Water-filtered infrared-A (wIRA) is a special form of heat radiation in the range 780-1400 nm which is used in clinical medicine for prevention and therapy because of its very good tolerability.[Hoffmann et al., 2016] [Piazena et al. 2014a] [Marti et al., 2014][Hoffmann , 2007]
In moderate climatic zones, in contrast to desert regions, the thermal radiation from the sun is perceived at the Earth’s surface as being pleasantly warm, causing no stinging or burning sensations in the skin due to the filtering of the radiation by water and water vapour of the atmosphere.[Hoffmann et al., 2016][Cobarg, 1995] The filter effect of water decreases those parts of infrared radiation (most parts of infrared-B and -C and the absorption bands of water within infrared-A), which would otherwise, by reacting with water molecules in the skin, cause an undesired thermal load to the surface of the skin (Figure).[Hoffmann et al., 2016][Piazena, 2015][Hoffmann, 2009][Stofft and Vaupel, 1996]
Technically, water-filtered infrared-A is produced by special radiators. Typically, the complete non-coherent and non-polarized broadband radiation of a 3000 Kelvin halogen bulb is passed through a cuvette containing water, which absorbs or decreases the described undesired wavelengths of the infrared radiation (Figure).[Hoffmann, 2016] The remaining wIRA radiation (in the range 780–1400 nm) has a high penetration capacity in tissue so that in comparison to conventional unfiltered infrared radiation a considerably higher amount of energy can be transferred deeply into the tissue while the thermal load to the skin surface remains low.[Piazena et al., 2014b][ Piazena and Kelleher, 2010]. Thermography shows different skin surface temperature with the same total irradiance: a water-filtered infrared-A radiator causes a lower skin surface temperature than conventional infrared radiators without water-filter.[Hoffmann, 2009] With equal skin surface temperature the total irradiance of infrared-A of a water-filtered infrared-A radiator is nearly 4–9-fold compared to conventional infrared radiators without water-filter. For certain clinically relevant wavelengths, such as 820 nm,[Karu et al. 2001a, 2001b, 2001c] the irradiance can be even greater (approximately 6–30-fold, see Figure).[Piazena et al., 2012] [Piazena, 2015].
wIRA increases tissue temperature, oxygen partial pressure and perfusion markedly.[Hoffmann et al., 2016][Al-Ahmad et al., 2015] [Hartel und Hoffmann, 2006][Mercer and de Weerd, 2005][Hellige et al. 1995]
The five main clinical effects of wIRA are: wIRA decreases pain, inflammation and exudation/hypersecretion, and promotes infection defense and regeneration, all in a cross-indication manner.[Hoffmann et al., 2017a][Walz et al., 2013] Therefore there is a wide range of indications for wIRA:
wIRA can be used i. a. for the promotion of healing of acute and chronic wounds (even an undisturbed “normal” wound healing can be improved: faster, with less pain and better cosmesis),[Aljasir et al., 2018][Zöller et al., 2016][Hoffmann et al., 2016][Däschlein et al., 2014][Künzli et al., 2013][Schumann et al., 2011][Mercer et al., 2008][Hartel et al., 2007][von Felbert et al., 2008] for the treatment of acute and chronic infections, in skin diseases (e. g. common warts,[Marini et al., 2006][Fuchs et al., 2004] Herpes simplex labialis, Herpes zoster, sclerodermia,[von Felbert et al., 2011][Foerster et al., 2009] acne papulopustulosa, actinic keratosis within a photodynamic therapy[Giehl et al., 2014][von Felbert et al., 2010][Hübner, 2005]), for the resorption improvement of topically applied substances,[Otberg et al., 2008] in acute and chronic musculoskeletal disorders (e. g. rheumatism[Hanusch et al., 2011] , arthrosis[Merle, 1995], arthritis[Lange et al., 2014], spondyloarthritis,[Lange et al., 2017][Zauner et al., 2014][Lange, 2012][Falkenbach et al., 1996] low back pain, lumbago, fibromyalgia[Brockow et al., 2007]), for regeneration after sports,[Hoffmann, 2002] in combination with ergometer training[Möckel et al., 2006] in complex regional pain syndrome (CRPS), in polyneuropathies, for keeping or increasing body temperature in neonatology[Singer et al., 2000] or postoperatively[Bräuer et al., 2004][Hellige et al., 1995] for compensation of a hypothermia.[Hoffmann et al., 2016][Notter et al., 2016]
In psychiatry, wIRA can be applied to stabilize the mental condition in patients with major mental disorders (depressions).[Hale et al. 2017][Janssen et al., 2016][Raison et al., 2015][Hanusch et al., 2013]
Due to the antimicrobial effect of wIRA this type of therapy can be used not only to improve wound healing but also to reduce oral bacteria which can lead for example to paradontitis, periimplantitis or root canal infections[Al-Ahmad et al., 2016][Al-Ahmad et al., 2015][Karygianni et al., 2014][Al-Ahmad et al., 2013] and wIRA can reduce chlamydial infections as well which can cause trachoma or infertility.[Kuratli et al., 2018][Rahn et al., 2016][Marti et al., 2015][Marti et al., 2014]
In addition wIRA can be used as local or systemic hyperthermia (whole body hyperthermia) in combination with photosensitizer (photodynamic therapy, PDT), with radiation therapy[Vaupel et al., 2018][Rich et al., 2016][Notter et al., 2016][Notter, 2014][Notter et al., 2011][Seegenschmidt et al., 1995][von Lieven and Dapper, 1994][Seegenschmidt, 1992] and chemotherapy in oncology.[Hoffmann, 2017b][Wehner et al., 2001][Wust et al., 2000]
The effects of wIRA are based on both its thermal effects (relying on transfer of heat energy) and thermic effects (temperature-dependent effects, occurring together with temperature changes) as well as on non-thermal and temperature-independent effects like direct effects on cells, cell structures and cell substances.[Aljasir et al., 2018][Gebbers et al., 2007][Fuchs et al., 2004]
Decrease of pain and inflammation and promotion of infection defense and regeneration can be explained both by thermal and non-thermal effects.[Hoffmann, 2017b] Concerning decrease of pain by wIRA an increased perfusion allows a better elimination of accumulated metabolites, as pain mediators, lactate, or bacterial toxins, and increases – together with an increased tissue temperature – metabolism (improved metabolisation of accumulated substances and improved regeneration); non-thermal effects include direct effects on cells and cellular structures and substances and perhaps as well on nociceptors;[Hartel et al., 2006] in addition wIRA relaxes muscles and decreases pain by this as well. Thermal effects include an increased energy production – which is decisive for a variety of processes including regeneration – by higher temperature and higher oxygen partial pressure.[Hartel et al., 2006]
A decrease of exudation/hypersecretion by wIRA can be explained by non-thermal effects.[Hoffmann, 2017b]
Concerning non-thermal effects the energy-rich wavelengths near to visible light – approximately 780–1000 nm (800–900 nm,[Albrecht-Bühler, 2005] 800 nm,[Ehrlicher et al., 2002] 820 nm,[Karu et al., 2001c] 830 nm[Chow et al., 2006]) – seem to be the clinically most important part of wIRA.[Winkel et al., 2014]
Non-thermal effects include an influence on the cytochrome C oxidase in the mitochondria: cytochrome C oxidase is known as universal photo acceptor for radiation of approximately 600–1000 nm with absorption maxima at 620, 680, 760 and 825 nm.[Karu, 2010][Karu, 2008] By absorption of radiation the cytochrome C oxidase can induce signalling cascades and therefore has regulatory function far beyond energy production,[Karu 2010][Karu, 2008][Karu, 2001c]
wIRA in therapeutic irradiation intensities and doses has been shown not only to be harmless to human skin, but also to have cytoprotective effects. [Piazena et al., 2014a][Jung et al., 2010][Jantschitsch et al., 2009][Gebbers et al., 2007][Frank et al., 2006][Burri et al, 2004][Applegate et al., 2000][Danno et al., 2001][Menezes et al., 1998][Danno and Horio, 1992] These aspects have been extensively discussed in the references[Piazena and Kelleher, 2010].
wIRA can be used in veterinary medicine as well.[Schabel, 2009]