Mineral Based Sunscreen Does Not Alter Core Temperature Responses To Heat Stress In Hot-dry Conditions In Young Adults

Outdoor athletes routinely eschew using sunscreen due to perceptions that sunscreen may impair thermoregulatory heat loss, but past studies examining the impact of sunscreen application on thermoregulatory responses have been equivocal. PURPOSE: To determine (1) the effects of a mineral based sunscreen application on critical environmental limits (i.e., temperature/humidity thresholds above which heat balance cannot be maintained) and (2) the rate of rise in core temperature during compensable and uncompensable heat stress in young adults. METHODS: Five subjects (1 M/4F; 25 ± 2 yr) completed two experimental visits each; one visit with a mineral-based sunscreen applied to all exposed body surfaces and a control visit with no sunscreen application. During experimental visits, subjects were exposed to progressive heat stress in an environmental chamber while walking on a treadmill at 3.5 mph, 4% grade. After a 30-min equilibration period, dry-bulb temperature (Tdb) was increased every 5 min while ambient water vapor pressure (Pa) was held constant at 12 mmHg until an upward inflection in gastrointestinal temperature (Tgi) was observed (i.e., the critical environmental limit). We compared critical environmental limits and the change in gastrointestinal temperature (ΔTgi) over time during compensable and uncompensable heat stress between the two conditions. RESULTS: Critical Tdb at which the Tgi inflection point occurred was not different between conditions (without sunscreen: 43.2 ± 3.3 °C; with sunscreen: 41.4 ± 2.5 °C; p ≥ 0.30). Similarly, the ΔTgi during compensable (without sunscreen: 0.12 °C/h; with sunscreen: 0.14 °C/h; p ≥ 0.65) and uncompensable (without sunscreen: 1.18 °C/h; with sunscreen: 0.72 °C/h; p ≥ 0.27) heat stress was not different between conditions. CONCLSUIONS: These data suggest that application of a mineral based sunscreen does not alter critical environmental limits or core temperature responses to heat stress in hot-dry conditions.