Warm and Humid Environment

The Particularity Of Warm And Humid Environment

Pr Olivier Hue Ph.D Laboratory ACTES, University of the French West Indies

The detrimental effect of high environmental temperature on aerobic performance has been well established (Galloway and Maughan 1997; Morris et al. 1998) and it is well known that exercise-heat acclimation involves a complex set of adaptations that serve to reduce physiological strain and improve ability to exercise in a hot environment (Armstrong and Maresh 1991). However, although considerable information has been gathered on the physiological adaptation to hot/dry climates, information on acclimation to hot/wet climates is still limited (Shapiro et al. 1998).

Earlier studies, cited by Pandolf (1998), have demonstrated that nearly complete heat acclimation for both hot/dry and hot/humid environments occurs after 7 to 10 days of exposure. In agreement with these earlier studies, Nielsen (1998) showed that physiological adaptations were similar in humid and dry heat when subjects exercised daily for 8-12 consecutive days.

Furthermore, 66% to 75% of the physiological adjustments were seen in 4 to 6 days (Pandolf et al. 1988; Sawka et al. 1996), and it has been suggested that high levels of aerobic capacity aid the individual to acclimate (Pandolf et al. 1988). The current recommendation is that competitive athletes expected to participate in an event involving heat stress should acclimate/train in the heat for at least 5 days beforehand to help maximize performance (Pandolf 1998).

The above results are in contrast to a few recent studies specifically comparing the effects of acclimation in a hot/dry climate to those created by a vapor barrier suit (NBC suit) that creates a hot/wet microclimate regardless of the ambient conditions (Shapiro et al. 1998). The results showed that under the same exercise intensity, the strain was higher (rectal temperature and heart rate were higher by about 1°C and 30 bpm, respectively) when subjects were tested with the suit. A similar study obtained the same results even after 10 days of acclimation, thus clearly demonstrating the difference between hot/dry and hot/wet acclimation conditions (Aoyagi et al. 1994).

Unfortunately, the data to date on hot-wet conditions, in which the high required evaporative cooling exceeds the evaporative capacity of the environment (Shapiro et al. 1998)—thereby inducing an irreversible decrease in performance (Nielsen 1996)—have only been gathered in laboratory (Aoyagi et al. 1994, Cheung and MacLellan 1998a, 1998b) or they have been extrapolated (Nielsen 1996). Despite the fact that these conditions are found naturally in a tropical climate, studies exploring the physiological responses elicited by a hot/wet climate during outdoor testing are very few.

The specificity of the tropical (i.e., hot and wet) environment is the high degree of humidity of the environment which do not permit adequate evaporation. Thus, in situations where the environmental evaporative capacity is limited, adherence to the strategy used in a hot/dry climate (i.e., reducing heat storage by increasing the sweating rate) creates a physiological disadvantage (Shapiro et al. 1008) that results in severe dehydration.

The only way to achieve a match between heat gain and heat dissipation during outdoor exercise is by reducing the metabolic heat production, thus the intensity.

Exercising in warm and humid environment

Case of swimming

A particularly interesting question concerns the extent to which swimming affects the process of acclimating to a tropical climate. The thermal balance of swimmers is well known to be regularly challenged because of the high heat transfer coefficient of water (Wade and Veghte, 1977).

Although most studies have reported the effect of cold water on thermoregulation (Chollet et al., 2000; Costill et al. 1985), swimming in high temperature increases heart rate in relation with hyperthermia and increased skin circulation (Geiser et al. 2001) and increases oesophageal temperature to the same extent as running in a hot environment (Geiser et al. 2001). It can thus certainly be considered as a discipline that induces a high thermoregulatory stress in hot/wet climate.

Even when the water temperature is comfortable in hot/wet climate, we recently demonstrated that aerobic performance of competitive young swimmers may be affected by the combination of the high convection and conduction capacity of water and the use of the silicone swim cap that is usually worn during training sessions and competitions (Figure 1). Indeed, swimming with a silicone cap on the head could be a stress for the central command of sudation, i.e., the hypothalamus, which is particularly sensitive in children ((Bar-Or, 1978; Meyer et al. 1992).

Physiological limitations might occur and long distance swim performance could be curtailed in children because of their naturally less efficient thermoregulatory processes (Bar-Or, 1978; Meyer et al. 1992; Fujishima et al. 2001). Thermoregulation during training sessions may decrease training intensities and thereby have a negative effect on future performance in competition.

We therefore recommend that young competitive swimmers and their coaches envisage removing the silicone swim cap during training sessions and competitions of 800-m or more in tropical environmental conditions.

Figure 1: Performance obtained by 11-12 years acclimated children in a 31°C water during a 800-m front crawl swimming, with (SC) and without swim cap (WSC)

Case of cycling

Although it has been demonstrated that cycling performance is largely decreased by both dehydration and/or hyperthermia when performed in laboratory (Ekblom et al. 1970; Barr et al. 1991; Nicol et al. 1991; Montain and Coyle, 1992) in which convective and evaporative cooling is likely to be less than those encountered in out-of-doors competition, the Elite cyclists ride at speed ranging from 20 to 50 km.h-1, generating an equivalent facing windspeed.

Adams et al. (1992) showed that subjects reached higher rectal and oesophageal temperatures when they exercised in windstill conditions compared with facing air velocity of 12.6 km.h-1. Recently, Saunders et al. (2005), comparing cycling at different air velocities in laboratory in warm and humid conditions, demonstrated that when air velocity is 33 km.h-1 or higher, the evaporative capacity of the environment is increased so that excess heat is reduced in proportion that a higher rate of fluid ingestion has no influence of heat storage, body temperature, sweat rate, heart rate or rate of perceived exertion.

These conclusions were corroborated by Hue et al. (2006) during a field study. It was demonstrated that physiological parameters or performance noted during a major sporting event realized in tropical conditions (i.e., The Tour de Guadeloupe, a multi-day (9 days) cycling race) were not affected differently than that noted during such event performed in neutral conditions. 

Case of running Because running is performed much more lower than cycling, the effect of air convection is of less importance. When analyzing in poor air velocity conditions, the performance during running is strongly affected by hot and humid climate (Costill et al. 1970; Gisolfi and Cipping, 1974).

However the lack of appropriate airflow substantially reduces the combined heat transfert coefficient (Gagge and Gonzales, 1996) and may over-estimate physiological strain (Cheuvront et al. 2004). Heat production in running has been described as depending on the athlete’s body mass and speed (Nielsen, 1996) and on running event of long time duration as the Marathon, the thermoregulation could be a problem (Kenefick et al. 2007).

Recently, studying the impact of weather on marathon-running performance, Ely et al. (2007) demonstrated a progressive slowing marathon performance as the WBGT increases from 5°C to 25°C, in men and women but more negatively for slower populations.

As noted above, the body mass is a factor associated with performance during running in hot-wet conditions. Because heat production in running depends on body mass and especially muscle mass and heat loss depends on surface area, an athlete’s body mass has an approximate two fold greater effect on heat production than on heat dissipation (Dennis et al. 1999).

Running speed also increase heat production more than heat dissipation and while heat production depends on absolute running speed, heat exchange by convection and evaporation is determined by the square root of the velocity air flow over the skin (Dennis et al. 1999) which implied that at 35°C and 60 % rh, the running speed at which a 45-kg athlete could maintain thermal balance is almost 7 km.h-1 faster than that of a 75-kg athlete (Dennis et al. 1999).

In fact, in hot and humid climate in which heat dissipation mechanism are at their limits, runners with lower body mass have a distinct thermal advantage: lighter runners produce and store less heat at the same running speed; hence they can run faster or further before reaching a limiting rectal temperature (Marino et al. 2000).

Other activities Adaptation or effect of hot and humid climate on intermittent exercise has also been studied. It was demonstrated that hot and humid climate reduced short duration sprint running (Maxwell et al. 1999) performance and sprint time in a 90-min protocol specific to soccer (Morris et al. 1998; 2000) or during cycling intermittent sprint protocol (Noakes et al. 2001). 

When sprint exercises are considered, this climate did not demonstrated negative effects when subject are euhydrated (Backx et al. 2000) and could even be considered as potentially positive in some circumstances (Racinais et al 2004): see the Introduction (Muscle contraction and Temperature) for more details.

Adaptation to hot and humid climate when exercising

Case of acclimated versus non acclimated people Heat acclimation (HA) refers to an increase in heat tolerance while working or exercising under stress conditions (Armstrong and Maresh, 1991, Nielsen, 1994). The physiological 

  Figure 2: Rectal temperature obtained in neutral environment (N), or 2, 7 and 14 days after arrival in tropical environment. (* different from neutral environment, a: time effect)

adaptation of HA include improved cardiac output, lowered heart rate, increase in stroke volume, sweat rate and blood plasma volume, decreased core temperature and mean skin temperature at rest (Wyndham et al. 1976), rectal temperature at rest (Buono et al. 1998) and oxygen consumption at a given work rate, earlier sweating during exercise and decreased sodium chloride losses in sweat and urine (Armstrong and Maresh, 1991). 

Although some of the processes have been demonstrated to occur within 8 and 14 days also in tropical climate (Hue et al. 2004; Voltaire et al. 2002; Figure 2), the full acclimatation expected by sportsmen (i.e., to be able to reproduce the same performance in a stressfull climate after acclimation than the performance obtained in neutral environment) failed to be of reality.

We demonstrated a significant lower performance during a maximal incremental outdoor running test in high level triathletes, after 8 and 14 days of acclimatation in tropical climate (Hue et al. 2004; Voltaire et al. 2002; Figure 3), putting into evidence the difficulty to fully acclimate in tropical environment. Indeed, the subjects of the studies were well-trained and trained daily more that 2 hours in tropical climate, two characteristics reported to facilitate the acclimatation process (Lind and Bass 1963; Pandolf et al. 1977).

Figure 3: The maximal running speed noted during an incremental running test was significantly lower 2 days after arrival in tropical climate (D2) and never attained the performance obtained in neutral environment (N) in high-level triathletes, at day 7 nor day 14.

Some authors demonstrated that acclimated people (i.e., people natives from or leaving in tropical climate for numerous at least 2 years (Bae et al. 2006)) showed heat tolerance with suppressed sweating (Matsumoto et al. 1993; Ogawa and Sugenoya, 1993; Lee et al. 1997; Bae et al. 2006) that provides the advantage of preserving body fluid and osmoregulation in order to better thermoregulate.

However, this adaptation was demonstrated not to be enough when exercising, at least during long time exercise performed at high intensity: Saat et al. (2005) demonstrated that natives from tropical climate had lower sudation and higher skin (and higher skin temperature (i.e., resulting in higher vapor pressure at the skin surface thus improved evaporation) and rectal temperature at 40% VO2max but Voltaire et al. (2003) demonstrated that natives to tropical climate performed a significantly lower 1h cycling performance at 80 % FCmax in tropical than in neutral climate (Figure 4).


Figure 4: Decrease in power output (P) during 60-min if cycling in tropical (red line) versus neutral (blue line) climates in natives from tropical climate. (i.e., the intensity was stand at 80% HR max)

Sex and/or anthropometric effects

Advantages of a smaller bodymass when distance-running in warm and humid conditions have been demonstrated (Dennis et al. 1999; Marino et al. 2000; Noakes et al. 2004): because the ratio between body surface area and mass is an important determinant of heat loss and gain when exercising in hot environment (Epstein et al. 1983) and because heat production during running depend on body (i.e., muscle mass) and heat loss depend on surface area, an athlete’s body mass has approximate two fold greater effect on heat production that on heat dissipation (Dennis et al. 1999). 

Notwithstanding the ethnical difference, Noakes et al. (2004) demonstrated better performance of African runners (i.e., with smaller body size) in warm and humid but not in cool environmental conditions than Caucasian ones (i.e., with greater body size); both being equally acclimated. The important effect of the body surface area/mass ratio let suppose that women could be in situation to better thermoregulate in hot and humid climate. 

Normally men displayed a better thermoregulation capacity, due to both a larger body mass that enables heat accumulation and a greater sweat rate.

In hot and humid climate, men are unable to benefit from their higher sweat rate due to the reduced evaporative capacity of humid air, however, their better sweat loss induce dehydration then loss in exercise performance and it has been demonstrated that the lowest performance of women than men in hot and humid running was only due to their lowest lean body mass (i.e. for similar body mass) and not to lowest thermoregulation (Wright et al. 2002); in luteal or follicular phase (Wright et al. 2002), at least in trained women (Kuwahara et al. 2005).

Optimisation of performance in hot and humid environment.

Because hot and humid environment decrease the performance (if no convection is available), some advertisements has to be given to athletes who aim to compete in hot and humid environment.

First, it has been demonstrated that well-trained subjects would acclimate better than less trained-ones, high aerobic capacities being evoked as potentially involved in acclimation process (Lind and Bass, 1963). Second, even 14 days of acclimation was not sufficient to completely acclimate (i.e., to have a performance in hot and humid environment similar to that of neutral climate) as demonstrated by Voltaire et al. (2002), it was demonstrated that at 8 days the acclimation process has emerged and was ended at the 14 (i.e; physiological adaptations).

Third, it has been demonstrated that 2 daily hours of training facilitate the acclimation process (Pandolf et al. 1977) and fourth, hyperthermia and dehydration are worst that hyperthermia alone (Sawka and Noakes, 2007).

Thus an athlete with high-aerobic capacity, well-trained and coming in the hot and humid climat at least 10 days before the sporting event, that will hydrate as recommended (Sawka and Noakes, 2007) could certainly performed as good as possible, considering the insurmountable deleterious effects of the hot and humid climat on long time sporting performance.


Training in tropical climate in order to optimize the following performance obtain when returned in neutral climate seems to be an interesting perspective. The tropical training could then become as much interesting that altitude one.

Indeed, hypervolemia has been demonstrated as one of the most important adaptation (and performance-related adaptation) that occur during aerobic exercise (Brun et al. 1998) and relative humidity is the most important factor of environmental stress (Gleeson, 1998), inducing a high level of sweating then lowering heart rate and internal body temperature in relation with hypervolemia (Nielsen, 1998).

Recently we demonstrated the positive effect (i.e., a 10% increase in performance in a 400-m front crawl swimming) of 8 days training in tropical climate for national to international-level swimmers (Hue et al. 2007). However, similarly to that noted for altitude training, the decay of acclimation to tropical climate is variable and occurs between 1 week and 1 month (Pandolf, 1998) and has to be carefully study in other sports.