Recognizing it
Watch for heavy sweating with cool, pale, clammy skin; a fast, weak pulse; muscle cramps; tiredness or weakness; dizziness or headache; and nausea. The person is usually still sweating and still mentally clear. Catching it here, and stopping activity, is what prevents the far more dangerous next stage.
The line to heat stroke
Heat stroke is a life-threatening emergency. The tells are hot skin (sweating may have stopped), a temperature that keeps rising, and, most importantly, changes in mental status: confusion, slurred speech, agitation, or collapse. If you see those, call emergency services and start cooling the person immediately with water, ice, or wet cloths while you wait.
Why the USVI raises the risk
High humidity means sweat evaporates poorly, so the body's cooling runs at a deficit, and full tropical sun adds radiant load a thermometer never shows. That combination is exactly what WBGT measures, which is why watching the WBGT band, taking shade and water breaks, and pacing exertion are the practical defenses. Infants, older adults, and people on certain medications reach exhaustion sooner.
This page is general education, not medical advice. In an emergency, contact local emergency services.