What changes in your body
With repeated heat exposure, the body starts sweating earlier and in greater volume, spreads sweat more evenly, loses less salt, and lowers the heart-rate and core-temperature cost of the same work. Most of this gain accrues over 7 to 14 days of gradually increasing exposure. Lose it, too: a week or two away from the heat and the adaptation fades.
Why the first days are the trap
An unacclimatized body is running its cooling system before the upgrades install. That is why heat-safety guidelines, for the military, for OSHA-covered workplaces, and for school athletics, ramp intensity up over the opening days rather than starting hard. In the USVI it matters most for two groups: new arrivals and tourists from cooler climates, and workers and athletesreturning after time away.
The practical rule
Until you have several days of heat behind you, treat everyWBGT band one level more seriously than the chart says, start outdoor activity gently, and lean hard on shade, water, and rest. Thelive map gives you the band; acclimatization tells you how much margin to add.