Health Risks and Interventions in Exertional Heat Stress

Abstract

Background: With climate change, heat waves are expected to become more frequent in the near future. Already, on average more than 25 000 "heat deaths" are estimated to occur in Europe every year. However, heat stress and heat illnesses arise not just when ambient temperatures are high. Physical exertion increases heat production within the organism many times over; if not enough heat is lost, there is a risk of exertional heat stress. This review article discusses contributing factors, at-risk groups, and the diagnosis and treatment of heat illnesses.

Methods: A selective literature search was carried out on PubMed. Current guidelines and expert recommendations were also included.

Results: Apart from muscular heat production (>70% of converted energy), there are other factors that singly or in combination can give rise to heat stress: clothing, climate/acclimatization, and individual factors. Through its insulating properties, clothing reduces the evaporation of sweat (the most effective physiological cooling mechanism). A sudden heat wave, or changing the climate zone (as with air travel), increases the risk of a heat-related health event. Overweight, low fitness level, acute infections, illness, dehydration, and other factors also reduce heat tolerance. In addition to children, older people are particularly at risk because of their reduced physiological adaptability, (multi-)morbidity, and intake of prescription drugs. A heat illness can progress suddenly to life-threatening heat stroke. Successful treatment depends on rapid diagnosis and cooling the body down as quickly as possible. The aim is to reduce core body temperature to <40 °C within 30 minutes.

Conclusion: Immediately effective cooling interventions are the only causal treatment for heat stroke. Time once lost cannot be made up. Prevention (acclimatization, reduced exposure, etc.) and terminating the heat stress in good time (e.g., stopping work) are better than any cure.

Figure 1

Heat metabolism: heat production and heat dissipation at rest and during increasing physical exertion (60 minutes on a cycle ergometer) in an unclothed human being at an ambient temperature of 22° C (adapted from [e7, e8])

Figure 2

Flowchart for use in a case of suspected exertional heat stroke (, – e59)

Figure 3

Symptoms of heat illnesses. Not all the symptoms may appear, and they may not appear in any given order. The various forms of heat illness may develop independently and apparently without warning; i.e., this is not a regular sequence of recognizable stages of illness (adapted from [e78, e81]).

Figure 4a

Rate of cooling of the human body using various cooling methods. The comparison is for general guidance only, since no standardized comparison is possible for the studies involved (, – e95, e97) in terms of initial body temperature and patient population characteristics (age, sex, body mass, body surface area to volume ratio, etc.). For the saline infusions, 2 × 1 L 0.9% saline solution was used.

Figure 4b

Model of reduction over time in core body temperature (adapted from [e96], assuming constant mean cooling rates [e93, e97]), the same initial temperature (43 °C), and the same time of treatment initiation. The chosen endpoint was the lower temperature threshold for cell damage in human beings (40 °C).