Heat intolerance is an unusual sensitivity to heat. People with heat intolerance may feel hot when others feel comfortable or cold. Heat intolerance causes may include conditions that cause dysautonomia, which affects the autonomic nervous system.

They may also have an unusual response to heat, such as intense sweating or anxiety. Heat intolerance is not a disease, but it can be a symptom of an underlying medical condition.

In this article, learn about the causes of heat intolerance, how to manage it, and when to see a doctor.

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Heat intolerance is a generic symptom that can refer to a wide variety of responses to heat.

Some people with heat intolerance merely dislike the heat. Others feel uncomfortably hot at temperatures that other people find comfortable.

People may be more susceptible to the effects of heat if they have a health condition such as:

  • diabetes mellitus
  • obesity
  • an infection
  • a skin disorder, such as eczema or psoriasis
  • sickle cell trait
  • cardiovascular disease
  • an alcohol use disorder

Certain medications, such as diuretics, may increase a person’s sensitivity to heat. Older adults and young children may also be more susceptible. Athletes and outdoor workers may also have a higher risk of heat-related illness.

Some people can develop serious or even life-threatening symptoms in response to heat. According to the Centers for Disease Control and Protection (CDC), heat-related illnesses, such as heat stroke, are responsible for more than 600 deaths in the United States each year.

People with heat intolerance need to be cautious in extreme heat, especially when they have other risk factors for heat-related illnesses.

The symptoms of heat intolerance can vary from person to person but may include:

  • feeling very hot in moderately warm temperatures
  • excessive sweating
  • not sweating enough in the heat
  • exhaustion and fatigue during warm weather
  • nausea, vomiting, or dizziness in response to heat
  • changes in mood when too hot

People with certain chronic illnesses, such as multiple sclerosis (MS), may find that their symptoms temporarily worsen in the heat. Some people with heat intolerance also have cold intolerance.

Heat exposure can lead to a range of complications, and people with conditions such as MS, which increase the risk of heat intolerance, may be particularly prone to these complications.

People with heat intolerance should be aware of signs and symptoms that could indicate:

  • Heatstroke: This is a medical emergency. Heatstroke can lead to seizures and a loss of consciousness and can quickly become fatal.
  • Heat exhaustion: In heat exhaustion, the body loses an excessive amount of water and salt, usually due to sweating. This can lead to dizziness, headache, nausea, and other symptoms.
  • Rhabdomyolysis: Prolonged heat exposure and exertion can lead lead to a rapid breakdown, rupture, and death of muscle tissue, known as rhabdomyolysis. This can result in a release of electrolytes and other substances that can, in turn, cause an irregular heart rate, seizures, and kidney damage.
  • Heat syncope: A person faints or becomes dizzy due to standing too long, dehydration, and heat exposure.
  • Heat cramps: Excessive sweating can lead to a loss of water and minerals in the body. A person may experience pain, muscle cramps, or spasms in the legs, arms, or abdomen.
  • Heat rashes: Sweating can result in areas of inflamed skin and clusters of pimples or small blisters known as heat rash or prickly heat.
  • Heat edema: Swelling and discomfort occur in the hands and feet due to blood vessel dilation and the pooling of fluids, for example, in the ankles.

Those with heat intolerance may have a disorder called dysautonomia that affects their autonomic nervous system.

The autonomic nervous system helps regulate automatic functions of the body, including the body’s response to heat.

Several medical conditions can cause dysautonomia, including:

Other causes of heat intolerance include:

  • Age: Infants, children under 4 years old, and older adults may be more sensitive to the heat. This sensitivity increases their susceptibility to heat-related illnesses, such as heat stroke.
  • Medication: Some medications change the body’s response to heat, for example, by decreasing sweat production. Anticholinergic drugs, which can treat many psychiatric conditions and Parkinson’s disease, may reduce sweating and increase heat sensitivity.
  • Sensory issues: Sensory processing disorder, as well as sensory issues that sometimes accompany autism, may make a person more sensitive to heat.
  • Neurological conditions: Medical conditions that affect the brain and spinal cord, such as spinal cord injuries and MS, can increase heat sensitivity by changing how the body or brain processes heat or by inhibiting the body’s ability to regulate temperature.
  • Endocrine system problems: The endocrine system helps the body regulate a wide range of functions. Disorders such as Graves’ disease, a thyroid condition, can increase heat sensitivity.
  • Being less physically fit: For some people, heat intolerance is a sign of poor cardiovascular and respiratory fitness. In 2014, researchers found that people who showed more signs of heat intolerance were also less physically fit.

People with heat intolerance should discuss their symptoms with a doctor, especially if the symptoms appear suddenly or get progressively worse.

To treat heat intolerance, doctors will focus on treating any underlying medical conditions. Treatment will vary widely depending on the underlying condition. For example, people with Graves’ disease may need radioiodine therapy to restore normal thyroid levels.

In many cases, heat intolerance is not fully preventable or treatable. A person with a spinal cord injury may have difficulty in extreme heat no matter what treatment they choose.

People may find that avoiding heat where possible and adopting strategies for safely managing any necessary time in hot conditions will help in the long term. Ways to manage heat intolerance include:

  • Avoiding direct sunlight. The sun tends to be at its hottest and brightest between 11 a.m. and 3 p.m.
  • Using air conditioning or a fan during the summer months.
  • Drinking plenty of fluids to avoid dehydration.
  • Wearing light-colored, loose-fitting clothing.
  • Avoiding alcohol in hot weather.
  • Taking a cool bath or swimming in a pool.
  • Wrapping a towel soaked in cold water around the back of the neck.
  • Avoiding strenuous activities during hot weather or in warm rooms.

People with heat intolerance should carefully monitor themselves for signs of heat-related illness, such as:

Seek immediate medical attention for:

  • an inability to sweat, even when very warm
  • a body temperature above 103°F
  • confusion
  • loss of consciousness

Managing medical conditions that cause heat intolerance can help prevent symptoms.

Talk to a doctor about staying safe in the heat and ask whether any medications are available to help the body regulate its temperature.

Some strategies that can reduce the risk of heat intolerance include:

  • Maintaining a healthy body weight. Heavier people may have more difficulty cooling their bodies.
  • Getting plenty of exercise to remain physically fit. People with good heart and lung health tend to respond better to the heat.
  • Limiting or avoiding alcohol and drug use. Excessive alcohol consumption and the abuse of some drugs, such as amphetamines, may increase heat sensitivity.
  • Keeping blood sugar levels in check. People with diabetes may be more vulnerable to the heat, especially when their blood sugar levels are too low or too high.
  • Drinking plenty of water. Extreme heat saps the body of water through sweating. If the body cannot sweat, it cannot stay cool, so staying hydrated is critical.
  • Taking time to acclimatize: People going to a hotter place on holiday, for instance, can spend more time outdoors as they become accustomed to the heat.

As many people enjoy outdoor activities, such as swimming and warm-weather festivities, those with heat intolerance may feel frustrated and excluded. However, the right treatment strategy and a few cooling measures may make the heat feel more manageable.

Heat intolerance can provide clues to a person’s overall health. It suggests that either the body may not be able to cool itself down properly, the brain may not be responding correctly to heat, or the heart and lungs may be struggling to work efficiently enough.

Anyone experiencing new or worsening heat intolerance should speak to a doctor, who can help diagnose the underlying problem.