Prickly Heat (also known as millaria) is a red, bumpy rash that comes on in areas of increased sweating, often areas that are covered by clothing. It often causes a prickling or itchy sensation and is due to blockage of the sweat ducts towards the top of the skin. It tends to occur more commonly in hot, humid environments. It can be prevented by avoiding excessive heat and by wearing loose fitting clothing with natural fibres such as loosely woven cotton or linen.
Polymorphous Light Eruption (PLE) is a different rash that tends to occur in areas of light exposure, particularly in the spring or early summer or at the start of a holiday. It is thought to affect up to one in ten people and causes a rash which can be itchy or painful and which appears within hours of sun exposure, often in areas which are covered during the rest of the year and then usually disappears over the next 1-7 days. The rash is red and may be bumpy, contain small blisters or even cause large blotches. If you are prone to PLE you could find it helpful to see a Dermatologist. Use of sunblock is important, but it must contain good UVA filters and be broad spectrum. Your Dermatologist can supply you with strong steroid creams or even steroid tablets which you could start at the first sign of trouble. Sometimes tablets or even ultraviolet light treatment before you go away can prevent the problem occurring.