Table 2.

Factors involved in the intraindividual and/or interindividual variability in sweating rate and sweat [Na⁺]

	WBSR	Local SR	Local sweat [Na+]	Comments
Day-to-day (CVs)	5–7%	6–22%	5–16% (WB: 11–17%)	Includes instrument variability (1–3%)
Regional differences
Across body (% difference)	NA	200–360%	80–120%	Range includes anatomical sites typically used/accessible in field testing (back, chest, forearm, thigh, and forehead)
Contralateral sides	NA	↔	↔	Forearms and scapulas
Exercise intensity (absolute VO2)				Impacts E req
High vs. moderate vs. low	↑	↑	↑	Directly related to metabolic energy expenditure (i.e. metabolic heat production)
Environmental conditions
Temperature (↑)	↑	↑	↑	Impacts E req; ↑ radiant heat gain and therefore ↑ T c
Solar radiation (↑)	↑	↑	↑/?	Impacts E req; ↑ radiant heat gain and therefore ↑ T c
Humidity (↑)	↑	↑	↑/?	↓ Water vapor gradient leads to ↓ evaporation of sweat, which ↑ T c and the need for higher SR than calculated from E req, but prolonged exposure can lead to hidromeiosis and decreased SR
Wind (↑)	↓	↓	↓/?	Impacts E req; ↑ convective/evaporative heat loss and therefore ↓ T c
Body mass
Larger vs. smaller	↑	?	?	Related to metabolic heat production and possibly sweating efficiency
Protective equipment	↑	↑	?	↓ Evaporative and radiant heat loss, ↑ metabolic heat gain and therefore ↑ T c
Sex
Men vs. women	↑	↑	↑/↔	SR differences related to higher body mass and metabolic heat production of men, rather than sex per se; less wasteful sweating by women in humid heat
Aging
Older vs. middle-aged vs. young adult	↓	↓	↔/?	Related to decline in fitness (and associated decline in cholinergic sensitivity), rather than aging per se
Maturation
Pre vs. post-pubertal	↓	?	↓	Related to lower sweat gland sensitivity; SR differences in males only, suggesting testosterone may be involved (although direct evidence is lacking)
Heat acclimation	↑	↑	↓	↑ Cholinergic and aldosterone sensitivity; gland hypertrophy; ↑ slope of relation between SR and T c; ↓ T c threshold for sweat onset
Aerobic capacity
Higher vs. lower VO2max	↑	↑	↔/?	↑ Cholinergic sensitivity; ↑ slope of relation between SR and T c; ↓ T c threshold for sweat onset
Hydration status
2–3% BML vs. euhydration	↓	↓	↑/?	Hypovolemia ↓ slope of relation between SR and T c; hyperosmolality ↑ T c threshold for sweat onset
Menstrual cycle
Luteal vs. follicular	↔	↓	↓/↔/?	Luteal phase ↑ T c threshold for sweat onset and ↓ slope of relation between SR and T c (thus LSR lower at a given T c); effect lessens with heat acclimation
Dietary sodium				Studies involved 8–14 days on strictly controlled, modified diets
Change from moderate to high intake (8–9 g Na+)	↔	↔	↑	↓ Circulating aldosterone
Change from moderate to low intake (1–2 g Na+)	↔	↔	↓	↑ Circulating aldosterone
Exercise duration (↑)
Low intensity	↔	↔	↔	Studies involved 3–7 h of exercise and low SR
High intensity	↓	↓	↓	Related to effects of hidromeiosis with prolonged heavy sweating
Race/ethnicity	↔	↔	↔	Indigenous environmental factors are more important than race or ethnicity per se. Heat habituation (lower, more efficient sweating) may occur in people indigenous to hot or tropical climates

See text for discussion and supporting references

BML body mass loss, CV coefficient of variation, E _req required rate of evaporation for heat balance, NA not applicable, [Na ⁺ ] sodium concentration, SR sweating rate, Tc body core temperature, VO ₂ oxygen uptake, VO _2max maximal oxygen uptake, WB whole body, WBSR whole-body sweating rate, ↑ indicates increase in the sweating response, ↓ indicates decrease in the sweating response, ↔ indicates no effect on the sweating response, ? indicates limited data available