Sun and heat allergies, like other allergies, are an immune system response to a substance that is normally harmless. True sun and heat allergies occur only occasionally in certain sensitive people. It is not clear why some people develop allergies to sun and heat, although there is evidence that some forms are inherited.

An allergic response to sunlight occurs when the immune system recognizes a part of sun-altered skin as a threat to the body and responds by triggering the allergic cascade. Sun allergies usually appear on the exposed skin as a rash or hives (urticaria). There are two forms of sun allergies: polymorphous light eruption and solar urticaria.

Some conditions that are referred to as sun allergies are actually skin reactions caused by a combination of an ingested or topically applied substance, and exposure to sunlight. These conditions are photosensitivity and photoallergic eruptions. They are sometimes called sun poisoning.

Heat allergies are similar to true sun allergies. They are caused by an immune system reaction to heat, and usually appear on the skin as hives. Heat allergies may be caused by the sun, or by sweating and hot showers. These hives can affect the entire body except for the armpits, palms of the hands and soles of the feet. Heat allergies also come in two forms: prickly heat and heat urticaria.

In certain rare cases, sun and heat allergies may cause anaphylactic shock, a severe and potentially life-threatening allergic reaction that requires emergency medical care.