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Exercise-heat tolerance (EHT) in children is influenced by many physiological factors, including sweat gland activity, cardiac output, exercise economy, ability to acclimate to heat, and maturation of organ systems. It is generally believed that children cannot tolerate hot environments as well as adults, although some children exhibit EHT that is superior to that of adults. There has been no research showing large exercise-induced differences between the core body temperatures of children versus adults, but differences in the time to onset of syncope and fatigue have been observed. This suggests that the greatest risk of heat illness for children is heat exhaustion (i.e., cardiovascular instability) and not heat stroke (i.e., hyperthermia). Therefore this review (a) examines the conclusions of previous studies to clarify misinterpretations of data, and (b) identifies research questions that require future study.

Athletes and researchers could benefit from a simple and universally accepted technique to determine whether humans are well-hydrated, euhydrated, or hypohydrated. Two laboratory studies (A, B) and one field study (C) were conducted to determine if urine color ( ${\text{U}}^{\text{col}}$ ) indicates hydration status accurately and to clarify the interchangeability of ${\text{U}}_{\mathrm{col}}$ , urine osmolality ( ${\text{U}}^{\text{osm}}$ ), and urine specific gravity ( ${\text{U}}^{\text{sg}}$ ) in research. ${\text{U}}_{\mathrm{col}}$ , ${\text{U}}_{\mathrm{osm}}$ , and ${\text{U}}_{\mathrm{sg}}$ were not significantly correlated with plasma osmolality, plasma sodium, or hemato-crit. This suggested that these hematologic measurements are not as sensitive to mild hypohydration (between days) as the selected urinary indices are. When the data from A, B, and C were combined, ${\text{U}}_{\mathrm{col}}$ was strongly correlated with ${\text{U}}_{\mathrm{hg}}$ and U„_sm. It was concluded that (a) ${\text{U}}_{\mathrm{col}}$ may be used in athletic/industrial settings or field studies, where close estimates of ${\text{U}}_{\mathrm{sg}}$ or ${\text{U}}_{\mathrm{osm}}$ are acceptable, but should not be utilized in laboratories where greater precision and accuracy are required, and (b) ${\text{U}}_{\mathrm{osm}}$ and ${\text{U}}_{\mathrm{sg}}$ may be used interchangeably to determine hydration status.