Based upon anecdotal observations, assessment of worker hydration status through field saliva testing has become more prevalent over recent years. This article addresses the key questions regarding this method, research findings and observations/feedback from the field. Let’s start with what’s being measured - what are saliva hydration test scores? Saliva hydration test scores The concentration of dissolved particles in saliva for a given sample volume (osmolarity) or given sample mass (osmolality) is what’s being measured, with osmolarity and osmolality used interchangeably. In basic terms, a lower concentration of particles in the saliva sample (lower score) reflects a greater fraction of fluid in saliva and therefore, a more “hydrated" sample. Figure 1.  A portable osmometer (MX3 Diagnostics) Variability of saliva hydration test scores Saliva as a potential marker of hydration status is not new (Friedberg and Doyle, 1959). Yet, it has been plagued by variability and limitations associated with laboratory-based analysis. The commercial release of the portable MX3 Hydration Testing System in 2018 (Figure 1) addressed the need for laboratory testing, however, reports of variability persist. Here’s a summary of the research findings regarding saliva as a marker of hydration: “Given the inherent variability and profound effect of oral intake, use of saliva osmolality as a marker of hydration status is d ubio us ” ( Ely et al., 2011 ). "…… its (saliva osmolality) large intra- and inter- individual variability limited its predictive power and sensitivity, rendering its utility questionable within a universal dehydration monitor” ( Taylor et al., 2012 ). “Saliva might be an effective index to evaluate hydration status but seems to be highly variable” ( Villiger et al., 2018 ) . “The diagnostic utility of saliva osmolality is affected by oral artefacts such as recent fluid consumption and factors influencing saliva flow rate which include neural control and inherent inter-individual variability. It is therefore of limited value in the assessment of hydration status” ( Lacey et al., 2019 ). "Considering the poor reliability and large number of confounding factors associated with salivary variables, the use of this technique is questionable” ( Barley et al., 2020 ). And finally, the following is from research assessing the reliability of the MX3 Hydration Testing System: “………. the categorization of hydration status according to SOSM (Saliva Osmolarity) from the MX3 HTS (Hydration Testing System) should be interpreted with caution given inter- and intra-individual variation in baseline saliva SOSM (Saliva Osmolarity) and potential interference from recent food, drink, gum, or tobacco consumption” ( Winter et al., 2024 ). While, worksite staff that use saliva as a hydration marker via the MX3 system may not be aware of the aforementioned quotes and associated research, in most cases, variability of test scores are not new to them. For example, Figure 2 represents back-to-back tests (two tests performed within ~40 seconds on a worker, no fluid consumption in the 5 mins prior or in-between tests) with the same MX3 osmometer in a climate-controlled clinic.  Figure 2.  Saliva osmolarity values from two tests performed within ~40 seconds on a worker, no fluid consumption in the 5 mins prior or in-between tests, with the same MX3 osmometer in a climate-controlled clinic Test 1, 115 milliosmoles (mOsm), test 2, 63 mOsm. Is this worker considered moderately dehydrated or hydrated? How are these scores interpreted? The paramedic that performed these tests stated they would routinely perform three or four tests in quick succession on a given worker and calculate an average of the scores to account for the observed variability. It’s this variability that has led some organisations with large field-based workforces to discourage or even ban the use of saliva hydration testing on their sites. So, what’s contributing to this variability? Saliva collection in the laboratory and the field Laboratory based studies utilise desktop osmometers that generally test 20 microlitre samples. That’s a sample of just 1/50 th  of 1mL. Prior to the availability of a portable osmometer, saliva had generally been collected via a swab placed under the tongue for two minutes (salivette method) or by accumulating saliva in the mouth for two minutes prior to expelling (spitting) into a tube (expectoration method). Both the salivette and expectoration sampling methods produce adequate saliva to sample the 20 microlitres from and they also produce comparable results ( Ely et al., 2014 ). Field testing is achieved with a portable, handheld osmometer that does not require expectoration or a salivette. The MX3 Hydration Testing System tests a 1 microlitre sample or just 1/1000 th  of 1mL of saliva sampled directly from the tongue with those being tested required to swallow all saliva and generate a fresh sample before each measurement, as per the MX3 manual (MX3 Hydration Testing System User Manual, 2022). It’s likely that the minute saliva sample and differences in the generation of the saliva sample account for some of the reported and observed variability. Other factors that influence saliva hydration testing Controlling for recent fluid consumption is a key concern for accuracy as determined through research conducted by the U.S. Army Research Institute of Environmental Medicine ( Ely et al., 2011 ). Saliva testing with a laboratory-based desktop osmometer one minute following a brief mouth rinse with water, produced a substantial decrease in test scores (~35mOsm) without any change in actual hydration status. The next test at 15 minutes post mouth rinse produced scores similar to the pre-mouth rinse value (see Figure 3), providing some guidance regarding the time course of fluid consumption impact on test scores.  Figure 3.  Mean saliva osmolality values (error bars represent standard deviation) prior to and at 1- and 15-minutes post brief mouth rinse with water. Hydration classifications according to MX3 Diagnostics The impact of a mouth rinse or consumption of a small volume of water on saliva hydration testing is an important point. And based upon our observations and site discussions, a point that is understood and used by workers to improve their saliva hydration scores. When informed of the impact of recent fluid consumption or mouth rinsing, a worksite staff member that routinely performs field-based saliva hydration testing posed the question “You’re not going to tell the workers how to game the system, are you?” The answer was “There’s no need to, they are already aware”. Given the frequency of testing, it should be no surprise that workers understand what factors contribute to acceptable test scores, including a quick sip of fluid ahead of their testing. How reliable are MX3 HTS saliva osmolarity scores? Reliability is basically a measure of consistency, or in other words, the degree to which you trust the score. Higher reliability translates to greater trust. The sole published study of MX3 HTS reliability assessed SOsm for 75 individuals via two tests conducted within five minutes of each other ( Winter et al., 2024 ). Importantly, tests were conducted indoors with participants providing a fresh saliva sample according to manufacturer instructions. In this regard, tests were more controlled than our observations of those collected in the field, likely resulting in conservative estimates of SOsm reliability when measured on worksites. Figure 4 plots the mean (average) of the two test scores and the difference between those scores. Despite the controlled testing, the researchers state that “it is with 90% confidence that any repeat measurements that fall within ± 21.3mOsm of an initial measurement may not represent a real change in SOsm". For example, if a workers pre-shift test score was 59mOsm, a score of 81 or greater, or 37 or smaller is required for you to be 90% confident that a true change in test score has occurred. But, the kicker here is that a similar reliability study needs to be conducted in the field using worksite test collection procedures. With less testing rigour, it’s highly likely that test reliability, and therefore, trust of test scores would worsen from that reported by Winter et al. (2024) . Figure 4. Difference in MX3 HTS SOsm test scores 1 and 2 plotted by the average of the two measurements. The thin horizontal lines represent ± 21.3mOsm ( Winter et al., 2024 ) Does it really matter if test scores are not consistent? From a scientific viewpoint, yes, it does. A lack of trust in a test result spawns the question, “Why measure it?” Based upon our observations, there appears to be two broad organisational groups performing saliva hydration testing. One group seem to have implemented saliva hydration testing as a means to remind workers to consider their hydration status. Whereas the other group seem to be using saliva testing as a key component of their heat stress management, with specific actions classified against MX3 HTS scores. Back to the original question “Does it really matter if test scores are not consistent?”, the answer is dependant upon which group an organisation falls into. For those using MX3 HTS scores to determine heat stress and in some cases fitness for work in the heat, a lack of test score reliability has greater implication than for those using it as a means to discuss fluid consumption with workers. Using a measure of hydration status as a surrogate of heat stress or heat strain is not supported by the evidence. We consider that approach an abuse of hydration testing, regardless of the measure. Many factors need to be considered in this regard, including work rate (body heat production) and environmental conditions (determine potential for body heat exchange with the environment), for example ( Vega-Arroyo et al., 2019 ). What do saliva hydration scores actually mean? We’ve received some feedback regarding a lack of context for SOsm scores, particularly for organisations that have historically measured urine specific gravity (USG) to estimate worker hydration status. Changes in body mass are considered a useful reference to provide context for other measures of hydration. So, below we provide SOsm and USG referenced against change in body mass from work by the US Army Research Institute of Environmental Medicine ( Cheuvront et al., 2010 ). To understand these graphs (Figures 5 and 6), the term euhydration refers to the hydration state where you’re neither hyperhydrated or dehydrated. We’ve heard euhydration referred to as “normal” hydration, whatever that means. For the results depicted in the graphs below, euhydration was achieved by habitual food intake and consuming 2L of sports drink from 1800 to 2200 on the evening prior to testing. No food or fluid was consumed from 2200 until after the 0630 testing of hydration markers was complete the following morning. Dehydration was achieved through 3-5h of physical activity in hot and dry conditions (40ºC / 20% RH). Figure 5.  Individual and mean values of saliva osmolality, plotted as a function of change in percentage body mass when euhydrated (orange circles) and dehydrated (red circles) ( Cheuvront et al., 2010 ). Figure 6.  Individual and mean values of urine specific gravity, plotted as a function of change in percentage body mass when euhydrated (orange circles) and dehydrated (red circles) ( Cheuvront et al., 2010 ). What’s all this mean for worker hydration status assessment? Firstly, given the inherent variability of SOsm and that the MX3 HTS utilises a minute volume of saliva for analysis, rigorous collection procedures are necessary to maximise reliability. This includes at least a 5-min period between fluid/food consumption, smoking/vaping etc and SOsm measurement. Where rigorous sample collection is not practical nor considered worthwhile, the value of saliva hydration status testing must be questioned.  Where the above is achieved, the results require careful interpretation. Variability in test scores between individuals has been a key theme throughout the saliva osmolality research. Test scores are not suitable to be compared between individuals and large changes (greater than ±21mOsm) are needed before you can trust an actual change has occurred in controlled settings. Research addressing the reliability of MX3 HTS as used on worksites is warranted. As is research on other saliva based measures of hydration, including Salhy (Figure 7) prior to their commercial release to permit organisations to make informed decisions regarding their use.   Figure 7.  Salhy Personal Hydration Test (www.salhy.com.au) References Barley OR, Chapman DW, Abbiss CR. Reviewing the current methods of assessing hydration in athletes. J Int Soc Sports Nutr. 2020;17(1):52. Cheuvront SN, Ely BR, Kenefick RW, Sawka MN. Biological variation and diagnostic accuracy of dehydration assessment markers. Am J Clin Nutr. 2010;92(3):565-573. Ely BR, Cheuvront SN, Kenefick RW, Sawka MN. Limitations of salivary osmolality as a marker of hydration status. Med Sci Sports Exerc. 2011;43(6):1080-1084. Ely BR, Cheuvront SN, Kenefick RW, Spitz MG, Heavens KR, Walsh NP, Sawka MN. Assessment of extracellular dehydration using saliva osmolality. Eur J Appl Physiol. 2014;114(1):85-92 Friedberg SJ, Doyle EM. Osmotic pressure of saliva. Clin Research. 1959;7:150. Lacey J, Corbett J, Forni L, Hooper L, Hughes F, Minto G, Moss C, Price S, Whyte G, Woodcock T, Mythen M, Montgomery H. A multidisciplinary consensus on dehydration: definitions, diagnostic methods and clinical implications. Ann Med. 2019;51(3-4):232-251. MX3 Hydration Testing System User Manual. 2022. Accessed 2 October 2025. Available at https://mx3diagnostics.com/wp-content/uploads/2024/05/MX3ManualLayout_Jan-2022.pdf . Taylor NA, van den Heuvel AM, Kerry P, McGhee S, Peoples GE, Brown MA, Patterson MJ. Observations on saliva osmolality during progressive dehydration and partial rehydration. Eur J Appl Physiol. 2012;112(9):3227-3237. Vega-Arroyo AJ, Mitchell DC, Castro JR, Armitage TL, Tancredi DJ, Bennett DH, Schenker MB. Impacts of weather, work rate, hydration, and clothing in heat-related illness in California farmworkers. Am J Ind Med. 2019;62(12):1038-1046. Villiger M, Stoop R, Vetsch T, Hohenauer E, Pini M, Clarys P, Pereira F, Clijsen R. Evaluation and review of body fluids saliva, sweat and tear compared to biochemical hydration assessment markers within blood and urine. Eur J Clin Nutr. 2018;72(1):69-76. Winter I, Burdin J, Wilson PB. Reliability and minimal detectable change of the MX3 hydration testing system. PLoS One. 2024;19(11):e0313320.

Figure 1.  Forearm immersion by basic trainees at Fort Jackson, Columbia, South Carolina (Credit - Robert Timmons/U.S. Army) Immersion of the forearms (Figure 1) is a cooling technique used predominantly in military and firefighting contexts. In conjunction with Dr Anthony Walker, we reviewed the core temperature cooling rates of forearm immersion, publishing our results in 2015. From the 12 studies reviewed, 10 - 24.9ºC (50 - 77ºF) forearm immersion yielded core temperature reductions of only 0.1 - 0.5ºC per 10 minutes, classifying this method as unacceptable (Table 1). We proposed the core temperature cooling rate classifications of Table 1 for time-limited firefighting operations, but they also apply to occupational settings where rest breaks are brief (up to 15 minutes) for commercial viability. Table 1. Proposed rates of cooling for firefighting operations to allow for the rapid safe re-entry of firefighters to emergency incidents Classification Core Temperature Cooling Rate Ideal >1.0ºC/10 mins Acceptable 0.7-1.0ºC/10 mins Unacceptable <0.7ºC/10 mins In 2016, a study by Yeargin and colleagues  achieved a cooling rate of 1ºC per 10 minutes for 5ºC (41ºF) forearm immersion. Such a cooling rate significantly exceeded prior findings as it represented the first trial of water temperatures below 10ºC. So, is forearm immersion a viable workplace heat stress control? The answer depends on the immersion water temperature, tasks to be performed after cooling and logistical constraints. Water Temperature In addition to the 12 aforementioned studies reviewed in 2015, research continues to confirm the slow cooling rates for 10ºC or warmer forearm immersion ( Nakamura et al., 2020 ; Iwahashi et al., 2023 ; Caballero et al., 2026 ). From these 15 studies, a total of 180 participants immersed their forearms in mean 15.0ºC (59ºF) water to lower their core temperature a mean of 0.35ºC per 10 minutes. It's probable that h igher pre-treatment core temperatures would improve reported cooling rates ( Brearley et al., 2023 ), but they are unlikely to approach acceptable cooling rates. Task Performance Evidence supporting forearm immersion is limited to one study that utilised 5ºC water Yeargin et al., 2016 ). To achieve a core temperature reduction of ~1ºC during a rest break, the required duration of 5ºC forearm immersion water is 10 minutes. In that time, workers may report numbness and discomfort of the hands and forearms while experiencing diminished manual dexterity ( Cheung et al., 2003 ) and ability to generate force ( Mallette et al., 2018 ) upon return to work. Logistics Provision of ample 5ºC water for cooling a workforce may require substantial ice or a reliable power source where water chillers are used. Given that water immersion facilities are likely to be shared (Figure 1), water sanitation requires consideration for repeated use and/or large workforces. Ice cold towels are an effective substitute for cold water immersion when treating exertional heat stroke in resource limited settings ( Rogerson and Brearley, 2024 ), however, they do not produce rapid cooling when applied to the forearms. Adams and colleagues (2021) reported cooling of just ~0.22ºC per 10 minutes when ice cold towels (1-3ºC) were rotated every three minutes. Figure 2. Basic trainees at Fort Sill, Lawton, Oklahoma, carrying bags of ice for immersion (Credit - U.S. Army) Final Point Forearm immersion requires cold water (~5ºC) to be effective, poses logistical challenges and may impair work performance post-cooling. If these factors can be managed, forearm immersion may be worth considering as a workplace heat stress control. Originally posted 20 October, 2021. References Adams WM, Morris EC, Walton SL, Karras EM. Comparing the Cooling Rates of Rotating Forearm Ice Towels and Passive Rest Following Exercise-Induced Hyperthermia. J Athl Train Sports Health Care. 2021;1:e1-6. Brearley M, Berry R, Hunt AP, Pope R. A Systematic Review of Post-Work Core Temperature Cooling Rates Conferred by Passive Rest. Biology. 2023;12(5):695. Brearley M, Walker A (2015). Water immersion for post incident cooling of firefighters; a review of practical fire ground cooling modalities. Extrem Physiol Med. 2015;4:15. Caballero R, Navarro S, Vanos J, Wardenaar FC. No differential effects among cooling strategies on post-exercise core temperature recovery in male athletes. Temperature. 2026;1-4. Cheung SS, Montie DL, White MD, Behm D. Changes in man- ual dexterity following short-term hand and forearm immersion in 10 C water. Aviat Space Environ Med. 2003;74:990–993. Iwahashi M, Chaen Y, Yanaoka T, Kurokawa Y, Hasegawa H. Cold water immersion of the hand and forearm during half-time improves intermittent exercise performance in the heat. Front Physiol. 2023;14:1143447. Mallette MM, Green LA, Gabriel DA, Cheung SS. The effects of local forearm muscle cooling on motor unit properties. Eur J Appl Physiol. 2018;118(2):401-410. Nakamura D, Muraishi K, Hasegawa H, Yasumatsu M, Takahashi H. Effect of a cooling strategy combining forearm water immersion and a low dose of ice slurry ingestion on physiological response and subsequent exercise performance in the heat. J Therm Biol. 2020;89:102530. Rogerson S, Brearley M. Suspected exertional heat stroke: A case study of worker cooling in a hot and humid field environment. Work. 2024;79(4):2103-2108. Yeargin S, McKenzie AL, Eberman LE, Kingsley JD, Dziedzicki DJ, Yoder P. Physiological and Perceived Effects of Forearm or Head Cooling During Simulated Firefighting Activity and Rehabilitation. J Athl Tra in. 2016;51(11):927-935.

In short Australian heatwaves are determined by the degree to which the current and forecast maximum and minimum temperatures differ from both recent and long term observations. Sole reliance on ambient temperature to determine heatwave forecasting seems appropriate for most climatic regions excluding those of the tropics and when other regions experience combined elevated ambient temperature and relative humidity. This has been anecdotally reported during 2024 east-coast low-intensity heatwaves, and using a real-world example, we demonstrate how heatwave classifications would differ through user of apparent temperature (that accounts for relative humidity) instead of ambient temperature. Furthermore, recent research supports the inclusion of relative humidity in heatwave calculations for Northern tropical regions (Nairn et al., 2022). Collectively, the evidence undermines the relevance of one-size-fits-all National heatwave warning system, such that caution should be exercised when classifying the risk of heat stress based upon one variable or metric, especially where heat and humidity are combined.  Figure 1.  A forecast map depicting low-intensity, severe and extreme heatwaves across Northern Australia (Bureau of Meteorology) Excess Heat Factor For many years, we've had concerns regarding how heatwaves are classified in humid regions of Australia, particularly in the tropics (Oppermann et al., 2017). Before we detail our concerns and use recent east-coast heatwaves along with published research to make our point, let's review how heatwaves are determined by the Bureau of Meteorology (BoM).  According to BoM, the forecast maximum and minimum temperatures for each three-day period in the coming week (e.g. Monday–Wednesday, Tuesday–Thursday) is compared to the 'normal' temperatures expected for that location at that time of year, and to observed temperatures over the last 30 days. Again, according to BoM, this comparison identifies regions where ambient temperatures are unusually high in relation to the local long-term climate and the recent past. The outcome of these comparisons is a metric called the Excess Heat Factor (EHF) that is used to determine the existence of a heatwave and its intensity as per BoM's classifications below. 1.  Low-intensity heatwaves are the most common—most people are able to cope with this level of heat. 2. Severe heatwaves are less frequent and are challenging for vulnerable people such as the elderly, particularly those with pre-existing medical conditions. 3. Extreme heatwaves are the rarest kind. They affect the reliability of infrastructure, like power and transport, and are dangerous for anyone who does not take precautions to keep cool—even those who are healthy. People who work or exercise outdoors are particularly at risk. We don't have an issue with use of the EHF. Our issue is related to the use of ambient temperature as the sole variable to calculate EHF. Of course, ambient temperature plays a major role in thermoregulation and heat stress more broadly. But, without consideration of other factors such as relative humidity, ambient temperature provides an incomplete summary of thermal conditions (Oppermann et al., 2017). For example, December maximum temperatures of 33ºC for Darwin, NT and Adelaide, SA are not as comparable once relative humidity is considered. So, why have BoM adopted an EHF based upon ambient temperature as a one-size-fits-all approach to heatwave classification in Australia? Presumably, the answer is that ambient temperature EHF is appropriate for the vast majority of the population for the vast majority of the time. Until it isn't (anecdotally), such as during the recent east coast “low intensity” heatwaves.  Excess Heat Factor and Public Health Prior to discussing these heatwaves, it's important to acknowledge the growing field of research that describes associations between ambient temperature EHF and public health outcomes, predominantly in temperate and subtropical climates (Bhatta et al., 2023). This work is critical to identify the heat-health burden and has seen increased research funding as a result. We are strong supporters of such research but a heatwave service that is good for most people, most of the time, is not good enough in our opinion. The recent east coast heatwaves have been generally classified as low intensity, below the threshold of severe or extreme, see Figure 2 for an example. Figure 2.  A recent heatwave for approximately half of QLD (Bureau of Meteorology). Elevated humidity during this heatwave was limited to coastal regions. The issue with Figure 2 was that this heatwave (based upon ambient temperature EHF) coincided with elevated relative humidity for coastal regions. The feedback we received from various work sites in the affected area was that the combined elevated heat and relative humidity conditions were oppressive, beyond that typically represented by a low intensity heatwave. We agreed based upon our observations from various work sites. In our opinion, it's worth considering the benefits of reporting EHF based upon combined heat and relative humidity (EHF Heat Index or apparent temperature) for regions with "humid" summers according to Figure 3. Figure 3.  Australian climate zones (Bureau of Meteorology) What About Atmospheric Moisture? Those in favour of the current (EHF ambient temperature) system cite that elevated minimum temperatures are indicative of elevated relative humidity and therefore, relative humidity is currently accounted for. We disagree, as does a key research paper. Led by Dr John Nairn, lead author of the first paper to describe the EHF, his research team examined the current ambient temperature EHF and heat index EHF (includes relative humidity) for representative cities within the six climate zones of Figure 3. They reported that: "results support ongoing use of a local temperature-only percentile-based heatwave index for detection of both dry and humid severe heatwaves in five of Australia’s six climate zones" (Nairn et al., 2022). This differs in Australia’s hot and humid tropical climate zone. This region experiences rare, very dry and very humid heatwaves. A comprehensive heatwave service in the tropics needs to operate the temperature-only and humidity-included versions of Australia’s EHF in order to capture these unusual heatwave events for an effective warning service" (Nairn et al., 2022). So, their data supports inclusion of relative humidity for Northern tropical regions (Figure 1) but not necessarily all of the recently heatwave impacted east-coast regions. Thanks to Christine Killip and the team from Weather Intelligence (a Katestone Company), we share a comparison of EHF ambient temperature v EHF apparent temperature for one site. You can read more about what the WI team intend to do with this new work by incorporating it into their Kite Weather Risk Forecasting platform   here . Figure 4.  Heatwave severity for Western Brisbane (Nov 2023 - Feb 2024) calculated Comparison of Excess Heat Factor based upon ambient temperature (blue line and markers) or apparent temperature (orange line and markers)  Analysis of all weather stations is well beyond the scope of this article, so data from Western Brisbane during recent months is included. The graph of Figure 4 represents heatwave severity using EHF based on ambient temperature (blue line and markers) compared to EHF based on apparent temperature (orange line and markers). The graph proves our point (and the anecdotal reports from workers) correct - the low intensity heatwaves of late January were classified as severe based upon apparent temperature. Final Point While it’s a small sample, the example of Figure 4 coupled with the evidence from Northern Australia (Nairn et al., 2022), is enough to caution individuals and organisations against classifying the risk of heat stress based upon one variable or metric, especially where heat and humidity are combined.  References Bhatta M, Field E, Cass M, Zander K, Guthridge S, Brearley M, Hines S, Pereira G, Nur D, Chang A, Singh G. Examining the Heat Health Burden in Australia: A Rapid Review. Climate. 2023;11(12):246. Nairn J, Moise A, Ostendorf B. The impact of humidity on Australia’s operational heatwave services. Climate Services. 2022;27:100315. Oppermann E, Brearley M, Law L, Smith JA, Clough A, Zander K. Heat, health, and humidity in Australia's monsoon tropics: a critical review of the problematization of ‘heat’ in a changing climate. Wiley Interdisciplinary Reviews: Climate Change. 2017;8(4):e468. Weather Intelligence.  https://weatherintelligence.global/weather-intelligence-new-heatwave-methodology/

In terms of heat stress management, there's an abundance of advice from a variety of sources. Government agencies including Health, Safe Work and Bureau of Meteorology all provide guidance to minimise the impact of hot weather. But what if such recommendations are based upon a lack of evidence, or worse, are contrary to the evidence. Consider this messaging from Healthdirect.gov.au (last reviewed March 2026): Avoid alcoholic, hot or sugary drinks (including tea and coffee) because these can make dehydration worse ( Healthdirect Australia Ltd, 2024 ). There's a lot to discuss in that sentence so we'll focus on the caffeine as the key ingredient of coffee (Figure 1) and to a lesser extent tea. Figure 1. Coffee and the chemical structure of it's primary ingredient caffeine Caffeine as a Diuretic Caffeine has been referred to as a diuretic since at least 1928, supported by research on high caffeine doses. Given that the vast majority do not consume such doses (an example is 6mg Caffeine per kg body mass; 100kg body mass = 6 cups of coffee - in one dose), the applicability of this research has been questioned and complemented with studies that better reflect typical consumption. Research assessing the diuretic effect of moderate doses of caffeine (an example is 4mg per kg body mass per day) suggests that coffee does not contribute to dehydration ( Killer et al., 2014 ). Authors of a meta-analysis of 16 studies agree, stating: “Concerns regarding fluid loss and potential adverse effects on fluid balance associated with caffeine ingestion are unfounded" ( Zhang et al., 2015 ). Final Point That's good news for those that enjoy a 'couple' of coffees at work. While many sources of information have been updated to reflect the evidence, frustratingly, more needs to be done to dispel the moderate dose caffeine - dehydration myth. References Healthdirect Australia Ltd. Hot weather risks and staying cool. https://www.healthdirect.gov.au/hot-weather-risks-and-staying-cool . December 2024. Last accessed March 2026. Killer SC, Blannin AK, Jeukendrup AE. No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population. PLoS One. 2014;9(1):e84154. Zhang Y, Coca A, Casa DJ, Antonio J, Green JM, Bishop PA. Caffeine and diuresis during rest and exercise: A meta-analysis. J Sci Med Sport. 2015;18(5):569-74.

We’ve demonstrated that despite its popularity as a heat stress control, resting in shade does not reverse the elevated core temperatures of heat-exposed workers in a timely manner ( Brearley et al., 2023 ). As a result, scheduled breaks may need to be extended and/or utilised more frequently, which may not be commercially viable. More effective cooling strategies during rest periods are an obvious solution here, but what about shade as a heat stress control during work? Figure 1.  Basic thermal imagery analysis of ground surface temperature of shaded and non-shaded areas on a clear day at ~38ºC Sunlight vs Shade It’s important to consider the differences between working while exposed to sunlight (solar radiation) compared to shade. Solar radiation is comprised of ~50% infrared rays that provide heat, ~42% visible rays that provide light and the remaining ~8% are ultraviolet (UV) rays known for their causal link to skin cancer. Blocking solar radiation provides protection from heat, light and UV rays, and is commonly achieved with relatively inexpensive shade sails/cloth. Typical shade sails block 80-95% of solar radiation. A simple method of visualising the impact of a shade sail (or similar) is to measure surface temperature in shaded and proximal non-shaded areas of a work site. Figure 1 demonstrates a ~20-25ºC surface temperature difference between the shade provided by a typical shade sail and non-shaded areas on a clear and dry ~38ºC day.   Table 1.  Classification system for human shade protection provided by knitted and woven shade fabrics as in Table 3.1 of the Australian Standard AS 4174:2018 Knitted and woven shade fabrics Australian standard AS 4174:2018 details the classification of shade sails/cloth solely based upon blocking of UV radiation, as per Table 1. Despite the lack of standardised reporting for infrared or light blocking, it’s generally assumed that as UV blocking increases, so too does infrared and light blocking. In our experience, ~340gsm shade sail fabric produces ~95% UV block but this may vary with shade sail quality and colour.  Figure 2.  Time to exhaustion while exercising at 70% maximum in 30ºC/50% relative humidity with various solar radiation loads ( Otani et al., 2016 ) Final Point Shade sails or equivalent can introduce a work site hazard during storms and high wind periods and flammability of the fabric may be a hazard for some sites. But in general, we consider shade a low cost - high return control. We’ve reported on the impacts of solar radiation on physical work performance previously (Figure 2, solar radiaton insight ), noting the steep decrease in time to exhaustion as the solar radiation load increases ( Otani et al., 2016 ). Clearly, working in shade is associated with delayed fatigue at a set workload. In our opinion, establishing shade for work sites is worthy of consideration to limit both the absorbance of heat by the work environment, and by the workers within that environment.     References Brearley M, Berry R, Hunt AP, Pope R. A systematic review of post-work core temperature cooling rates conferred by passive rest. Biology. 2023;12(5):695. Otani H, Kaya M, Tamaki A, Watson P, Maughan RJ. Effects of solar radiation on endurance exercise capacity in a hot environment. Eur J Appl Physiol . 2016;116:769-779.

We've detailed issues with Australian recommendations for Exertional Heat Stroke (EHS) treatment, and summarised evidence for alternatives to the commonly recommended application of cold compresses on the shallow arteries in this paper   (Brearley, 2019). In short, rotating cold compresses on the shallow arteries (Figure 1) does not remove adequate body heat to rapidly lower the elevated core temperature associated with EHS. In their 2009 review of cooling methods, McDermott et al. stated: ".......... the use of ice packs or ice bags for the treatment of EHS should be discontinued, because the extraction of heat from the body is ineffective for the body temperatures typically associated with EHS" ( McDermott et al., 2009 ). Figure 1. An illustration of the application of ice packs to the shallow arteries. This method provides unacceptably low cooling rates for treatment of exertional heat stroke (Credit - Healthwise) Cold Compress Alternatives Of the cold compress alternatives, cold water immersion is considered the gold standard, with rotation of cold, wet towels (ice towels) over the entire body regarded as an inferior, yet, effective EHS treatment. Importantly, the ice towels are considered a feasible option for use by workers in remote settings, requiring only an ice box/esky and access to ice, water and cotton towels or similar. Since the outcome from EHS is dependent upon the duration and severity of hyperthermia, medical treatment seeks to minimise the area under the core temperature curve to ensure survival and prevent long-term medical complications. Returning core temperature to less than 39ºC (102 º F) within the 30 minute treatment window or 'golden half hour' post collapse is the objective here. Ice Towels Based upon the evidence, rotation of ice towels over the entire body is an effective treatment due to reported cooling rates of 1.1ºC per 10 minutes (Table 1). Note that time to return core temperature to 39ºC will vary with severity of EHS (peak core temperature). For severe EHS cases (>41.5ºC/~107 ºF ), a core temperature decrease of ~3ºC is required within 30 minutes of collapse. Table 1.  Core temperature cooling rates for selected cooling modalities Cooling Modality Cooling Rate (°C/10 mins) Reference Ice Water Immersion (2°C) 3.50 Proulx et al., 2002 Tarp Assisted Cooling with Oscillation (9.2°C) 1.70 Hosokawa et al., 2016 Ice Towels/Sheets 1.60 DeGroot et al., 2023 Ice Towels 1.10 Armstrong et al., 1996 Combined Cooling * 0.36 Kielblock et al., 1986 Passive Rest (underwear only) 0.27 Kielblock et al., 1986 Passive Rest (insulated clothing) 0.04 Brearley et al., 2023 * Cold packs on groin, neck, and axillae plus splashing body with water while evaporating with compressed air In our experience, the initial response to ice towels as an EHS field treatment was mixed, with many organisations stating they would defer treatment choice to their contracted medical provider. The key point here is that for workers in remote settings and/or on sites remote from facilities, professional medical assistance may not be available within the 30 minute window, even when stationed on-site. Further, the stated 30 minute window does not refer to the time to initiate cooling, rather, it's the time limit to have achieved a substantial core temperature reduction (<39ºC). Hence, the outcome of EHS treatment in remote settings is more likely dependant upon co-workers provision of effective first aid, inclusive of cooling, while medical assistance is en route.  EHS Ice Towel Case Study In conjunction with Dr Shane Rogerson of Energy Queensland ( Rogerson and Brearley, 2024 ) , we describe the practicality of the ice towel method in an industrial setting (~4000 field workers distributed throughout QLD), where criteria for use include cost effectiveness, portability, scalability, and implementation by a single worker under the stress of an emergency. The paper also describes the emergency application of the ice towel method for a worker suffering suspected EHS on a remote job site, while awaiting paramedics. For this work, Energy Queensland won the Best Solution of a Workplace Health and Safety Risk award at the 32nd Annual National Safety Awards of Excellence (2025). Development and implementation of this technique is summarised in a two minute video, authored by Dr Rogerson (Figure 2). Figure 2.  A video overview of the ice towel treatment for EHS (Credit - Energy Queensland) References Armstrong LE, Crago AE, Adams R, Roberts WO, Maresh CM. Whole-body cooling of hyperthermic runners: Comparison of two field therapies. Am J Emerg Med. 1996;14:355-58. Brearley M. Are Recommended Heat Stroke Treatments Adequate for Australian Workers? Annals of Work Exposures and Health. 2019, 63:263-66. Brearley M, Berry R, Hunt AP, Pope R. A Systematic Review of Post-Work Core Temperature Cooling Rates Conferred by Passive Rest. Biology. 2023;12:695. DeGroot DW, Henderson KN, O’Connor FG. Cooling modality effectiveness and mortality associate with prehospital care of exertional heat stroke casualities. J Emerg Med. 2023;64:175-80. Hosokawa Y, Adams WM, Belval LN, Vandermark LW, Casa DJ. Tarp-Assisted Cooling as a Method of Whole-Body Cooling in Hyperthermic Individuals. Ann Emerg Med. 2016;69:347-52. Kielblock AJ, Van Rensburg JP, Franz RM. Body cooling as a method for reducing hyperthermia. An evaluation of techniques. S Afr Med J. 1986;69, 378-80. McDermott BP, Casa DJ, Ganio MS, Lopez RM, Yeargin SW, Armstrong LE, Maresh CM. Acute whole-body cooling for exercise-induced hyperthermia: a systematic review. J Athl Train. 2009;44(1):84-93. Proulx CI, Ducharme MB, Kenny GP. Effect of water temperature on cooling efficiency during hyperthermia in humans. J Appl Physiol. 2002;94:1317-23. Rogerson S, Brearley M. Suspected exertional heat stroke; A case study of worker cooling in a hot and humid field environment. Work. 2024; 79(4):2103-2108.

Commercially available heat stress kits (see Figure 1 for example) contain instant ice packs. According to the associated information of the kits, the listed items are for the prevention of heat-related illness, including heat stroke. While the kits do not state they are intended for treatment, instant ice packs have been observed within worksite first aid facilities and/or response vehicles in anticipation of treating exertional heat stroke in the field. Figure 1. Commercially available heat stress kit for industry and its composition, including four instant ice packs Whereas standard ice packs require a means of producing and/or storing ice plus prevention of melting during transportation, instant ice packs simply require breaking the barrier between water and the solid chemical, traditionally ammonium nitrate but more recently, ammonium chloride. Once this ‘activation’ occurs, most brands claim the pack will remain consistently cold for up to 20 minutes of clinically recommended application. Most brands suggest pack usage for sports injury, sprains and minor pain ailments. Exertional Heat Stroke (EHS) treatment is rarely mentioned in this regard. Yet, based upon the theoretical basis for use on an EHS patient and recommendations to do so within Australian first aid training curriculum (no differentiation between standard ice and instant ice packs), it seems intuitive to have a supply of instant ice packs for resource limited settings. Their low cost, ease of storage and transport likely enhance their use.   Use by NSW Ambulance Arguably the strongest support for instant ice packs comes from emergency medical response teams. Use of instant ice packs by NSW Ambulance (Australian Paramedical Organisation) for heat stroke treatment was revealed during the 2024 inquest into the death of Keith Titmuss, the 20-year-old Manly Sea Eagle rugby league athlete that suffered EHS during a training session and died on November 23, 2020.  Figure 2.  Sentry Medical instant ice pack and NSW Ambulance Protocol E3 Hyperthermia Treatment During the inquest into his death, Senior Staff Specialist in Emergency Medicine, Associate Professor Anna Holdgate, testified on behalf of NSW Ambulance that their ambulances carry 4 to 6 Sentry Medical instant ice packs (Lee, 2024). While the size was not provided, they are likely similar to the commercially available instant ice pack of Figure 2. The inquest also identified that the NSW Ambulance protocol for treatment of heat stroke requires removal of patient from the heat source followed by the steps detailed in Figure 2. Note that NSW Ambulance Protocol E3 Hyperthermia does not differentiate between classical and EHS. Lack of Evidence Despite use by NSW Ambulance, rotation of instant ice packs on the neck, arm pits/axillae and groin confer cooling rates so low ( Kielblock et al., 1986 ), that they are deemed ‘unacceptable’ for heat stroke treatment. McDermott et al. (2009) stated in their review paper that: “……….the use of ice packs or ice bags for the treatment of EHS should be discontinued, because the extraction of heat from the body is ineffective for the body temperatures typically associated with EHS”.  While it’s apparent that the instant ice packs lack the cooling power to rapidly reduce core temperature, how do they compare to standard ice packs? The respective cooling power of instant ice and standard ice packs was compared by pack immersion in two litres of water (Phan et al., 2013). Despite the packs being matched on the basis of physical size, the standard ice packs (570g) were ~2.4 times heavier than the instant ice packs (238g; 147mL water and 91g ammonium nitrate). This disparity likely contributed to the superior cooling rate of the standard ice packs (3.8 times that of the instant ice packs). When compared on the basis of mass, the standard ice packs had 60% more cooling power than the instant ice packs. Effective treatment of EHS requires a cooling rate sufficient to rapidly lower core body temperature, typically achieved through methods such as cold water immersion or the application of ice-cold towels ( Rogerson and Brearley, 2024 ), which far exceed the cooling power provided by either type of ice pack. Three years post the passing of Keith Titmuss, NSW Ambulance seemingly agreed. During the 2024 inquest, Assoc. Prof. Holdgate stated: “So of the various methods of cooling, that method (ice pack application to the neck, arm pits/axillae and groin) is probably one of the least effective methods of cooling. So it doesn't - won't cause any harm if it can be done, but it may not cause much benefit”. Enough said.   Take Home Message Cooling by ice packs applied to the shallow arteries, whether chemically cooled or formed by frozen water, is not supported as a primary treatment for EHS. For organisations that plan to use ice packs in their management of EHS, we suggest reviewing the evidence ( Brearley, 2019 ).   References Brearley MB. Are Recommended Heat Stroke Treatments Adequate for Australian Workers? Ann Work Expo Health. 2019;63:263-266. Kielblock AJ, Van Rensburg JP, Franz RM. Body cooling as a method for reducing hyperthermia. An evaluation of techniques. S Afr Med J. 1986;69:378-380. Lee, D. Inquest into the death of Keith Titmuss. Coroner’s Court of New South Wales. File 2020/333632  McDermott BP, Casa DJ, Ganio MS, Lopez RM, Yeargin SW, Armstrong LE, Maresh CM. Acute whole-body cooling for exercise-induced hyperthermia: A systematic review. J Athl Train. 2009;44:84-93. Phan S, Lissoway J, Lipman GS. Chemical cold packs may provide insufficient enthalpy change for treatment of hyperthermia. Wilderness Environ Med. 2013;24(1):37-41. Rogerson S, Brearley M. Suspected exertional heat stroke: A case study of worker cooling in a hot and humid field environment. Work. 2024;79(4):2103-2108.

It's very important as physical work rate determines body heat production. In turn, this internal heat production combined with the external heat load of the environment determines worker heat stress. Quantifying the conditions workers are exposed to is relatively simple for a fixed worksite, and achievable through portable monitors/weather stations for mobile sites. Many organisations implement such measures to inform their heat stress management and that's a positive. But what about work rate? In our opinion, the role of work rate in occupational heat stress deserves more attention, so we make this point through the research of Tatterson et al. (2000). Which of the following indoor work environments is more likely to result in worker heat stress? Hotter conditions = more heat stress, right? Figure 1. Which of the following indoor work environments is more likely to result in worker heat stress? Comparison of Environments The 32ºC (90ºF) environment has substantially less potential for body heat dissipation, so similar work rates between conditions will result in greater body heat storage, higher core temperature and heat stress compared to the 23ºC (73ºF) environment (Figure 1). But what if workers adjusted their work rate to compensate for the hotter conditions - could they produce similar heat stress between conditions? Tatterson et. ( 2000 ) studied 11 elite Australian cyclists completing 30-min time trials in the conditions previously described, summarised as 23ºC (green dots) and 32ºC (red dots) on the graphs of Figure 2. Despite the different conditions, core temperatures were similar throughout the time trial (Figure 2A), with slightly higher skin temperature and sweat rate in the 32ºC conditions. The similar overall heat stress was due to the lower work rate (reported here as 6.5% lower cycling power output in Figure 2B) and therefore body heat production, in the 32ºC conditions. Figure 2. Core temperature and power output of elite road cyclists during a 30-min time trial in 23ºC and 32ºC Pacing of Effort Sure, work-as-normal for most workers does not require periods of maximal workload as per this elite cycling research example. Yet, similar to the cyclists, workers that self-pace their effort are constantly adjusting work rate based upon factors such as experience, health and internal feedback. We wrote this about pacing in a paper on electrical utility worker heat stress: "By understanding the tasks required, number of staff available, time frame for completion, anticipated climatic conditions and factoring in their personal experience, physical fitness, and acute physiological status, workers can initiate behavioural and work load adjustments, thereby selecting an appropriate pace to complete the task and prevent excessive body heat storage. It is reasonable to expect that regular exposure of the utility workers to hot conditions has enabled pacing strategies to be routinely applied and modified ( Brearley et al., 2015 )." Final Point The Tatterson et al. (2000) example highlights the role of work rate in the development of heat stress. While knowledge of expected and actual environmental conditions are important, also understanding the work rate required to complete a task, and that harsher environmental conditions are likely to lower the sustainable work rate and therefore, productivity of individual workers, allows alterations of schedule, redeployment of resources and tailoring of controls to mitigate heat stress. Originally posted 4 March 2023. References Brearley M, Harrington P, Lee D, Taylor R (2015). Working in hot conditions-a study of electrical utility workers in the northern territory of Australia. J Occup Environ Hyg. 12(3):156-162. Tatterson AJ, Hahn AG, Martin DT, Febbraio MA (2000). Effects of heat stress on physiological responses and exercise performance in elite cyclists. J Sci Med Sport. 3(2):186-193.

A common question asked of us by workers relates to electrolytes and specifically, sodium (the salty part of salt), while working in the heat. Some workers report that sodium supplementation is necessary to replace sodium excreted in sweat, thereby preventing muscle or 'heat' cramps. As for outdoor and/or heat-exposed manual workers, ultramarathon runners endure long periods of exposure and physical activity, albeit at a higher work rate. It was interesting to read the recently published work of Hoffman and White (2020) that reported the majority of the 1152 ultra-runners sampled indicated that sodium supplements should be available during races and ~60% reported that they prevent muscle cramping. This was despite Hoffman et al. (2015) reporting that the majority of sodium for ultrarunners was generally sourced from their food and fluid consumption. Importantly, Hoffman and co also reported that there was no difference in sodium consumption between those that cramped (or 'near-cramped') compared to the non-crampers. Note that this analysis is from relatively small groups (total n=23, 9 v 14). For this sample, sodium consumption was also not related to the incidence of nausea. So, what does this mean for heat exposed workers with high sweat rates? Hoffman's work provides more evidence that food and fluid consumption are likely to provide ample sodium and that supplementation may not be warranted for most workers. For excessive sweaters and those with confirmed (by medical staff) electrolyte deficiency, supplementation may be advised. That could be as simple as adding salt to the post-shift meal. We provide workers with the numbers regarding sodium content of common foods v electrolyte-containing beverages to demonstrate why food consumption during a hot work shift is important in spite of the loss of appetite reported by approximately 1 in 4 workers across Northern Australia ( Carter et al., 2020 ). Most are surprised to learn that the 'electrolyte-containing' beverages are relatively low in electrolytes when compared to commonly consumed sandwiches. References Carter S, Field E, Oppermann E, Brearley M. The impact of perceived heat stress symptoms on work-related tasks and social factors: A cross-sectional survey of Australia's Monsoonal North. Applied Ergonomics. 2020;82:102918 Hoffman MD, Stuempfle KJ, Valentino T. Sodium Intake During an Ultramarathon Does Not Prevent Muscle Cramping, Dehydration, Hyponatremia, or Nausea. Sports Med Open. 2015;1(1):39 Hoffman MD, White MD. Belief in the Need for Sodium Supplementation During Ultramarathons Remains Strong: Findings from the Ultrarunners Longitudinal TRAcking (ULTRA) Study. Appl Physiol Nutr Metab. 2020; 45(2):118-22

Given the advances in consumer wearable technology, it's a fair expectation that those advances would translate to wearables suitable for workers. Sure, occupational settings are more demanding in terms of the 'ruggedisation' required to operate in the hot and humid microclimate proximal to the body. But if heat-exposed workers are to be managed with input from a wearable device, the key requirement is accuracy. So, here's our take on three wearables currently on the market. CORE by greenTEG AG CORE claims to be the "only wearable non-invasive, continuous, and accurate Core Body Temperature monitoring solution", estimating core temperature from a rechargeable 5x4cm heat flux and skin temperature sensor (see above). The sensor is attached below the armpit by adhesive patch or a wearable band (like a heart rate monitor). The battery lasts for multiple days when transmitting and for weeks in stand-by mode. The product website claims that "the accuracy of CORE has been validated in an independent clinical study" and provides a "download" validation link that opens five slides from CORE spruiking accuracy but without the associated published independent research. Our disappointment was short-lived as independent research has been published regarding CORE accuracy. The study employed both cool and warm/hot environmental conditions for exercise that produced almost 12,000 data points, including many >38.5ºC, with the researchers stating that: "The results obtained do not support the manufacturer’s claim that the CORE sensor provides a valid measure of core body temperature". You can read the full paper here . Kenzen Utilising a similar sized sensor (~5x6cm) to CORE that attaches to the upper arm with a band, Kenzen claims its system provides "continuous safety monitoring (that) is both real-time and highly accurate". Evidence is provided to support this claim but not in the form of independent research published in a peer-reviewed journal. The research is a Kenzen sponsored abstract from the Experimental Biology 2021 Meeting. Read the abstract here . There's a large discrepancy between an abstract and a full research paper. Furthermore, the analysis of data described within the abstract is questionable. Data was collected every 5 minutes and was seemingly averaged each hour as per the graph below. For a product that claims to provide real-time safety monitoring, reporting accuracy based upon hourly data is a strange move. The accuracy of those real-time measurements should be reported and until that's done by independent research, the claim of accuracy cannot be substantiated. Cosinuss° Unlike the CORE and Kenzen options, Cosinuss° is an "in-ear" sensor with a battery life of up to 24hrs. Cosinuss° states "Our ergonomic wearables offer a more cost-effective and accurate alternative to the competition for measuring the following: blood oxygen saturation, core body temperature, heart rate, heart rate variability, respiratory rate and position/acceleration". Cosinuss° accuracy has been evaluated by an independent but small field trial of firefighters, with published results. While the research would have benefitted from additional participants and more profound heat stress (maximal core temp of 38.4ºC), the authors concluded that: “The validity of the Cosinuss° (C-med version) was not confirmed in this study ( Roossien et al. 2020 )". Interestingly, despite generally under-predicting core temperature, there was an instance where Cosinuss° core temperature was 40.1ºC. Note that Cosinuss° produce a range of in-ear monitors and that the results reported for C-med may not be representative of all models. Final Point Despite our negative conclusions regarding the accuracy of the CORE, Kenzen and Cosinuss° (based upon available information), we have a vested interest in core temperature wearables, as they may contribute to management of worker heat stress in the field. We currently utilise ingestible pills that provide accurate core temperature data with the primary downside of being invasive. Other limitations include ingestion lead-time prior to monitoring, occasional delays for pill connectivity and worker monitoring capped by the associated hardware and therefore, cost. There's a need for non-invasive options and a lot of development has been, and will continue to be done in this space. We look forward to reporting if and when worker core temperature wearables are "field-ready". References Moyen N, Bapat R, Tan B, Esfahani M, Mundel T. Accuracy of a wearable device to non - invasively predict continuous core body temperature. The FASEB Journal. 2021;35 Roossien C, Heus R, Reneman M, Verkerke G. Monitoring core temperature of firefighters to validate a wearable non-invasive core thermometer in different types of protective clothing: Concurrent in-vivo validation. Appl. Ergon. 2020;83103001 Verdel N, Podlogar T, Ciuha U, Holmberg H-C, Debevec T, Supej M. Reliability and Validity of the CORE Sensor to Assess Core Body Temperature during Cycling Exercise. Sensors. 2021; 21(17):5932

Prior to the COVID-19 pandemic, prolonged face mask use was reserved for a small proportion of society. Following their widespread use, face masks have been anecdotally linked to a range of outcomes, including an exacerbation of heat stress. In response, Morris et al. (2020) addressed the impact of face mask use by assessing core body temperature, cognitive performance and a range of perceptual responses during light exercise in hot conditions (40ºC/20% RH). Participants reported that perceived shortness of breath was substantially worse with face mask use, whereas core temperature, cognition, under mask skin temperature, whole-body thermal discomfort and facial thermal discomfort were not adversely impacted. Critics could argue that the exercise duration was not sufficient (45mins), the workload was too low (100W) and that the study had too few participants (n=8). But inadequate heat stress was not an issue given that mean core temperatures were in the 38.2-38.4ºC (~101 º F) range. Results may vary with more prolonged face mask use, but based upon these findings, the anecdotal reports of elevated heat stress are yet to be supported by evidence. Note that there are outliers and we expect to see additional research related to this topic. Reference Morris NB, Piil JF, Christiansen L, Flouris AD, Nybo L. Prolonged facemask use in the heat worsens dyspnea without compromising motor-cognitive performance. Temperature. 2020 Oct 11:1-6.

Figure 1. How heat domes occur (Credit - Washington Post) Following a North American hot weather event in June 2021 (Henderson et al., 2022; Jain et al., 2024; Wettstein et al., 2024), the term “heat dome” was officially recognised by the American Meteorological Society in 2022. They define it as: “An exceptionally hot air mass that develops when high pressure aloft prevents warm air below from rising, thus trapping the warm air as if it were in a dome.” Heat domes have also been likened to the trapping of heat in a similar manner to a lid on a saucepan. The heat dome is visually summarised by Figure 1.  Figure 2.  January 24-28, 2026 Australian heatwave maps (Credit - Bureau of Meteorology) To the end of 2025, the term "heat dome” had been infrequently used in Australia, despite the impact of the 2021 North American event. For example, it was the deadliest weather event in Canada’s history, with 619 heat-related deaths during the 7-day heat dome in British Columbia. But the record-breaking Australian heatwave of late January 2026 (Figure 2), that was due to a heat dome, exposed a large population to severe heat (Figure 3) and saw usage of the term increase (Figure 4). With projections for more frequent and severe heatwaves, heat domes are likely to become more familiar. Figure 3.  January 26, 2026 maximum temperatures (Credit - Australian Broadcasting Corporation) Figure 4.  An example heat dome headline (Credit - Weatherzone) References Henderson SB, McLean KE, Lee MJ, Kosatsky T. Analysis of community deaths during the catastrophic 2021 heat dome: early evidence to inform the public health response during subsequent events in greater Vancouver, Canada. Environmental Epidemiology. 2022;6(1):e189. Jain P, Sharma, AR, Acuna DC. et al. Record-breaking fire weather in North America in 2021 was initiated by the Pacific northwest heat dome. Commun Earth Environ. 2024;5,202. Wettstein ZS, Hall J, Buck C, Mitchell SH, Hess JJ. Impacts of the 2021 heat dome on emergency department visits, hospitalizations, and health system operations in three hospitals in Seattle, Washington. JACEP Open. 2024;5(1):e13098.