you have …
To reduce pain, we need to reduce credible evidence of danger & increase credible evidence of safety.
~ Lorimer Moseley. Explainer: what is pain and what is happening when we feel it? TheConversation.com.
The brain probably interprets a safe source of warmth as good “credible evidence of safety,” for basic psychological reasons related to the environmental and social conditions we evolved in. Cold kills! Hypothermia has been at or near the top of the list of threats to our safety throughout all of prehistory and most of history.8 And we also associate warmth with contact and intimacy — another powerful goodness.
So a nice controlled source of warmth is probably just about the most basic reassuring thing there is. And that’s always good for pain.9 You might mistake this for a psychological effect, and it is in a sense, but it’s more useful to look at it as “applied neurology”: leveraging what we know about how pain neurology works. It’s more akin to triggering a reflex than a mind game.
Some like it hot
How hot is too hot? It depends on your tolerance. Some like it a lot hotter than others. Comic by Jake Likes Onions
Heating depth: how far does it go?
Roughly a degree celcius or two at a couple centimetres depth, give or take, depending on how and where it’s done.
Scientists have tested this. For instance, in 1998, Draper et al heated subjects triceps muscles with hot packs for fifteen minutes, and then checked their temperature with a needle probe — like a very thin meat thermometer (don’t worry, they were anaesthetized).10 They found an average increase of 3.8˚C at a depth of one centimetre, and .78˚ at three centimetres.
I think this data clearly shows that superficial heating is an easy way to modestly increase tissue temperature up to a couple centimetres — which is most of the volume of most muscles — for whatever that’s worth. The 3.8˚C increase in the muscle shallows is something, but .78˚ change at 3cm depth — and a lot of muscle is that deep — is clinically trivial, well within the range of healthy variations in core body temperature.
Heat for trigger points
A trigger point is a small patch of acutely sensitive soft tissue, a poorly understood little patch of sensory misery. They are probably contracted, stagnant, swampy11 sections of muscle tissue — but that’s just a theory.12
However they work, no one doubts that these sensitive spots in muscle are common. They can cause anything from stiffness and dull aching to show-stopping agony. The pain often spreads in confusing patterns, and they grow like weeds around other painful problems and injuries, making them interesting and tricky and incredibly common.
And heat seems to be a particularly good therapy for trigger points. Never formally tested! But plausible.
Pure speculation about why heat might be good for trigger points
Trigger points are probably aggravated by stress, and (as noted above) being warm is a pleasant and comforting sensation, as long as we aren’t overheated to begin with. But it probably goes beyond that …
Relaxation reduces resting muscle tone. You can have “tight” muscles without actually being in frank spasm. There are many degrees of muscle tone between deep relaxation and a charlie horse. Many otherwise healthy people live in a state of uncomfortably high muscle tone, their muscles always a little clenched and exhausted, probably with some specific areas even worse from awkward working postures. This state is inherently uncomfortable, like being tired from exercise — but without the endorphins — and it may be fertile ground for trigger points. If so, any reduction in muscle tone may be quite helpful.
Heat for soreness after exercise
Perhaps the most popular therapeutic reason to get into a hot tub is to try to beat the pain of DOMS (delayed-onset muscle soreness) — that nasty 24–28 hours of muscle pain you get after unfamiliar exercise. But DOMS is mostly invincible. Researchers have proven over and over again that there really is no effective treatment for it.
Except, maybe, heat?
For a long time, “warm underwater jet massage” — hot tub jets — was the only treatment that had ever shown any potential, in a 1995 study.13 But it was a small and flawed piece of research — and most people know from personal experience that a soak in a hot tub may “take the edge off,” but hardly constitutes a miracle cure for DOMS. I mostly ignored that evidence.
In 2006, we got some good science news: more persuasive data, surprisingly good results in treating DOMS in the low back with a “heat wrap,” a wearable device that applies heat for hours at a time.14 Another small study, but this time more persuasive one. Eureka? Proof needs more data, but this makes it well worth trying heat on your sore muscles after exercise. Happy heating!
Is Tiger Balm hot?
Ointments and balms like Tiger Balm15, RUB A535, and Deep Heat are not really hot, but they do have two things in common with heat — they feel hot, and they cause superficial capillaries to open up as wide as they can.
All these products contain a chemical irritant or rubefacient (and there’s your word-of-the-day). Rubefacients feel hot because they give you a mild chemical burn! Capsaicin is the most classic rubefacient, the active ingredient in most of these products, and it’s literally spicy: it’s the same stuff that makes chilli peppers spicy, on your tongue or your back?
So rubefaciants piss off your skin, in other words.
And that is the only sense in which a rubefacient is heating anything. The product name “Deep Heat” always makes me roll my eyes, because a more truthful name would be Shallow Irritation! “Chemicals that are mild skin irritants may make a patient feel warm,” wrote Borrell et al, “but they will not produce any in vivo temperature rises or any of the physiological effects of heat.”16 The effect is just too superficial.
And is a mild chemical burn helpful in any way? The main rationale for a rubefacient is that it’s a counter-irritant — a neurological distraction from your pain, which is the more general and well-known principle of diffuse noxious inhibitory control. Counter-irritation is a real thing, but it’s not a powerful thing. You could call it another tool for the pain treatment toolbox, but it’s not an impressive tool, maybe just like a small screwdriver or a tack hammer.
Or maybe a crowbar? Capsaicin has actually gotten a lot of high fives and thumps on the back from science.17 It seems likely that some people, for unclear reasons, respond to them much better than others, which boosts the worth-a-try factor.
Some of the effects of capsaicin are more exotic than expected, too — like actually making nerve endings shrivel and retreat.18 Interesting stuff.
Note that many rubefacients also have touted other medicinal and pain-killing ingredients and mechanisms. For instance, garlic is supposed to be good for all kinds of things, salicylates are definitely medicinal in some applications. But of course any such benefits are just bonus prizes that have nothing to do with heating or even rubefaciants, and it’s a rogue’s gallery of mostly unexciting possibilities.
Local heating versus systemic heating
There are many ways to heat yourself up, but two broad categories: local and systemic.
Local heating means specific heating: applying a hot water bottle, heating pad, heated gel pack or bean bag to a specific place on the body.
Systemic heating means raising the entire body temperature with a bath or jacuzzi, steam bath, or piping hot shower — basically creating an artificial fever!
For most local heating, I recommend seeking out a large-sized (14 × 27) Thermophore, the Rolls Royce of heating pads: large, heavy, thick, and moist. Yes, moist — they produce a moist heat with “a special tightly-woven fleece blend cover which retains moisture from the air.” If you leave a Thermophore on a plastic surface, there will be beads of water under it five minutes later. The moisture captured from the air by the Thermophore conducts heat far more effectively than a dry heating pad. Luxurious! Both genuine Thermophores (roughly USD $50-100) and cheaper knock-offs (and probably nearly as good) can be ordered online from Relaxus.com, and are also available in some medical supply stores. Of course, Amazon.com has them, too.
Full-body heating in hot tubs, showers, saunas and steam rooms is also often helpful factor with conditions where emotional stress and/or knots in your muscles are the cause of your troubles, or a significant complicating factor — low back pain is the most obvious example. See Hot Baths for Injury & Pain for tips on getting the most out of your bathtub or Jacuzzi. Believe it or not, many people do not really know how to take a bath! There are several simple tips that can definitely wring more therapeutic value out of the experience. (Adding Epsom salts to a bath is not one of them, however — it’s popular, but probably useless.24)
Warming with infrared radiation: infrared saunas, especially “far” infrared saunas
Red & infrared radiation on a hand in an infrared scanner. Photo by Yu Chieh Ho.
Infrared radiation is almost light — so close that it is often called infrared light, even though it is actually beyond the part of the electromagnetic spectrum that we can normally see. It has a longer wavelength than any visible light. And infrared is radiation is warming — it jostles our molecules — which is why it is also often called heat radiation.
Infrared saunas sound fancy, like they are emitting some kind of special radiation, not just heating rays but healing rays. In fact, an infrared heater is just a … heater. All heaters are infrared heaters, because all radiative heating is infrared. Calling it an “infrared” heater is kind of like calling a lamp a “visible radiation” lamp. If you were to put a space heater in a small cedar panelled room, you would have yourself an infrared sauna.
If any infrared radiation is a healing ray, it’s “far” infrared radiation (farther from visible light, the longest infrared wavelengths). More expensive saunas use far infrared, and it is a bit different. The main advantage of using far infrared is entirely practical: they require minimal shielding, because the heating elements themselves stay almost magically cool while still heating up whatever they paint with their radiation (like you). So they can be built into the walls of the sauna without scorching them (or you). And that’s quite useful.
Manufacturers and resellers universally tout the penetrating quality of far-infrared heating. Supposedly longer wavelengths “shine” right through superficial layers of tissue, heating deeper tissues directly, while sunlight heat is almost all in the near infrared that supposedly heats only the outer layer. (And we all know how unsatisfying warm sunshine is. 🙄)
In general, all infrared radiation penetrates tissue, just like visible light. But heat is heat, and those deeper tissues will get warm in a normal sauna too — it just takes longer. This is much like the difference between a microwave oven and a conventional oven. Either way, food gets heated all the way through.
But it’s specifically far infrared heating that is usually touted as being penetrative, and I have been unable to find a good source for this claim. The only widely cited source is a 2012 paper that makes the same claim but does not support it in any way,25 making it a classic example of a bogus citation. Even if it true, it is not actually supported by that reference.
Meanwhile, I have found precisely the opposite claim from other more credible experts, who say that far infrared is actually the least penetrative, and even shorter wavelengths don’t penetrate very far, only a few millimetres.26 (They don’t actually cite a source for their statement either, ugh, but the paper is much calmer and more trustworthy overall. If any of my readers can shed any light on this — a better source, perhaps — I’d be grateful.)
If that’s the best I can learn after a lot of searching, then the average sauna-shopper cannot possibly be expected to know what to believe. But for the sake of argument, let’s just say that far infrared is penetrative and then ask the really important science question here: so what if it does?
You want microwaves with that?
Far-infrared heating is, in fact, almost microwave heating. They are immediately adjacent to each other on the EM spectrum.
Healing rays: is far infrared radiation special?
The only reason to think that “penetrating” heat matters is the idea that there’s something about far infrared radiation that is actually helpful to our biology in some way. And while there is endless speculation and belief that effects like this exist — for instance, there are very closely related beliefs about lasers, infrasound, ultrasound, and electric current — even though most enthusiastic experts concede that we don’t actually know. For instance, Vatansever and Hamblin (cited above) conclude their over-excited review like this:
If it can be proved that non-heating FIR has real and significant biological effects, then the possible future applications are wide ranging.
If. If! I can think of an awful lot of things that would be wonderful if they could be proved! The fact is, no one really has any idea, because not nearly enough good quality research has been done on this topic.
And there’s a more practical problem: claims about far-infrared are generally paired with the idea that it has to be “pure” far-infrared, exclusively limited to a specific, narrow range of wavelengths. As with tanning beds, these heaters can be made well and deliver the type of radiation that was promised — for whatever it’s worth — but the consumer has no way of knowing. And these are expensive products. The potential for corruption and abuse is obvious, and of course regulation is effectively nonexistent.
Finally, a safety note: it’s not like a high-tech person-warming oven is completely risk-free.27 Pure thermal injury is possible on the mundane end. But there are more exotic risks, too: if you accept the possibility of positive biological effects, then you have to embrace the possibility of negative ones too. Amateur and overexcited biophysicists often forget this.
Did you find this article useful? Please support independent science journalism with a donation. See the donation page for more information and options.
About Paul Ingraham
I am a science writer, former massage therapist, and I was the assistant editor at ScienceBasedMedicine.org for several years. I have had my share of injuries and pain challenges as a runner and ultimate player. My wife and I live in downtown Vancouver, Canada. See my full bio and qualifications, or my blog, Writerly. You might run into me on Facebook or Twitter.
What’s new in this article?
March — Added substantial section about infrared heating.
2016 — Added evidence of efficacy of NSAIDs for common injuries.
— A series of substantial upgrades, details unlogged.
2007 — Publication.
- Garra G, Singer AJ, Leno R, et al. Heat or cold packs for neck and back strain: a randomized controlled trial of efficacy. Acad Emerg Med. 2010 May;17(5):484–9. PubMed #20536800. ❐ This 2010 study showed quite clearly that both ice packs and hot packs were only a little bit beneficial for neck and back pain, and about equally so. But a small therapeutic effect is still valuable, and there are almost certainly situations where it works even better — after all, these were people with acute pain bad enough that they went to the hospital. They may have been a bit beyond the help of a hot pack! ⤻
- Ingraham. Masking Symptoms Is Under-Rated: Masking symptoms,” especially with medications, is often maligned. But sometimes symptoms need masking! ❐ PainScience.com. 803 words. “Masking symptoms,” especially with medications, is often maligned because it doesn’t “treat the root cause.” But masking symptoms can be a good idea, and it should not be eschewed just because it doesn’t have a real healing effect … because there are very few real healing effects! “Healing” is mainly about removing impediments to natural recovery, like stress on tissues. It’s not dictated by some mythical power to speed healing, but by a strong understanding of the nature of the problem and what pisses it off and impedes recovery. Focus on facilitating natural recovery, and don’t knock a little “symptom relief” along the way. ⤻
Muscle tension can cause a surprising amount of pain. A leg or foot cramp is a common and extreme example, where no one has any doubt of the cause of pain. But imagine a muscle spasm much less strong, but lasting for days and days — or years! Although superficially a simple concept, there are actually several physiological mechanisms by which muscle can become shortened and painful for a long time, some well understood and straightforward (spasticity from neurological diseases), while others are quite mysterious (like spasms peole are born with, as in torticollis or wry neck).
One excellent example of neuropathic spasticity is the “MS hug”: a symptom of multiple sclerosis that feels like a painfully tight band around the chest, often experienced long before diagnosis. Although the feeling of constriction is the classic symptom, many patients also just experience widespread pain in the chest wall.⤻
- Several drugs make muscles hurt, but the most notorious are the statins (cholesterol-reducing drugs) and bisphosphonates (for osteoporosis). In some people, these act like muscle poisons. ⤻
- Here’s an example of what can happen when you heat an inflamed injury: When I was still in school, and my father had not yet learned to call me before asking a doctor about his aches and pains, he went to a drop-in clinic following a traumatic knee injury. The physician on duty prescribed heat! That was wrong, and the results spoke loud and clear: my dad’s knee swelled dramatically, outrageously, causing severe pain and immobility. ⤻
- Some rips aren’t as sudden and obvious as you might expect, and trigger points can flare up faster than you expect. They also probably occur together: a TrP may be why the muscle tore in the first place, or it could crop up in the aftermath, or it could simply feel like a tear. ⤻
- In low back pain — infamous for being frustratingly chronic — strains and trigger points are both common, but there are also many other potential sources of pain. So it’s much more difficult to know if heat is a good idea than it is with, say, thigh pain. ⤻
- Obviously heat kills too, but much, much less frequently. Humans can survive almost any amount of heat as long as we can get some shade and water and don’t have to overexert ourselves. Cold kills relatively effortlessly! ⤻
- Ingraham. Pain is Weird: Pain science reveals a volatile, misleading sensation that is often more than just a symptom, and sometimes worse than whatever started it. ❐ PainScience.com. 13200 words. Modern pain science shows that pain is a volatile, complex sensation that is thoroughly tuned by the brain and often overprotectively exaggerated, so much so that sensitization often becomes more serious and chronic than the original problem. All of this has complicated “all in your head” implications: if the brain controls all pain, does that mean that we can think pain away? Probably not, but we do have some “neurological leverage” of great value — we can influence pain, if we understand it. ⤻
- Draper DO, Harris ST, Schulthies S, et al. Hot-Pack and 1-MHz Ultrasound Treatments Have an Additive Effect on Muscle Temperature Increase. J Athl Train. 1998 Jan;33(1):21–4. PubMed #16558479. ❐ PainSci #54111. ❐ ⤻
- Shah JP, Danoff JV, Desai MJ, et al. Biochemicals associated with pain and inflammation are elevated in sites near to and remote from active myofascial trigger points. Arch Phys Med Rehabil. 2008;89(1):16–23. PubMed #18164325. ❐
This significant paper demonstrates that the biochemical milieu of trigger points is acidic and contains many pain-causing metabolites. For much more information about this, see Toxic Muscle Knots.⤻
- “Trigger points” are controversial, although published expert opinions criticizing the concept remain relatively scarce. This article does not get into the controversy, but it’s important to acknowledge it. The bottom line is that people do suffer from a painful phenomenon (very sensitive spots), and no one disputes that — they just dispute what to call it, how it works, and how to treat it! If you want to learn more about the controversies, see: Trigger Point Doubts: Do muscle knots exist? Exploring controversies about the existence and nature of so-called “trigger points” and myofascial pain syndrome ⤻
- Viitasalo JT, Niemela K, Kaappola R, et al. Warm underwater water-jet massage improves recovery from intense physical exercise. Eur J Appl Physiol Occup Physiol. 1995;71(5):431–8. PubMed #8565975. ❐ ⤻
- Mayer JM, Mooney V, Matheson LN, et al. Continuous low-level heat wrap therapy for the prevention and early phase treatment of delayed-onset muscle soreness of the low back: a randomized controlled trial. Arch Phys Med Rehabil. 2006 Oct;87(10):1310–7. PubMed #17023239. ❐ ⤻
- Ironically, tigers really hate Tiger Balm. ⤻
- Borrell RM, Parker R, Henley EJ, Masley D, Repinecz M. Comparison of in vivo temperatures produced by hydrotherapy, paraffin wax treatment, and Fluidotherapy. Phys Ther. 1980 Oct;60(10):1273–6. PubMed #7443789. ❐ ⤻
- Derry S, Sven-Rice A, Cole P, Tan T, Moore RA. Topical capsaicin (high concentration) for chronic neuropathic pain in adults. Cochrane Database Syst Rev. 2013;2:CD007393. PubMed #23450576. ❐ ⤻
- Kennedy WR, Vanhove GF, Lu SP, et al. A Randomized, Controlled, Open-Label Study of the Long-Term Effects of NGX-4010, a High-Concentration Capsaicin Patch, on Epidermal Nerve Fiber Density and Sensory Function in Healthy Volunteers. J Pain. 2010 Jun;11(6):579–587. PubMed #20400377. ❐ ⤻
- There are four kinds: acetaminophen/paracetamol (Tylenol, Panadol), plus three non-steroidal anti-inflammatories (NSAIDs): aspirin (Bayer, Bufferin), ibuprofen (Advil, Motrin), and naproxen (Aleve, Naprosyn).
- Science Based Pharmacy [Internet]. Gavura S. How risky are NSAIDS?; 2015 Jul 25 [cited 16 Aug 18]. ⤻
- Hung KK, Graham CA, Lo RS, et al. Oral paracetamol and/or ibuprofen for treating pain after soft tissue injuries: Single centre double-blind, randomised controlled clinical trial. PLoS One. 2018;13(2):e0192043. PubMed #29408866. ❐
In a test of ibuprofen vs paracetamol for hundreds of soft tissue injuries, there was no significant difference in the modest benefits or side effects, contrary to some past evidence and the widely held medical belief that ibuprofen is better for injuries (inflammation). It’s possible that placebo is powering the effects of both medications, and that's why they were equal, but it's impossible to know without a control group for this study. Note that the superiority of these medications is probably variable and not as clearly established as you might think.⤻
- FDA.gov [Internet]. Acetaminophen and Liver Injury: Q & A for Consumers; 2009 Jun 4 [cited 16 Aug 31].
“This drug is generally considered safe when used according to the directions on its labeling. But taking more than the recommended amount can cause liver damage, ranging from abnormalities in liver function blood tests, to acute liver failure, and even death.”⤻
- Machado GC, Maher CG, Ferreira PH, et al. Efficacy and safety of paracetamol for spinal pain and osteoarthritis: systematic review and meta-analysis of randomised placebo controlled trials. BMJ. 2015;350:h1225. PubMed #25828856. ❐ PainSci #54220. ❐ ⤻
- Epsom salt in your bath is cheap and harmless and it makes the water feel “silkier,” but it’s unlikely that it has a therapeutic effect on aches and pains. Magnesium supplementation might be helpful for some patients with some kinds of pain, but not many others, and it’s doubtful that it can soak through the skin, and it definitely doesn’t “detox” anything. The soothing heat of a nice bath is probably the main source of health benefits. The case for the healing powers of Epsom salt is mostly made by people selling the stuff, or recommending it as casually and imprecisely as an old wives’ tale. See Does Epsom Salt Work? The science of Epsom salt bathing for recovery from muscle pain, soreness, or injury. ⤻
- Vatansever F, Hamblin MR. Far infrared radiation (FIR): its biological effects and medical applications. Photonics Lasers Med. 2012 Nov;4:255–266. PubMed #23833705. ❐ PainSci #52578. ❐
This is a lousy paper. The authors clearly have rose-coloured glasses on when it comes to infrared radiation. They repeat the assertion that far infrared radiation can “penetrate up to 1.5 inches (almost 4 cm) beneath the skin”… but, appallingly, provides no additional detail or source, even though it’s a very important point. And yet it appears that their statement has been taken as gospel by most everyone in the infrared sauna business. This is a familiar scenario, and there have been quite a few cases where publication of a paper like this was arranged by an industry specifically so that it can be cited in marketing materials. Just sayin’.⤻
- International Commission on Non-Ionizing Radiation Protection. ICNIRP statement on far infrared radiation exposure. Health Phys. 2006 Dec;91(6):630–45. PubMed #17099407. ❐
The three infrared spectral bands roughly distinguish between different penetration depths into tissue, which are strongly dependent upon water absorp- tion. IR-A radiation [near infrared] penetrates several millimeters into tissue. IR-B penetrates less than 1 mm, and at the penetration depth (1/e) is least (approximately 1 m) at wavelengths near 3 m, where water has its highest absorption peak. IR-C [far infrared] does not penetrate beyond the uppermost layer of the dead skin cells, the stratum corneum.