Heat-Related Illness (Hyperthermia)

What is heat-related illness?

Heat-related illness (also commonly called hyperthermia) refers to a group of conditions that happen when your body has trouble cooling itself down.

Normally, your body has safeguards in place to cool you down when you get too hot. For example, your sweat glands produce sweat that evaporates from your skin, lowering your body temperature before it can get to dangerous levels. But sometimes, the weather is so hot and humid, or your body produces so much internal heat (for example, from a vigorous workout) that your natural cooldown system gets overwhelmed.

Picture trying to cool an entire house with a single fan on a hot day — it just won’t work. There’s too much heat building up and not enough cooling action from the fan. It’s similar with heat-related illnesses. Your body gets overwhelmed by heat and needs some extra help to cool itself down.

Heat-related illnesses occur on a spectrum from mild to severe. Mild heat-related illnesses — like heat rash and heat cramps — usually go away with rest or at-home treatments. But moderate or severe heat-related illnesses — like heat exhaustion and heat stroke — require prompt medical attention to avoid serious complications.

What is hyperthermia?

Hyperthermia is a medical term that means your internal (core) body temperature is higher than normal. This generally means your temperature is higher than 98.6 degrees Fahrenheit (37 degrees Celsius). Some people have a baseline temperature that’s a bit higher or lower than that. Anything above your baseline is hyperthermia.

Not all heat-related illnesses cause a spike in your body temperature, or hyperthermia. But the most serious forms — heat exhaustion and heat stroke — typically do. That’s why people often use the terms hyperthermia and heat-related illness interchangeably.

What are the four types of heat illness?

The four main types of heat illness, in order from mildest to most severe, include:

• Heat rash. This is a group of tiny, itchy bumps on your skin that develop when sweat gets trapped beneath your skin. Usually, you can manage heat rash at home by keeping your skin cool and dry and using anti-itch creams.
• Heat cramps (also called exercise-associated muscle cramps). These are muscle cramps in your legs, arms, belly or elsewhere. They typically occur when you’re exerting yourself in the heat and sweating a lot. Heat cramps are a mild form of heat-related illness, and you’ll usually have a normal or slightly elevated body temperature. But these cramps are a warning that more severe illness could occur if you don’t take steps to cool down.
• Heat exhaustion. This is a moderate form of heat illness that can quickly turn into heat stroke without treatment. It happens when you lose too much water and/or salt, typically when exerting yourself in the heat. Your internal body temperature is elevated but below 104 degrees F (40 degrees C). Symptoms include dizziness, nausea, vomiting and headache.
• Heat stroke. This is a life-threatening heat illness. It’s when your internal body temperature gets very high — typically above 104 degrees F. You have symptoms of heat exhaustion plus signs of brain dysfunction (encephalopathy), like confusion, behavior changes and slurred speech. Heat stroke can occur from doing physical activity in the heat (exertional heat stroke) or simply from being in a hot environment for too long (classic/non-exertional heat stroke).

What are the symptoms of heat-related illness (hyperthermia)?

Hyperthermia symptoms vary according to the specific condition you have. The infographic below lists the signs and symptoms of the four main types of heat illness.

Heat stroke, unlike other heat illnesses, affects your brain function. Besides physical symptoms, it causes changes to your thinking and behavior. Often, bystanders are the ones who notice heat stroke symptoms in a person and call for medical help. If you have heat stroke, you may be confused, delirious or unconscious and have difficulty seeking help for yourself.

What causes heat illness?

Heat-related illnesses happen when your body’s usual mechanisms for managing heat get overwhelmed. This can occur due to hot and humid weather, metabolic heat (heat your body makes) or both. As a result, your body can’t get rid of heat efficiently enough to keep your temperature at a normal level. Sometimes, your temperature stays normal but you develop other, uncomfortable symptoms — like a rash or muscle cramping.

What are the risk factors?

You may have an increased risk of heat-related illness if you:

• Exercise in the heat — and the longer the workout, the greater your risk.
• Have a job that requires intense physical activity in a hot environment (outdoors or indoors). Examples include farming, construction work, firefighting and military duty.
• Are dehydrated.
• Take medications that affect your body’s ability to cool itself.
• Have a medical condition — like anhidrosis or deep burns — that interferes with sweating.

How is heat-related illness diagnosed?

Not all heat illnesses receive an official diagnosis. You may recognize the signs and symptoms of milder conditions, like heat rash or heat cramps, and manage things on your own. But you may choose to see a provider for those conditions if symptoms are severe or won’t go away. If you have signs of heat exhaustion or heat stroke, seek medical care immediately.

In general, a healthcare provider will diagnose your condition by doing a physical exam and talking to you about your symptoms — including what makes them better or worse. Heat exhaustion and heat stroke are often diagnosed and treated in the emergency room. There, providers check your vital signs and review your symptoms. They’ll begin treatment right away to prevent complications.

What is the treatment for heat-related illness?

Treatment depends on the type of heat-related illness. You can usually manage heat rash and heat cramps on your own. You should seek medical attention for heat exhaustion and heat stroke while taking measures to cool your body.

The table below lists common treatments for each type of heat illness.

How can I prevent hyperthermia?

You can lower the risk of your body overheating by:

• Wearing loose-fitting, breathable clothing in the heat.
• Staying hydrated at all times — whether you’re active or not.
• Keeping your home at a cool, comfortable temperature with fans and/or air conditioning.
• Finding places to go during hot weather (like malls or community centers) if your home is too hot.
• Limiting or avoiding physical activity in hot conditions. If you can’t avoid it, take frequent breaks in the shade or a cool, indoor location.
• Taking a cool shower after time in the heat.
• Speaking with your healthcare provider about your risks for heat-related illness (like with certain medical conditions or medications). They’ll work with you to lower these risks whenever possible.

What can I expect if I have a heat-related illness?

Your outlook depends on many factors, including:

• The type of heat-related illness you have and its severity.
• How soon you receive treatment.
• Your age and overall health.

Some heat illnesses — like heat rash — are typically mild and won’t affect your outlook much. But heat exhaustion can quickly turn into heat stroke, which can be life-threatening. Your healthcare provider can tell you more about what to expect in your situation.

When should I seek medical care?

Call 911 or your local emergency services number if you have signs of heat exhaustion or heat stroke. The sooner you get treatment, the better your chances of recovering.

You may not need medical care for heat cramps or heat rash. But if symptoms bother you a lot or get worse, contact a healthcare provider. They can rule out other causes and make sure you get proper treatment.

What questions should I ask my healthcare provider?

Talking about heat-related illness with your provider is the first step toward preventing it. Here are some questions you may want to ask your provider to stay one step ahead of the heat:

• Do any of my medical conditions or medications raise my risk for heat-related illness?
• How can I lower my risk?
• Do you have tips for exercising in the heat?
• What should I do to lower my risk of heat illness at work?
• How much water should I drink each day?
• How much water should I drink when I’m active?
• How do you recommend I get enough electrolytes?
• What symptoms should prompt me to call you?

If you’re recovering from a heat illness, it may help to ask your provider:

• How long will I need to recover?
• When can I resume my usual physical activities?
• How can I prevent this from happening again?
• Will I have any lasting effects or complications?

What are other causes of hyperthermia besides heat exposure?

You can have a raised body temperature without having a heat-related illness. Hyperthermia can also happen due to:

• An infection. When you’re sick, your body (specifically, your hypothalamus) cranks up the heat to help fight off the infection. The result is the raised body temperature we commonly call a fever. A fever is part of your body’s normal immune response to invaders like viruses and bacteria.
• Medications. Some medications can raise your body temperature, potentially to dangerous levels. Examples include certain anesthesia medications (malignant hyperthermia), antipsychotic medications (neuroleptic malignant syndrome) and medications that increase serotonin levels in your body (serotonin syndrome). Healthcare providers monitor your use of these and other medications to watch for harmful effects and will adjust your treatment plan as needed.

Hyperthermia vs. fever — what’s the difference?

Hyperthermia is the medical term for an elevated body temperature. This can have many causes, including infections and heat exposure.

When an infection causes a raised temperature, you have a fever. Your hypothalamus triggers activity within your body that makes your temperature go up. This is a bit like turning up the setting on a thermostat. It’s intentional and your body is in control. Once the infection clears, your hypothalamus turns the setting back down so you return to a normal temperature.

When you’re overheated from hot weather or physical exertion, your body temperature rises, but we don’t say you have a fever. This is because your hypothalamus isn’t raising your internal “thermostat.” Instead, heat overwhelms your body, and your body’s normal cooling mechanisms (primarily sweating) struggle to keep up. It’s like on a hot day when you have the A/C on full blast but the room temperature still isn’t lowering to where you want it to be.

So, a fever is always hyperthermia, but hyperthermia doesn’t always mean a fever.

What are heat-sensitive diseases?

Heat-sensitive diseases are conditions that heat exposure triggers or worsens. Examples include:

• Abnormal heartbeat (arrhythmia).
• Asthma.
• Chronic obstructive pulmonary disease (COPD).
• Coronary artery disease.
• High blood sugar (hyperglycemia).
• Ischemic stroke.
• Kidney failure.
• Respiratory tract infections.

If you have one of these conditions or any risk factors, talk to your healthcare provider about ways to protect yourself in hot weather.

A note from Cleveland Clinic

Hot weather can be lovely and relaxing. But with the heat come certain risks that might not always be obvious. Heat-related illness can happen to anyone. Even if you’ve exerted yourself in the heat before and felt fine, that doesn’t mean you’ll avoid heat illness in the future. It’s always important to stay hydrated, limit physical activity in extreme heat and take steps to protect others around you.

If you can’t avoid heat exposure due to work or other obligations, talk to your healthcare provider. They can help you come up with a plan for lowering your risk of heat illness. They’ll also explain which signs or symptoms to look out for and when to seek medical care.