Risk factors for heatstroke – it’s personal!

Zachary J. Schlader, PhD, FACSM

In many ways the history of understanding the human body’s responses to exposure to hot environments, often termed heat stress, is founded on observations in the workplace. Arguably, this work started with observations of the regular occurrence of sunstroke in South African gold miners in the late 18th and early 19th centuries. Sunstroke, now known as heatstroke, is the most severe form of heat illness and is described by extreme elevations in internal body temperature and nervous system alterations (including confusion, coma, disorientation). When diagnosis and/or treatment is delayed, heatstroke is often deadly or can cause long-term health challenges. Equally, if not more, important is that observations in the workplace led to the development of strategies to prevent heatstroke. For example, in the South African mines, adaptations caused by repeated exposure to heat, known as heat acclimation, were shown to reduce the incidence of heatstroke in gold miners.

With the frequency and intensity of hot weather increasing due to climate change, there is a heightened risk of developing heat illness across all populations. The experiencing of warmer weather occurs largely independent of geographical location, socioeconomic status, body size, race, biological sex, or health status. However, the risk of deleterious health outcomes caused by heat exposure is not universal, with many groups and individuals being at higher risk of heat illness than others.

The term heat illness describes a series of disorders ranging from relatively mild to life threatening. Mild heat illnesses can mostly be treated with rest and removal from the hot environment. Heatstroke is the most severe heat illness. Without aggressive body cooling, heatstroke can be deadly.

Heatstroke is diagnosed through clinical signs and symptoms and is often the likely diagnosis when internal body temperature exceeds 40.5°C (105°F) and nervous system alterations are observed (including confusion, coma, disorientation), amongst others. There are two forms of heatstroke: classic heatstroke and exertional heatstroke. Both forms of heatstroke are caused by extreme elevations in internal body temperature. Classic heatstroke occurs during resting exposures to extreme heat and more frequently affects the very young, older adults, and/or those with underlying illness. By contrast, exertional heatstroke more often occurs in healthy adults, such as workers engaging in intense manual labor, military personnel, and athletes.

The primary environmental factors that influence the risk of developing heatstroke are high air temperature and elevated levels of humidity (or the amount of water suspended in the air) (Box 1). Hot air temperatures challenge the body’s ability to stay cool and high humidity impairs the evaporation of sweat, which is the body’s primary and extremely powerful mode of cooling. Thus, even a warm but very humid environment can increase the risk of internal body temperature rising to dangerous levels.

The differentiating factor between classic and exertional heatstroke is physical activity. Physical activity causes the increased generation of body heat and can further elevate internal body temperature. Thus, physical activity is an important situational risk factor for heatstroke (Box 1). As such, while exertional heatstroke is more likely to occur during heat exposure, it can also occur in conditions much cooler than that which could increase the risk of classic heatstroke.

In addition to environmental and situational risk factors, there are numerous personal risk factors that are associated with a higher risk of developing heatstroke (Box 2). These personal risk factors largely include aspects related to medical history such as a prior viral illness or sunburn, use of certain medications, or a diagnosis with cardiovascular disease. Other important personal factors may be considered morphological in nature, including being of advanced age and/or being overweight or obese, or be classified as more behavioral in nature, which includes adopting a sedentary lifestyle, excessive alcohol intake, lack of fluid intake (or dehydration), or lack of sleep. Thus, not all people exposure to the same environment are at risk of developing heatstroke.

Collectively, when a group of people are exposed to hot and humid conditions care must be taken to fully appraise the situational and personal factors underlying the risk of heat illness, particularly the most severe form – heatstroke.

Box 1: Environmental and situational risk factors for heatstroke:
• High air temperatures
• Elevated humidity (the amount of water suspended in the air)
• Physical activity

Box 2: Personal risk factors for heatstroke:
• Prior viral illness.
• Sunburn.
• Use of certain medications.
• Underlying cardiovascular disease.• Advanced age.
• Overweight or obesity.
• Sedentary lifestyle.
• Excessive alcohol intake.
• Lack of fluid intake (dehydration)
• Lack of sleep.