Because your sweat glands have not acclimatized: miliaria is blocked, inflamed sweat ducts - backed-up sweat raises the itchy bumps - and it thrives in hot humid climates, the exact shock of flying a temperate body into tropical heat with sunscreen layers and all-day sweating.
Classic pattern: days 2 to 4 of the trip, under waistbands and in skin folds, resolving once skin cools and dries. Care: loose fabrics, cool showers, AC breaks, no heavy creams on the rash, calamine for itch. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper .
What to do
Common questions
Why do I get heat rash on vacation?
Because heat rash is what happens when your sweat glands meet a climate they have not acclimatized to - and a vacation delivers exactly that shock. The mechanism, per DermNet: miliaria is caused by blockage and inflammation of the eccrine sweat ducts - sweat that cannot escape backs up into the skin and raises the itchy pinprick bumps (prickly heat) or clear tiny blisters, depending on how deep the blockage sits. It thrives in hot, humid, tropical conditions - precisely the vacation trifecta of a body flown from a temperate spring into 35-degree humidity, sunscreen and occlusive clothing layers, and all-day sweating. The vacation pattern is classic: days 2 to 4 of the trip, under waistbands, in skin folds, on the chest and back. It resolves on its own once the skin cools and dries - usually days. Management: loose breathable clothing, cool showers, air conditioning breaks, avoid heavy occlusive creams on the rash, and calamine or a mild hydrocortisone for the itch. The prevention play for the next trip: lightweight fabrics, shade-and-AC midday, and letting skin dry fully after swimming before re-layering. Rash that spreads fast, gets angry, or comes with fever - that is past prickly heat and into infection territory.