Allergic reaction heat rash

Close up of a shoulder with a rash being inspected by someone in surgical gloves. It could be allergic reaction heat rash.

Human beings can’t survive without a stable body temperature. So we have a highly effective (most of the time) thermostat to regulate it. You get hot and your body produces sweat. The sweat uses your body heat to evaporate and your temperature should hopefully go down.

Feeling hot and sweaty is uncomfortable, but if you have cholinergic urticaria you could develop hives in addition to other symptoms. Cholinergic urticaria is a type of chronic inducible urticaria. This means the hives tend to last more than 6 weeks and are usually triggered by something. In the case of cholinergic urticaria anything that raises your body temperature, such as stress or eating spicy foods is what triggers the hives. Some people can also experience acute urticaria, where hives appear than disappear within 24 hours. This type of urticaria doesn’t usually last more than 6 weeks.

Cholinergic urticaria usually first appears in people between the ages of 10 and 30 and affects males more than females.If you have other forms of chronic hives, this can also increase your risk of developing heat allergy rash.

The word cholinergic describes the part of the nervous system that controls muscle contraction, heart rate and the width of blood vessels.

What causes allergic heat rash?

Cholinergic urticaria can develop when you’re hot enough to sweat. For some people reaching a certain temperature may trigger the release of histamine – a chemical your body produces during an allergic reaction. It can cause inflammation and heat allergy rash symptoms.

Any actions you take that raise your body temperature have the potential to cause a breakout of hives. This includes taking a hot bath or shower.

Other causes of allergic heat rash could include:

  • Exercising
  • Eating hot or spicy foods
  • Emotional stress
  • Using a blanket to keep warm
  • Wearing warm clothes
  • Sitting in a sauna

Simple guide to skin allergies

A close up of hands. The person is moisturizing their skin because of skin allergies.

Allergic heat rash signs and symptoms

When you’re experiencing cholinergic urticaria you may notice:

  • Wheals, or small raised bumps
  • The bumps feel itchy or cause a burning sensation
  • Flushing around the bumps
  • Mild swelling from bumps joining together

You may first see symptoms on the and neck, which could then spread to the limbs and face. Usually, symptoms clear within 90 minutes, providing you take action to cool down.

A rare complication of heat allergy rash is anaphylaxis. Researchers don’t fully know why some people with heat allergy rash go on to develop anaphylaxis, but they think it’s probably related to exercise-induced anaphylaxis. This happens when anaphylactic symptoms occur after you’ve done physical activity. Food-dependent anaphylaxis can also occur after exercising.

Usually, it develops after a person has eaten something they’re possibly allergic to an hour before doing physical activity.

Anaphylaxis is a severe systemic allergic reaction. It can come on suddenly and be life-threatening so it should be treated as a medical emergency. You must seek medical help immediately (more about treatment here).

Signs and symptoms of anaphylaxis include:

  • Widespread skin reactions, including hives and itching and flushed or pale skin
  • Low blood pressure (hypotension)
  • Constriction of your airways and a swollen tongue or throat, which may cause wheezing and trouble breathing
  • A weak and rapid pulse
  • Nausea, vomiting or diarrhea
  • Dizziness or fainting
Quote Image

For some people reaching a certain temperature can trigger the release of histamine – a chemical your body produces during an allergic reaction.

How is allergic heat rash diagnosed?

Even if you have mild symptoms, you should still speak with your healthcare provider. Once they diagnose cholinergic urticaria, they could offer more suitable advice on avoidance strategies and treatments.

Generally, they use these tests when diagnosing cholinergic urticaria:

Exercise challenge test: A healthcare provider will observe if you break out in a heat allergy rash after exercising. You would usually spend 10–15 minutes on the treadmill or bike until you reach the point of sweating.

Passive warming test: If the exercise challenge test suggests you have cholinergic urticaria, you may have a passive warming test as well. This involves raising your body temperature with a warm bath, for example.

Sometimes your healthcare provider may use a methacholine injection to trigger hives. But only a third of people show a positive result so this test can’t be used alone in a cholinergic urticaria diagnosis.

You would usually spend 10–15 minutes on the treadmill or bike until you reach the point of sweating.

How is allergic heat rash diagnosed?

This all depends on the severity of your symptoms. If they’re mild, you may only have to make a few lifestyle changes, such as avoiding spicy foods and intense exercise. Stopping strenuous physical activity if you’re an athlete may be more difficult.

Antihistamines can often relieve allergic heat rash. But more severe symptoms could require different treatments. If you’ve ever developed anaphylaxis from getting too hot, your healthcare provider is likely to prescribe epinephrine auto-injectors. This is more likely with a history of exercise-induced anaphylaxis.

Your doctor will review your existing medications when considering prescribing epinephrine as certain antihypertensive drugs may impact its effectiveness. Carry two auto-injectors at all times. And go to the hospital immediately even if the first dose helped you to feel better in case of a delayed secondary reaction.

Anaphylaxis is a medical emergency.

A woman holding her phone to her ear and pointing sternly at the viewer.

Is cholinergic urticaria rash different from prickly heat?

Cholinergic urticaria is different from prickly heat or miliaria. Prickly heat is a sweat rash and happens when sweat is trapped in the skin. Prickly heat tends to occur in babies or anyone that is sweating a lot, for example from fever.

Usually, prickly heat goes away on its own within 24 hours. But finding ways to keep the skin cool could help reduce symptoms a little quicker. For mild cases, you could prevent heat rash from developing by wearing loose fitting clothing and staying in the shade. Or, try to work out at cooler times of the day to avoid excessive sweating. This is especially important during hot weather.

What causes allergic heat rash to flare?

Anything that can raise your body temperature enough for you to sweat could cause a flare. Usually, lifestyle changes are enough to prevent flares from happening as frequently. This may mean avoiding spicy foods and hot baths.

Can exercise cause allergic heat rash?

Yes, exercise can cause allergic heat rash because it raises your body temperature. In fact, it’s the most common trigger of cholinergic urticaria.


Heat allergy rash is a type of skin rash and tends to clear up on its own within 90 minutes, if you stop doing what’s making you feel hot. The release of histamine is what causes hives to appear, researchers are still trying to understand exactly how body temperature affects this.

Find a doctor
Think you may have an allergy? Maybe you’d like to manage your symptoms better. Let us help you find an allergist nearby so you can get the help you need.
find a doctor icon

Common causes of allergic heat rash include exercising, taking hot baths and eating hot or spicy foods. Usually, making lifestyle changes, such as not doing vigorous exercise can prevent hives from developing, but this can be more difficult for athletes. In these instances, a healthcare provider may suggest treating symptoms when they appear with antihistamines.

Allergic heat rash is different from prickly heat because it’s not caused by blocked sweat glands and typically goes away after a person cools down. If you’ve noticed your heat hives are taking longer to disappear than usual, or you’re experiencing other symptoms, please do speak with a healthcare provider. Additionally, if you’re struggling to breathe seek urgent medical treatment as this can be a symptom of anaphylaxis.

Share your story
If you’ve read all the way to the end of this article, thank you. We’d love to know what you think. Have you tried any of the tips we’ve suggested? Or do you have any of your own that you’d like to share with others? Head over to our Facebook or Instagram page or email us and share your story.
klarify takes allergy science and makes it simple, and we have rigorous process for doing this. We use up-to-date and authoritative sources of information. Medical experts review our content before we share it with you. They and the klarify editorial team strive to be accurate, thorough, clear and objective at all times. Our editorial policy explains exactly how we do this.

Last medically reviewed on 15/11/2022