Sun & Heat Allergies – Diagnosis, treatment and prevention

Sun Heat Allergies


Sun and heat allergies involve skin rashes that occur on exposure to sunlight and/or hot temperatures. True sun and heat allergies are rare. Sensitivity to sunlight (photosensitivity) or heat is much more common, and can be the result of using certain medications or topical skin products. Skin rashes caused by photosensitivity are known as photoallergic eruptions.

Sun and heat allergies cause skin reactions in the form of hives and rashes. Hives are red, raised and itchy areas of the skin that can appear singly or in batches. They can vary dramatically in size.

About sun and heat allergies

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.

Polymorphous light eruption

Polymorphous light eruption (PMLE) is the most common form of sun allergy. The condition is more commonly seen in women.

The symptoms of PMLE include:

  • Itchy, burning rash on sun-exposed portions of the neck, chest, arms and lower legs

  • Periods of chills, headache, nausea and feeling ill

  • Red, flat or raised areas, fluid-filled blisters or tiny areas of bleeding under the skin

The itchy rash associated with PMLE appears on sun-exposed skin within the first two hours of sun exposure. The rash can take many forms, though its appearance is usually consistent during subsequent appearances in the same individual. PMLE symptoms usually first begin during early adulthood, but can appear at any age. Symptoms appear during the early spring, and gradually decrease as the skin becomes less sensitive to sunlight. Also known as hardening, this type of desensitization usually lasts through the end of summer. The PMLE rash may disappear entirely during the summer, and then return at full intensity the following spring. Some patients experience temporary remissions.

PMLE typically appears on limited areas at first. However, it often becomes more extensive during subsequent summers. PMLE-related lesions usually heal without scarring.

Actinic prurigo is the hereditary form of PMLE that occurs among those of American Indian background of North, South and Central America. The symptoms are more intense than those of PMLE and often first appear during childhood. Several generations of the same family may have a history of this skin condition. Actinic prurigo does not improve with time.

The symptoms of actinic prurigo are the same as PMLE but are concentrated on the face, especially around the lips. Symptoms are sometimes located in the eye membranes (conjunctiva) causing brown spots and extreme sensitivity to light. The rash will usually disappear within two to three days if sun exposure is avoided.

Long-term protection from sunlight is necessary to avoid worsening both conditions. However, many people with PMLE improve significantly within seven years of first displaying symptoms.

Photosensitivity and photoallergic eruption

Photosensitivity is the most common type of reaction to the sun. It is not a true allergy. Photosensitive individuals often develop a rash because of a strong sensitivity to sunlight. In rarer cases, an individual will be sensitive to indirect sunlight or even fluorescent indoor lights. Certain medications or topical creams can increase the skin’s susceptibility to sunlight in these individuals.

Photoallergic eruption is an allergic reaction to a combination of an ingested or applied chemical to the skin, and sunlight. Some medications cause people to become sensitized to the sun and experience allergic reactions after taking the drugs and being exposed to sunlight. Other triggers of photoallergic eruption are lotions or medications that are applied to the skin followed by exposure to the sun. It is the combination of ultraviolet rays (an invisible form of radiation) from the sun and the chemicals in the substances that trigger the allergic reaction.

There are many medications that may cause sun sensitization in certain individuals, including:

  • Birth control pills
  • Antibiotics
  • Antihistamines
  • Nonsteroidal anti-inflammatory drugs (NSAIDs)
  • Antidepressants
  • Herbal remedies (e.g., St. John’s Wort)
  • Cold medications
  • Blood pressure medications

Other triggers include:

  • Cosmetics
  • Perfumes
  • Plants
  • Sunscreen

The symptoms of photosensitivity/photoallergic eruption include an itchy, red rash or tiny blisters. In some cases, the rash may spread to skin covered by clothing. Symptoms may not appear until one or two days after exposure and will usually disappear after the problem chemical/medication is identified and its use is discontinued.

A combination of covering the skin while in the sun and using sunscreen will help prevent the occurrence of rashes in sensitive individuals. However, sunscreen should not be used if it is an allergen capable of causing a photoallergic eruption.

Some serious diseases (e.g., lupus, porphyrias) cause extreme sun sensitivity. For patients with these diseases, it is essential to avoid sun exposure as much as possible.

Solar urticaria

Solar urticaria is a rare skin condition that most often affects young women. It is a true sun allergy that involves a response from the immune system. It produces hives on sunlight-exposed skin soon after exposure.

Hives are welts that appear on the skin surface singly or in groups. They can vary in size from as small as a pencil eraser to as large as a dinner plate and may overlap to create large areas of swelling. Hives are itchy and they may also burn and sting. Mild cases of hives are usually temporary and not a threat to a person’s health. However, there may be a recurrence of hives from one local area to another during the same allergic reaction. If hives worsen or persist for several days, the patient should contact a physician.

With solar urticaria, hives usually appear on sun-exposed skin within minutes of exposure. Individual hives typically last for several hours. Though these hives will often disappear without treatment, hives usually return on further sun exposure. Because of the constant threat of recurrence, this condition can be quite disabling and difficult to manage.

Heat allergy

Heat allergy is an allergic reaction of the skin to heat that causes a rash or hives to erupt on the skin. Hives related to heat allergy are usually localized to the area of heat exposure, but may sometimes be generalized all over the body. The generalized form of heat allergy-related hives may take years to disappear completely.

Skin reactions to heat are similar to skin reactions to sun. There are two common types of heat reactions:

  • Prickly heat. A nonallergic skin rash from heat often experienced by infants in the diaper area or in areas covered by clothing. It is caused by clogging within the narrow sweat ducts located just under the skin. It often results from sweat forming under hot skin and friction from clothing. Prickly heat will usually clear up by keeping the skin cool and dry.
  • Heat urticaria. An allergic skin reaction that causes hives to appear on skin exposed to heat. Heat urticaria may require desensitization through controlled exposure to heat as a treatment. This type of condition is very rare.

While extremely rare, a heat allergy can cause the potentially fatal condition known as anaphylactic shock. This condition requires emergency medical treatment. The signs and symptoms of anaphylactic shock include:

  • Difficulty breathing
  • Low blood pressure
  • Dizziness

Diagnosis, treatment and prevention

The diagnosis of sun and heat allergies begins with a physician consultation. The physician will develop a detailed medical history. The physician may ask questions about any family history of allergies and asthma, and inquire about exposure to possible irritants, current medications, over-the-counter drugs, topical agents or any herbal products. It is important to discuss the use of all current medications or herbal remedies even if they are not used daily. The physician will examine the skin and ask questions about when the skin reaction occurs in relation to sun and heat exposure.

The physician may want to perform more detailed tests including:

  • Skin biopsy. Laboratory test that examines a small piece of skin that has been removed by the physician.

  • Blood tests. Tests that determine the amount of a given substance in the blood and identify abnormally high or low levels. Blood tests can identify any underlying diseases or conditions that may be causing the skin reaction.

  • Phototesting. Test that exposes a small area of skin to measured amounts of ultraviolet light (an invisible form of radiation emitted by the sun, as well as sun lamps and tanning beds). If skin symptoms appear, the test will confirm that symptoms are related to sun exposure. Symptoms will usually appear within 30 minutes of exposure.

  • Allergy tests. Tests that help to identify specific allergens. They may be skin tests, where the skin is pricked or injected with an allergen. If a red and swollen area of the skin appears at the test site, the test is positive for that allergen. Blood tests (e.g., RAST test) may be used to measure the levels of IgE in response to the introduction of a specific allergen.

Avoidance is the best treatment and prevention method for rashes or hives induced by exposure to sunlight or heat. For people who are sun-sensitive, sun protection is necessary regardless of weather conditions, and even in the shade. Covering the skin with clothing while exposed to sunlight will help to avoid the development of hives. Sunscreen or sunblock should be applied thoroughly on all areas of the skin that could be exposed to the sun (unless the sunscreen is an allergen capable of causing a photoallergic eruption).

Since sunscreen does not completely block the sun’s rays, patients using sunscreen may still develop a reaction when exposed to the sun. Therefore, sunscreen should be used in conjunction with the avoidance of sunlight and not as a substitute for it.

Heat exposure can be avoided by covering the head and wearing cool, loose-fitting clothing when exposed to heat. Avoidance of hot baths or showers or other conditions involving warm temperatures is necessary. For people who have severe reactions and live in extremely hot climates, relocation may be beneficial.

Photoallergic eruptions can be treated by avoiding the sun while taking any medications that aggravate the condition. If other types of medications are available, discussion with a physician may identify alternate treatments that may not cause a photoallergic reaction.

A tepid shower or oatmeal bath may temporarily relieve the itchiness and irritation associated with mild hives. Cool compresses may also help with this type of discomfort.

Medications may be recommended or prescribed to treat moderate hives. The standard medications are antihistamines, which are available in both prescription and non–prescription forms. For more severe cases, corticosteroids may be recommended for the rapid reduction of swelling and itching. In extreme cases of solar urticaria, antimalarial drugs may be prescribed for their anti–inflammatory properties. Extreme cases of actinic prurigo may require a prescription of thalidomide for its immune system suppression properties.

Desensitization to sunlight or heat for PMLE, solar and heat urticaria are possible treatments for severe cases. Desensitization involves limited and controlled exposure to the allergen in a medical setting. A short dose of ultraviolet light is used to desensitize patients with PMLE and solar urticaria. Treatment of PMLE with a six week course of desensitization often provides a patient with protection for the entire summer. Limited and controlled exposure to heat may be used for heat urticaria.

Severe hives that create the potentially life-threatening allergic reaction known as anaphylactic shock should be treated under emergency medical care. Epinephrine injections (allergy kit) are required to reduce swelling and rapidly open breathing passages.

Questions for your doctor

Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions related to sun and heat allergies:

  1. Do my symptoms suggest that I have an allergy or sensitivity to sun or heat?

  2. What methods will you use to determine the cause of my symptoms?

  3. What are my treatment options?

  4. Could my medications be causing my symptoms?

  5. Does the allergy pose a danger to my overall health?

  6. How long will it take for my rash to go away?

  7. Should I expect to have similar reactions again in the future?

  8. How can I prevent reactions to the sun/heat in the future?

  9. If I wear sunscreen, is it safe to spend time in the sun?

  10. Are my children more likely to develop sun or heat allergies because I have the condition?
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