A number of heat illnesses exist including:

- Heat stroke - Defined by a body temperature of greater than due to environmental heat exposure with lack of thermoregulation. Symptoms include dry skin, rapid, strong pulse and dizziness.

- Heat exhaustion - Can be a precursor of heatstroke; the symptoms include heavy sweating, rapid breathing and a fast, weak pulse.

- Heat syncope - Fainting or dizziness as a result of overheating.

- Heat edema

- Heat cramps - Muscle pains that happen during heavy exercise in hot weather.

- Heat rash - Skin irritation from excessive sweating.

- Heat tetany - Usually results from short periods of stress in intense heat. Symptoms may include hyperventilation, respiratory problems, numbness or tingling, or muscle spasms.

Heat illness or heat-related illness is a spectrum of disorders due to environmental exposure to heat. It includes minor conditions such as heat cramps, heat syncope, and heat exhaustion as well as the more severe condition known as heat stroke.

An early stage of hyperthermia can be "heat exhaustion" (or "heat prostration" or "heat stress"), whose symptoms include heavy sweating, rapid breathing and a fast, weak pulse. If the condition progresses to heat stroke, then hot, dry skin is typical as blood vessels dilate in an attempt to increase heat loss. An inability to cool the body through perspiration may cause the skin to feel dry.

Other signs and symptoms vary. Accompanying dehydration can produce nausea, vomiting, headaches, and low blood pressure and the latter can lead to fainting or dizziness, especially if the standing position is assumed quickly.

In severe heat stroke, there may be confused, hostile, or seemingly intoxicated behavior. Heart rate and respiration rate will increase (tachycardia and tachypnea) as blood pressure drops and the heart attempts to maintain adequate circulation. The decrease in blood pressure can then cause blood vessels to contract reflexively, resulting in a pale or bluish skin color in advanced cases. Young children, in particular, may have seizures. Eventually, organ failure, unconsciousness and death will result.

Symptoms of heat exhaustion include nausea, dizziness, irritability, headache, thirst, weakness, high body temperature, excessive sweating, and decreased urine output.

In humans, hyperthermia is defined as a temperature greater than , depending on the reference used, that occurs without a change in the body's temperature set point.

The normal human body temperature can be as high as in the late afternoon. Hyperthermia requires an elevation from the temperature that would otherwise be expected. Such elevations range from mild to extreme; body temperatures above can be life-threatening.

Heat exhaustion is a severe form of heat illness. It is a medical emergency. Heat exhaustion is caused by the loss of water and electrolytes through sweating.

Signs and symptoms vary depending on the degree of hypothermia, and may be divided by the three stages of severity. Infants with hypothermia may feel cold when touched, with bright red skin and an unusual lack of energy.

Symptoms of mild hypothermia may be vague, with sympathetic nervous system excitation (shivering, high blood pressure, fast heart rate, fast respiratory rate, and contraction of blood vessels). These are all physiological responses to preserve heat. Increased urine production due to cold, mental confusion, and hepatic dysfunction may also be present. Hyperglycemia may be present, as glucose consumption by cells and insulin secretion both decrease, and tissue sensitivity to insulin may be blunted. Sympathetic activation also releases glucose from the liver. In many cases, however, especially in alcoholic patients, hypoglycemia appears to be a more common presentation. Hypoglycemia is also found in many hypothermic patients, because hypothermia may be a result of hypoglycemia.

Occupational heat stress is the net load to which a worker is exposed from the combined contributions of metabolic heat, environmental factors, and clothing worn which results in an increase in heat storage in the body. Heat stress can result in heat-related illnesses, such as heat stroke, hyperthermia, heat exhaustion, heat cramps or heat rashes. Although heat exhaustion is less severe, hyperthermia is a medical emergency and requires emergency treatment, which if not provided can even lead to death.

Heat stress causes illness but also may account for an increase in workplace accidents, and a decrease in worker productivity. Worker injuries attributable to heat include those caused by: sweaty palms, fogged-up safety glasses, and dizziness. Burns may also occur as a result of accidental contact with hot surfaces or steam. In United States, occupational heat stress in becoming more significant as the average temperatures increase but remains overlooked. There are few studies and regulations regarding heat exposure of workers.

Heat syncope is fainting or dizziness as a result of overheating ("syncope" is the medical term for fainting). It is a type of heat illness. The basic symptom of heat syncope is fainting, with or without mental confusion. Heat syncope is caused by peripheral vessel dilation, resulting in diminished blood flow to the heart and dehydration.

Faintness, dizziness, headache, increased pulse, restlessness, nausea, vomiting, and brief loss of consciousness.

Heat cramps, a type of heat illness, are muscle spasms that result from loss of large amount of salt and water through exercise. Heat cramps are associated with cramping in the abdomen, arms and calves. This can be caused by inadequate consumption of fluids or electrolytes. Frequently, they don't occur until sometime later, especially at night or when relaxing. Heavy sweating causes heat cramps, especially when the water is replaced without also replacing salt or potassium.

Although heat cramps can be quite painful, they usually don't result in permanent damage, though they can be a symptom of heat stroke or heat exhaustion. Heat cramps can indicate a more severe problem in someone with heart disease or if they last for longer than an hour.

In order to prevent them, one may drink electrolyte solutions such as sports drinks during exercise or strenuous work or eat potassium-rich foods like bananas and apples. When heat cramps occur, the affected person should avoid strenuous work and exercise for several hours to allow for recovery.

The main symptoms of heat stress are perspiration, increased heart rate, and dehydration. Other general symptoms include painful muscle cramps, extreme weakness, nausea, dizziness, headache, breathing fast and clammy, pale, cool, and/or moist skin or red, dry skin.

Polymer fume fever or fluoropolymer fever, also informally called Teflon flu, is an inhalation fever caused by the fumes released when polytetrafluoroethylene (PTFE, known under the trade name Teflon) reaches temperatures of 300 °C (572 °F) to 450 °C (842 °F). When PTFE is heated above 450 °C the pyrolysis products are different and inhalation may cause acute lung injury. Symptoms are flu-like (chills, headaches and fevers) with chest tightness and mild cough. Onset occurs about 4 to 8 hours after exposure to the pyrolysis products of PTFE. A high white blood cell count may be seen and chest x-ray findings are usually minimal.

The polymer fumes are especially harmful to certain birds whose breathing, optimized for rapidity, allows toxins which are excluded by human lungs. Fumes from Teflon in very high heat are fatal to parrots, as well as some other birds (PTFE Toxicosis).

The characteristics of a burn depend upon its depth. Superficial burns cause pain lasting two or three days, followed by peeling of the skin over the next few days. Individuals suffering from more severe burns may indicate discomfort or complain of feeling pressure rather than pain. Full-thickness burns may be entirely insensitive to light touch or puncture. While superficial burns are typically red in color, severe burns may be pink, white or black. Burns around the mouth or singed hair inside the nose may indicate that burns to the airways have occurred, but these findings are not definitive. More worrisome signs include: shortness of breath, hoarseness, and stridor or wheezing. Itchiness is common during the healing process, occurring in up to 90% of adults and nearly all children. Numbness or tingling may persist for a prolonged period of time after an electrical injury. Burns may also produce emotional and psychological distress.

A catastrophic illness is a severe illness requiring prolonged hospitalization or recovery. Examples would include coma, cancer, leukemia, heart attack or stroke. These illnesses usually involve high costs for hospitals, doctors and medicines and may incapacitate the person from working, creating a financial hardship. They are the type intended to be covered by high-deductible health plans. Research indicates that the unusual economic environment of the delivery of catastrophic illness care encourages the use of innovative therapies. Medicare contains a benefit for catastrophic illness.

The symptoms and signs, as described by physician John Caius and others, were as follows: the disease began very suddenly with a sense of apprehension, followed by cold shivers (sometimes very violent), giddiness, headache, and severe pains in the neck, shoulders and limbs, with great exhaustion. After the cold stage, which might last from half an hour to three hours, the hot and sweating stage followed. The characteristic sweat broke out suddenly without any obvious cause. Accompanying the sweat, or after, was a sense of heat, headache, delirium, rapid pulse, and intense thirst. Palpitation and pain in the heart were frequent symptoms. No skin eruptions were noted by observers including Caius. In the final stages, there was either general exhaustion and collapse, or an irresistible urge to sleep, which Caius thought to be fatal if the patient was permitted to give way to it. One attack did not offer immunity, and some people suffered several bouts before dying. The disease tended to occur in summer and early autumn.

Heat urticaria presents within five minutes after the skin has been exposed to heat above 43 degrees Celsius (109.4 degrees Fahrenheit), with the exposed area becoming burned, stinging, and turning red, swollen, and indurated.

A number of complications may occur, with infections being the most common. In order of frequency, potential complications include: pneumonia, cellulitis, urinary tract infections and respiratory failure. Risk factors for infection include: burns of more than 30% TBSA, full-thickness burns, extremes of age (young or old), or burns involving the legs or perineum. Pneumonia occurs particularly commonly in those with inhalation injuries.

Anemia secondary to full thickness burns of greater than 10% TBSA is common. Electrical burns may lead to compartment syndrome or rhabdomyolysis due to muscle breakdown. Blood clotting in the veins of the legs is estimated to occur in 6 to 25% of people. The hypermetabolic state that may persist for years after a major burn can result in a decrease in bone density and a loss of muscle mass. Keloids may form subsequent to a burn, particularly in those who are young and dark skinned. Following a burn, children may have significant psychological trauma and experience post-traumatic stress disorder. Scarring may also result in a disturbance in body image. In the developing world, significant burns may result in social isolation, extreme poverty and child abandonment.

Drug-induced fever is a symptom of an adverse drug reaction wherein the administration of drugs intended to help a patient causes a hypermetabolic state resulting in fever. The drug may interfere with heat dissipation peripherally, increase the rate of metabolism, evoke a cellular or humoral immune response, mimic endogenous pyrogen, or damage tissues.

Patients who suffer from acute OI usually manifest the disorder by a temporary loss of consciousness and posture, with rapid recovery (simple faints, or syncope), as well as remaining conscious during their loss of posture. This is different from a syncope caused by cardiac problems because there are known triggers for the fainting spell (standing, heat, emotion) and identifiable prodromal symptoms (nausea, blurred vision, headache). As Dr. Julian M. Stewart, an expert in OI from New York Medical College states, "Many syncopal patients have no intercurrent illness; between faints, they are well."

Symptoms:

- Altered vision (blurred vision, "white outs"/gray outs, black outs, double vision)

- Anxiety

- Exercise intolerance

- Fatigue

- Headache

- Heart palpitations, as the heart races to compensate for the falling blood pressure

- Hyperpnea or sensation of difficulty breathing or swallowing (see also hyperventilation syndrome)

- Lightheadedness

- Sweating

- Tremulousness

- Weakness

A classic manifestation of acute OI is a soldier who faints after standing rigidly at attention for an extended period of time.

Patients with chronic orthostatic intolerance have symptoms on most or all days. Their symptoms may include most of the symptoms of acute OI, plus:

- Nausea

- Neurocognitive deficits, such as attention problems

- Pallor

- Sensitivity to heat

- Sleep problems

- Other vasomotor symptoms.

Sweating sickness, also known as "English sweating sickness" or "English sweate" (), was a mysterious and highly contagious disease that struck England, and later continental Europe, in a series of epidemics beginning in 1485. The last outbreak occurred in 1551, after which the disease apparently vanished. The onset of symptoms was dramatic and sudden, death often occurring within hours. Although its cause remains unknown, it has been suggested that an unknown species of hantavirus was responsible for the outbreak.

Chronic critical illness is a disease state which affects intensive care patients who have survived an initial insult but remain dependent on intensive care for a protracted period, neither dying nor recovering. The most characteristic clinical feature is a prolonged requirement for mechanical ventilation. Other features include profound weakness associated with critical illness polyneuropathy, increased susceptibility to infection, metabolic changes and hormonal changes. There may be protracted or permanent delirium, or other marked cognitive impairment. The physical and psychological symptoms of the disease are very severe, including a propensity to develop post traumatic stress syndrome.

Strict definitions of chronic critical illness vary. One definition is the requirement for mechanical ventilation for 21 days or more. It is estimated that 5-10% of patients who require mechanical ventilation as part of their initial illness will go on to develop chronic critical illness. Overall prevalence has been estimated at 34.4 per 100 000 of the population. Most adult patients do not survive chronic critical illness, and furthermore even those who are discharged from hospital frequently die soon after discharge. One-year mortality in adults is 48-68%. However, children fare better with two-thirds surviving to 5 years or beyond.

The burning heat is usually limited to the soles of the feet, but may extend up to the ankles or lower legs of some patients. The burning can sometimes be accompanied by feelings of 'pins and needles' or tingling in these regions. Nighttime is when almost all sufferers of this syndrome report the heat symptoms being the worst, with the condition getting better as morning comes. Those who have psychosomatic disorders sometimes display psychological symptoms along with the burning of feet associated with the syndrome. For most, there is no erythema of the skin of their feet during the heat sensations, and almost never is there accompanying tenderness along with it.