Search strategy

A literature search was conducted using Medline to identify published papers on the topic of heat-related injury/heat illness, and heat-related death among athletes. Keywords included heat illness, heat injury, child/adolescent, sports and activity. The search was restricted to articles in the English language and was focused on children and adolescents.

What is heat injury?

Heat injury occurs when excessive internal thermal energy is generated or absorbed by the human body. Heat injury covers a wide range of conditions, ranging from swelling, rash and muscle cramps in its mildest forms, through to heat exhaustion and heat stroke in its more severe forms.2 3

These events are often referred as heat “illness;” however, since an uncontrolled energy transfer is occurring, heat illness is in fact a type of injury. We are mainly used to injuries that result from kinetic energy (such as a blow to the head causing concussion), but thermal energy is an equally potent source of energy for injury causation.

Why are children and adolescents vulnerable to heat injury?

There are three main reasons why children and adolescents are considered to be more vulnerable than adults to heat injury: physiological, exposure and social/behavioural context.

Physiological

It is widely believed that children are at increased risk of heat injury for physiological reasons. The reasons typically cited include:

• children’s bodies have a higher surface-area-to-volume ratio than adults, which increases their heat absorption rate under very hot climatic conditions;

• they have a lower rate of perspiration rate than adults, and thus a reduced ability to cool through sweating;

• their metabolic rate, and consequent heat production, per kilogram of body mass is higher than that of adults at the same work rate.4 5

Recently, these long-held beliefs have been challenged. It has been argued that these concepts are not well supported by exercise physiology data.6 It is clear that exercise physiology literature lacks definitive well-controlled adult–child comparisons,6 and thus it is difficult to assess the validity of these long-held beliefs.

Exposure

Children and adolescents may be at increased risk simply because they are more likely to be exposed to vigorous physical exercise during the warm summer months. For example, each year, August preseason football practices around the US expose 17 times more high school age athletes than collegiate athletes to physical activity in hot conditions. This is because there are 1.1 million high school football players compared with 65 000 collegiate football players nationally. Additionally, children and adolescents tend to attend (and staff) outdoor summer sports camps, while adults seek refuge in indoor environments.

Social/behavioural

Primary prevention strategies for heat injury include frequent rest and hydration breaks (see “Prevention measures” section below). However, there is considerable variation between individuals in their thirst response and need for water,7 and between coaches in their practices regarding provision of water breaks.8 A study of youth in summer sports camps found that personal knowledge about the importance of hydration did not correlate with actual hydration status,7 possibly indicating the importance of social and structural factors in moderating hydration and rest break behaviour. Children and adolescents who need water or rest, but are being supervised by adult coaches, may be reluctant to interrupt structured exercise drills to take a break. They may also be prone to peer pressure to “tough the heat out.” Up to 56% of youth in summer sports camps experience significant or severe dehydration.7

Extent of the heat injury problem

Mortality

In the USA, the National Center for Catastrophic Sports Injury Research reports that from 1995 to 2008, a total of 39 football players died from heat stroke (29 high school, seven college, two professional and one sandlot).9 For the high school players, this translates to a death rate of 0.20 per 100 000 player-seasons. Based on the data collected by this centre, it seems that the majority of heat-related deaths in US children and adolescents occur in football, followed by cross-country and other running sports, but it should be noted that this reporting system is influenced by media coverage and volunteerism. Any physical activity in warm or hot conditions has the potential to induce heat injury, including indoor activities. For example, in 1997, three collegiate wrestlers died from heat-related conditions while trying to lose weight in preparation for match weigh-in.

Morbidity

For every death, there are undoubtedly many more non-fatal cases. However, accurate enumeration of non-fatal head illness cases is problematic. The innovative internet-based RIO injury surveillance system reported an incidence rate of non-fatal heat illness for high school football of five per 100 000 athlete-exposures over the 2005/2006 and 2006/2007 academic years.10 This rate was 11 times higher than the rate for all other sports combined. The second highest rate was in volleyball (one per 100 000 athlete-exposures).

A limitation of the RIO data is that this source includes only events that result in one or more days lost from participation in sport. Since an athlete could completely recover from mild heat injury within 24 h,2 it is likely that the true number of heat injury events exceeds those captured by this data source. Indeed, survey data from a group of 450 high school football programmes indicate a preseason risk of 1.4 heat injuries per school,8 which is higher than the RIO rate. However, these data come from a completely self-selected sample (a group of schools that self-selected to complete an online survey on heat injury) and thus potentially overestimate the true incidence rate.

Aetiology, pathophysiology and treatment

Heat injury occurs when internal body temperature rises faster than the body’s ability to dissipate heat. Once temperatures exceed 40°C (104°F), cell failure and breakdown begin to occur, resulting in organ dysfunction and eventually system failure. In the worst cases, death will result if injury is not recognised and/or the athlete is not cooled quickly. Thus, the two key elements to treating heat injury are (1) to quickly identify affected athletes and (2) to rapidly cool their whole body.

There is some debate about the most effective method for rapidly lowering core body temperature,11 12 but the best available evidence suggests that immediate immersion in ice-water is the fastest way to initiate core cooling.13 Emergency medical services should be contacted immediately if core body temperature exceeds 40°C (104°F), if there is persistent vomiting and resulting inability to rehydrate, or evidence of altered mental status.2 It is imperative that cooling is initiated before emergency transportation occurs.4

The damage from heat injury is largely reversible, but only up to a certain point. The key factors influencing reversibility are (1) the extent to which the core body temperature was elevated above 40.5°C and (2) the length of time the core temperature was elevated above 40.5°C.4 If the athlete spends too much time at too high a temperature (eg, more than an hour at 41.5°C), organ damage, coma and eventually death will result.4 This is why BOTH rapid identification of affected athletes AND rapid initiation of whole-body cooling are critical to recovery.

The presentation of heat injury is very heterogeneous between athletes, and can involve nausea, incoherence, fatigue, weakness, vomiting, cramps, weak rapid pulse, flushed appearance, visual disturbances, unsteadiness and collapse.2 9 This makes timely identification of cases a considerable challenge.

Risk factors for heat injury

There are no well-conducted analytical epidemiological studies, such as cohort or case-control studies, that have attempted to identify risk factors for heat injury. The available information on risk factors is entirely based on case series data, clinical observation and data from exercise physiology studies. There are few recent well-controlled exercise physiology studies of heat and exercise in children that are directly applicable to real-world field conditions.6 Actual observational studies of children exercising under field conditions are typically limited to heat injury precursor conditions, such as mild dehydration or subclinical increases in core body temperature.14 15 16 Thus, our knowledge of risk factors for heat injury is largely dependent on a group of exercise physiology studies, many of which have been limited to adults and were conducted in laboratory settings that may or may not simulate real field conditions.

Extrinsic risk factors

Fundamentally, hot and humid climatic conditions are the single most critical predisposing risk factor.4 Things that impede the body’s ability to radiate heat, or increase heat absorption, are also considered to be risk factors.4 These include too high a level of exertion, too much clothing, dark-coloured clothing, insufficient rest breaks and lack of shade.2 Global warming has also been proposed as a risk factor.17

Intrinsic risk factors

Poor acclimatisation to exercise in hot climatic conditions is considered to be an important risk factor,2 17 as are poor physical fitness and obesity.2 21 A wide range of medical conditions and medications are also thought to be implicated.2 These include alcohol, amphetamines, benzodiazepines, prior history of heat injury, sickle cell trait and recent febrile illness.2 It seems that there is considerable intrinsic variation between children in their response to heat and exercise, but the causes and determinants of this variation are unclear.6

Prevention measures

As with risk factors, there are no controlled studies of interventions, and prevention recommendations have been based on clinical observation and physiological information. Nevertheless, a recent series of systematic reviews, consensus statements and expert opinions have generated core set of prevention recommendations,2 4 5 9 18 19 20 and these are summarised in table 1. As with risk factors, much of this information comes from studies of adults, and the extrapolation to youth populations is largely untested in empirical terms.

Logically, frequent rest breaks, reductions in exercise in very hot conditions and adequate hydration are expected to reduce risk in adolescents.5 19 20 Some studies have found that children will voluntarily drink more fluid during exercise in warm or hot environments when provided with a “sports drink” (eg, Gatorade) than when plain water is given,22 23 but other studies have not replicated this finding.15 This variation likely reflects social factors and individual differences in behavioural patterns of voluntary water consumption. Irrespective of whether they increase consumption, electrolyte-based drinks are preferred, since overconsumption of water can disrupt the body’s electrolyte balance, resulting in hyponatraemia.

As indicated above, both heat and humidity play an important role in the onset of heat injury. Wet bulb globe temperature is an index that combines ambient temperature and ambient humidity data into one overall index. Published guidelines for exertion at various levels of wet bulb globe temperature are available.2 4 18 Currently, only 7% of US high school football coaches utilise wet bulb globe temperature.8

Since the initial signs of heat injury may be subtle, the advent of reliable internal heat sensors is an important recent technological development. Internal heat sensors are small electronic devices that can be ingested by the child prior to exercise. They provide rapid and timely telemetry data on internal body temperature to a handheld or belt-worn external sensor. These devices provide a valid, reliable and non-invasive means of monitoring core body temperature in young athletes.24 25 However, these monitors provide essentially only one day of measurements before breaking down within the body.

There have been repeated calls in the literature for more education of clinical and non-clinical personnel at youth sports events (table 1). However, a recent survey indicated that fewer than 50% of high school football programmes have a planned routine for regular breaks for water and rest during high temperature conditions, and over one-third of programmes do not provide athletes with any educational materials on heat injury.8 Thus, research is needed to develop better strategies for effectively communicating safety messages about heat injury to children and adolescents, their coaches and their care givers. Such research should also consider social context, in terms of parental, coach and peer attitudes to sport safety and athlete care.

Further research

Although a detailed body of exercise physiology, case observations and expert opinion exists in the literature, the field is bereft of research in several important areas. These include:

• epidemiological incidence studies of non-fatal heat injury;

• health communication and health behaviour studies;

• ongoing monitoring of knowledge, attitudes and behaviours towards heat injury;

• analytical epidemiological studies evaluating interventions and identifying risk factors.

Epidemiological incidence studies

It is surprising that there are no reliable data on the incidence of non-fatal heat injuries in youth sport. Studies are needed that utilise an injury definition that does not involve time lost from participation.

Health communications and health behaviour research

There is a rich base of physiological knowledge, case reports and expert opinion that has led to the formation of prevention strategies for heat injury.2 4 5 9 18 19 20 However, lengthy academic papers are not readily understood by children, parents and coaches, and there is no research on the most effective means to communicate this knowledge to the general sports community. The social context in which safety messaging is provided is frequently important, and this needs to be studied as an important modifier of prevention messaging. In particular, data are needed on general attitudes about safety expressed by coaches to youth athletes and how these can reinforce or negate prevention messaging. The most effective “package” for communicating information on heat injury to children, parents and coaches has not yet been identified.

Ongoing monitoring of heat injury knowledge, attitudes and behaviours

There is a need for survey data on the knowledge, attitudes and reported behaviours of children, parents and coaches towards heat injury. Ideally, these surveys should be nationally representative and ongoing.

Analytical studies of risk factors and intervention effectiveness

Although there is considerable consensus of expert opinion on prevention strategies for heat injury,2 4 5 9 18 19 20 none of these interventions have been subject to evaluation through analytical observational designs (such as prospective cohort or case-control studies). Furthermore, although these prevention recommendations are grounded in physiology and case series observations, their actual implementation in the field needs to be studied and evaluated to ensure that the major risk factors have been correctly identified and are adequately addressed by these interventions. Given the wide variations in the implementation of these measures in settings such as US high school football programmes,8 there is ample opportunity for observational evaluations of the collective and individual effectiveness of these strategies in maintaining acceptable core body temperature preventing heat injury.

Conclusions

Many excellent resources exist in the biomedical literature about heat injury in children and how to prevent it. However, research is needed on the most effective means of communicating this knowledge to children, parents and coaches. Additionally, the literature needs to be strengthened through routine behavioural surveys of knowledge and behaviour in children, parents and coaches, and through analytical designs that test the effectiveness of prevention strategies.