Using Physiological Evidence to Inform Extreme Heat Public Health Guidance (Core Science Keynote)

During heat waves, public health messaging for mitigating the health effects of extreme heat exposure, irrespective of the city or country, often offers similar advice: stay hydrated, drink cold water, do not consume caffeinated drinks, keep cool (which usually means using air conditioning), and do not use an electric fan if it gets too hot – usually defined as >35°C (95°F). But how grounded in physiological evidence is this advice [1]?

Over the past several years researchers in the Thermal Ergonomics Laboratory at the University of Sydney along with their international collaborators have conducted a series of laboratory-based experiments aimed at developing a body of physiological data that can serve as empirical evidence upon which heat wave guidance can be built. In a state-of-the-art climate chamber, we have replicated a range of major historical heat wave conditions, from very hot-dry events such as the recent heat wave in California to cooler but much more humid events such as the Chicago (1995) or Montreal (2018) heat waves. At the same time, we have measured how different groups of people (young, older adults, older adults with different co-morbidities) physiologically respond to these simulated heat wave exposures (i.e. rise in deep core temperature, increase in cardiovascular strain, rate of dehydration) when using different personal cooling strategies.

So far, we have found that best cooling methods are dependent on the type of heatwave and a person’s ability to sweat. For example, contrary to the existing public health guidance on fan use in heatwaves, we have shown that fans can be very effective [2] cooling devices at air temperatures up to at least 42°C with 50% relative humidity for young healthy people [3], but less so in older [4] adults (>65 y) because of age-related reductions in the ability to sweat [5]. However, in very hot (46°C), but dry (<15%RH) heat waves all sweat evaporates anyway and more heat is just added to the body with fan use resulting in accelerated body heating even in young healthy people. These limitations can likely be offset though by wetting the skin with a sponge and/or placing the feet in a bucket of cold water. This work is ongoing and in the next 2 years, along with collaborators in Dallas, TX and Montreal, Canada, we will extend our studies to focus on developing an understanding (which is desperately lacking) of how different prescription medications and health disorders (e.g. coronary artery disease) affect the ability to keep cool in a heat wave.

Other studies by our research team have demonstrated that drinking cold water does not typically provide a net cooling effect relative to body temperature water due to a parallel reduction in sweating [6] mediated by independent thermoreceptors residing in the stomach or abdomen [7]. The consequent reduction in skin surface evaporation with cold water ingestion is in lock-step with the parallel increase conductive heat exchange with cold ingested fluids leading to a net-zero change in total heat loss. It follows that decreasing the temperature of a drink (or adding ice to it) only provides a cooling effect under specific circumstances such as when sweat evaporation is diminished due to high levels of ambient humidity [8].

Recent investigations also indicate that heat wave messaging focusing foremost on “staying hydrated” and/or “drinking before getting thirsty” may be somewhat redundant, at least in young healthy adults, as ad libitum water ingestion, while not fully rehydrating a person, enables them to prevent dehydration-related exacerbations in thermal and cardiovascular strain during a 3-h exposure (with moderate physical activity) to hot/humid heat wave conditions. It was also found that ad libitum drinking of iced (5°C) water replaced less of the fluid lost from sweating than cold tap water (20°C). On-going studies are now assessing different strategies for preventing the development of dehydration while preserving water supplies – this may be particularly relevant for settings with limited clean drinking water.

References:

1. Jay O, Capon A. (2018) Lancet Planet Health;2(1):e10
2. Ravanelli N, Hodder S, Havenith G, Jay O (2015) JAMA; 313(7): 724–25
3. Ravanelli N, Gagnon D, Hodder S, Havenith G, Jay O (2016) Int J Biometeorol; 61(2):313-23
4. Gagnon D, Romero S, Cramer M, Jay O, Crandall C (2016) JAMA; 316(9):989-91
5. Gagnon D, Romero S, Cramer M, Kouda K, Poh P, Ngo H, Jay O, Crandall C (2017) Med Sci Sports Exerc; 49(11):2333-42
6. Bain A, Lesperance N, Jay O (2012) Acta Physiologica; 206(2):98-108
7. Morris N, Bain A, Cramer M, Jay O (2014) J Appl Physiol; 116(8):1088-95
8. Jay O, Morris N (2018) Sports Med; 48(Suppl 1):17:29