Severe discomfort or even drowning incident may lurk due to potential hypothermia or hyperthermia during prolonged exposure to noxious water temperatures. Therefore, particular emphasis should be given to thermal perception, as it can predict the thermal load that the human body receives when exposed to various climatic conditions. However, there is no thermal sensation model for water immersion as a "gold standard." This scoping review aims to highlight the empirical thermal assessment from olden times and propose a simple, cost-effective thermal perception index during whole-body water immersion in healthy individuals. A standard literary search is performed on PubMed and Google Scholar. The words “Immersion,” “Water,” “Bath” are used either as solely searched terms and MeSH terms or in combination with the text words thermoregulatory response,” “thermal perception,” “cold water,” “warm water,” “hot water,” “thermoneutral water,” “skin temperature,” and “body temperature.” Twelve published articles fulfill the review inclusion/exclusion criteria. The body literature maps the guidelines for the classification of water temperature, primarily used in various related health science disciplines. It investigates further the multifaceted nature of thermal perception. Also, we mention the thermoregulatory responses to identify the frame in which a theoretical anthropocentric thermal model could be developed. This scoping review's most important message is that our six-point thermal sensation scale we developed with the following specific water temperature ranges: icy <12oC, cold 12-24oC, cool 24-29oC, neutral 29-38oC, warm 38-43oC, and hot >43oC, may become a potential universal use for aquatic leisure activities because of being based on human neurophysiology and thermoregulation. Each selected range of temperature has been chosen depending on how a subject expressed his own thermal sensation after a 10 min water immersion and with what is known about the activation of transient receptor potential ion channels (TRP). The common reaction between icy and hot zone is the activation of nociceptive thermoreceptors, such as TRPA1 and TRPV1, causing unbearable pain that steers to the inability to complete a prolonged water immersion. Both cold and warm zone alters the core temperature and other cardiovascular factors significantly, mainly due to TRPM8’s action for cold and TRPV4, TRPM2’s action for warm stimuli. None other than shivering thermogenesis is observed in the cool zone - the function of TRPM8 in synergy with TRPC5 may be responsible for attenuated thermoregulatory responses. In terms of TRPs, the complex neutral zone is a dynamic intermediate state not presenting shivering thermogenesis, sweating activity, and changes in core temperature. Undoubtedly the classification of water temperature assists in charting the human physiological responses. Our findings enlighten the significance of thermal sensation as an indicator for health and aquatic safety and as a principal factor for establishing a behavioral thermal model applicable for water immersion.
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Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών
