Breaking Out in Hives: 4 Causes You Wouldn't Expect

If you break out in hives, medically known as urticaria, your skin itches and is covered in red or skin-colored welts. Hives typically crop up when you are allergic to a substance—like pet dander, pollen, or latex—triggering your body to release histamine into your blood.

While it may seem like you are breaking out in hives for no apparent reason, there is usually an underlying cause. Environmental factors, certain health conditions, workouts, or emotional changes usually cause unexplainable hives. Here's what you need to know about hives that happen out of nowhere.

1. The Great Outdoors

Insect bites and exposure to pollen can make you break out in hives. You may not have realized that direct sunlight, cold temperatures, or swimming in cold water can also bring on hives.

Hives caused by exposure to heat, cold, or pressure (such as with tight athletic clothing) are a type of physical urticaria because the cause is physical. This doesn't necessarily mean you're allergic to the elements. "It's really more that your skin is very sensitive," Marilyn Li, MD, a Los Angeles-based allergist and immunologist, told Health.

While avoiding these triggers could prevent hives from returning, Dr. Li said a healthcare provider might be able to provide an antihistamine that could solve weather- or temperature-based problems. That way, you can enjoy a hike on a sunny summer day or hold a cold beverage without worrying about a potential hives outbreak.

2. An Underlying Illness

Chronic hives can be a telltale sign of a more significant problem, and extensive blood work may be necessary to pinpoint the underlying issue.

Patients with lupus, lymphoma, thyroid disease, and hepatitis might have hives as a symptom of their illness, Debra Jaliman, MD, a New York City-based dermatologist and the author of Skin Rules, told Health. Since these are typically chronic hives, medications are the most reliable form of relief.

If you've contracted a virus—including SARS-CoV-2, the virus responsible for COVID-19—that could also cause hives.

3. Your Daily Workout

While your morning jog or post-work weightlifting regimen can boost endorphins, your exercise routine could also be causing you to break out in hives.

According to Dr. Jaliman, the body produces acetylcholine—a chemical that can inhibit cell breakdown—as a response to exercise. For some people, acetylcholine will disrupt skin cells and either irritate the skin and create a rash the same way histamine does or contribute to the release of histamine.

Dr. Li said sweat could also result in a breakout for those prone to hives. The sweat doesn't cause hives but indicates your body's heat rising. It's known as cholinergic urticaria when hives occur due to increased body temperature.

For some, excess warmth on the skin from a workout or other body-heat-inducing activity like a hot bath is enough to produce hives.

4. Emotionally Charged Situations

Intense emotional responses can trigger a hives outbreak. For those with chronic hives—or hives that persist for more than six weeks at a time—stress can result in worsened hives and more intense itching. Also, the relationship can happen the other way around: hives can impair emotional functioning.

Why Am I Breaking Out in Hives at Night?

When you're lying in bed attempting to sleep, you might find that you break out in hives—and dermatographism could be responsible. Also called dermographism urticaria, dermatographism affects 2% to 5% of individuals but is the most likely cause of physical urticaria. It happens following skin pressure or trauma, and one possible reason for skin pressure may be due to your skin coming into contact with bedding or sheets.

Another unexpected cause of nighttime hives could be related to the timing of your work shift. One study found that night shift workers were at a higher risk for experiencing chronic spontaneous urticaria (CSU). Researchers suggested that circadian rhythms could play a role in the association but that more research would be necessary.

When To See a Healthcare Provider

For those with chronic hives from heat-induced or illness-induced activities, Dr. Li suggested visiting a healthcare provider to pinpoint the symptoms and receive an antihistamine prescription. "The solutions are individualized and many times may involve just taking a little more medication," Dr. Li said.

In most cases, hives will go away with time—unless you experience a severe case that may require medication or a shot. However, seek emergency medical attention right away if you find that hives are accompanied by symptoms such as:

• Face, mouth, or tongue swelling
• Feelings of faintness
• Shortness of breath
• Swallowing issues
• Throat swelling or tightness
• Wheezing

A Quick Review

Allergens typically cause hives. However, unexpected causes may include environmental temperatures, stress, health conditions such as lupus or thyroid disease, and exercise. If your hives don't go away or you experience other symptoms such as throat swelling or feeling faint, seek medical care.