You do not have to be doing anything strenuous to become dehydrated in a hot, humid climate. A day of walking, time outdoors and prolonged heat exposure can increase fluid losses. The reason it catches people out is that the early symptoms arrive dressed as something else. The headache gets blamed on the sun, the tiredness on the flight, the short temper on traffic. This is a guide to reading those signals in order, from the ones that are merely annoying to the ones that need