Conditions such as stroke and myocardial infarction owe some of their remarkable improvements in outcomes and mortality to primary, secondary and tertiary preventative strategies. Personalised and artificial intelligence-guided medicine promise further individualised treatments and better outcomes. However, the prevention of heat illness has failed to keep up.
Heat illness is defined as systemic symptoms associated with a raised core temperature due to exposure to high environmental temperatures (‘classical heat illness’) or strenuous physical activity (‘exertional heat illness’). The most severe form of the classical heat illness spectrum, classical heatstroke (CHS), is common and poses severe risks such as multi-organ failure (Walter et al, 2016), with a mortality risk of over 60% reported in intensive care (Bouchama et al, 2022). The World Health Organization (WHO) (2018) estimates that heatwaves are responsible for tens of thousands of direct and indirect excess deaths each year. The number of deaths has been predicted to rise by over 2.5-fold in the next 60 years (Hajat et al, 2014).
Ambient temperature forecasts are routinely used, which go some way to indicating the risk of developing heat illness. This makes physiological and physical cogency. The difference between the ambient temperature and the temperature of the individual influences heat loss by thermal radiation, accounting for the majority, up to 60% (Rao and Rajan 2008) of heat dissipation. Not taken into account though are other methods of heat dissipation, such as convective heat loss, due to movement of cooler air across the skin, evaporation of water to vapour from the skin surface, for example, from sweating, the warming of subsequently exhaled air, conduction to surfaces in direct contact with skin or clothing, and storage of heat within the body.
More reliable heat indices, for example, wet bulb globe temperatures (WBGT), which take into account humidity, wind speed, and visible and infrared radiation in addition to temperature, are therefore needed and physiologically more cogent. High humidity, for instance, assessed by WBGT, adversely affects evaporation and mortality (Barreca, 2012); a WBGT of 35°C, indicating high temperatures and humidity, minimises heat dissipation by both thermal radiation and evaporation, and has been suggested as the maximum tolerated temperature, beyond which hyperthermia will occur (Raymond et al, 2020). This concept is not fully assessed by ambient thermometry alone. Even these measures do not consider individual risk factors including extremes of age, susceptible genotypes and phenotypes, activity levels, clothing, and certain medications.
Individualised preventative medicine represents an exciting era—for some conditions. Others, such as heat illness continue to hide in the shadows. While hiding in the shadows isn't a bad way to prevent heat illness, we look forward to better defining the risk posed by environmental conditions on the population, to enable us to do something impactful with the result, before tackling individual risk.