Fever (Temperatures)

Fever (Temperatures)

Defining fever

When a person’s body temperature temporarily increases, this is known as fever (also called controlled hyperthermia, pyrexia or elevated temperature). Often as a result of an illness, a fever is a symptom that something abnormal is happening in the body. How serious a fever is can be influenced by a person’s age, the underlying cause (which can sometimes be non-infectious in nature) and the temperature increase degree. A slightly elevated temperature may not always be as serious for an adult as it is likely to be for a young child (infant or toddler).

Fever is typically accompanied by a wave of fatigue or chills, and is often a response to an infection (viral or bacterial) or inflammation (due to tissue injury or illness). Non-infection causing fevers can be as a result of poisons, drugs, injuries to or abnormalities of the brain, heat exposure or endocrine diseases (problems with the glandular or hormonal system).

A typical fever (mild) should clear within a matter of days and can be treated with an assortment of available over-the-counter medications (or antipyretics). Sometimes a mild fever can clear on its own without the need for any medicinal treatment.

It has been noted that fever can also play a key role in helping the body’s immune system to naturally fend off a variety of infections (i.e. body temperature increases as a natural immune response to infection). The symptom of fever is one of the immune system’s natural attempts to get the better of an infection that is apparent in the body. The immune system alerts to the presence of an infection and ‘uses elevated body temperature’ to help resolve it. If body temperature (fever) is too high, the body cannot help to resolve whatever is wrong (i.e. neutralising some kind of bacterium or virus in the body) and the fever may result in serious side effects including dehydration, hallucinations and even convulsions.

A higher than normal increase in temperature can affect anyone of any age. A fever, alone, is not an illness, but does indicate that something not working as it should in the body. An infection of some kind is usually the underlying condition causing the symptom of fever. When severe, a fever is often a warning sign that the body is seriously unwell (a person may have a serious health condition) and needs prompt medical attention.

What is a normal body temperature?

A normal body temperature does vary from one person to the next, but is typically around 36 to 37 degrees Celsius (or 98.6 degrees Fahrenheit). Body temperature also tends to fluctuate at different times of the day – it is lower in the mornings and increases towards late afternoon and evening. Our body temperature is normally at its highest at around 6pm and at its lowest at around 3 in the morning. Some other factors which influence fluctuations in body temperature are intense exercise and for women, in particular, a menstrual cycle.

As a person’s body temperature increases, he or she may experience a temporary sensation of cold until the temperature plateaus (reaches its peak). A person experiencing a fever rarely does so without the presence of other symptoms. Most fevers are accompanied by a set of very specific symptoms. Together, a set of symptoms, including fever, can help a medical professional to be able to distinguish an underlying cause and diagnose a condition. The correct course of treatment can then be implemented so that a person can return to optimum health.

How does body temperature work?

The hypothalamus (pronounced hi-poe-THAL-uh-muhs) is a part of the brain which usually controls temperature in the body. The hypothalamus links the endocrine and nervous systems via the pituitary gland (hypophysis). This part of the brain is, in effect, the body’s own thermostat, which maintains a normal temperature through heating (such as increased metabolism) and cooling (such as sweating) mechanisms. The hypothalamus is effectively in the driver’s seat using key temperature sensors to control heating.

As skin temperature rises above its baseline, sweating begins and can increase rapidly. If body temperature drops below baseline, the control mechanisms initiate activity to conserve heat in the body and increase production of heat or warmth. This is done by means of the cessation of sweating, shivering (to increase the production of heat in the body’s muscles), the secretion of norepinephrine, thyroxine and epinephrine (hormones and neurotransmitters that regulate fight or flight, thyroid function and metabolism), and vasoconstriction (decreasing the flow of heat to the skin).

Fever-producing substances, or pyrogens, can enter the body and upset the natural flow of temperature, thus causing fever. Pyrogens are foreign to the body and typically come from a source outside of it (such as viruses, fungi, bacteria, illicit drugs or toxins), and stimulate additional pyrogens once inside. These effectively send a ‘message’ to the hypothalamus (known as neural feedback mechanisms) to increase body temperature. The effect is that the body’s normal heating mechanisms react by shivering and constricting blood vessels. The body is then primed to reach a new temperature that is higher than its normal baseline in order to eliminate temperature sensitive viruses and bacteria.

The body also produces its own pyrogens in response to inflammation causing fever (this is a natural response). These are known as endogenous pyrogens or cytokines.

Temperature devices (thermometers) can be used to determine a body temperature measurement. These devices can be inserted into a person’s mouth, rectum, axilla (under the armpit), skin or ear. Other devices which a medical professional may use have temperature-sensing probes that can record temperature while in use. These include laryngoscopes, rectal probes and bronchoscopes. A mercury thermometer is the most common device used, but digital thermometers with disposable probe covers are also regularly used. Rectal temperature measurements are generally more accurate (reflecting the core temperature) than those taken orally.

What temperature classifies as a fever?

  • For babies or infants, a young child or teenager: When the body temperature exceeds 37.5 degrees Celsius (99.5 degrees Fahrenheit).
  • For an adult: When the body temperature exceeds between 37.2 and 37.5 degrees Celsius (99 to 99.5 degrees Fahrenheit).

Fevers can be classified as:

  • Low-grade: Temperatures range between 37.7 and 38.3 degrees Celsius (100 and 101 degrees Fahrenheit). If a low-grade fever doesn’t clear within 4 to 7 days, medical treatment should be sought out. If any fever persists, you must consult with you doctor for a thorough check-up.
  • Intermediate: Temperatures of 38.8 degrees Celsius (102 degrees Fahrenheit) for an adult and ranging between 39.4 to 40 degrees Celsius (103 to 104 degrees Fahrenheit) for an infant (0-6 months of age). If a fever reaches this stage, it is best to seek assistance from a medical professional as soon as possible. The more prompt you are at addressing the fever and having it checked, the better.
  • High-grade: Temperatures range between 39.4 and 41.6 degrees Celsius (103 and 107 degrees Fahrenheit) or higher. Extremely high fever is known as hyperpyrexia and can be very dangerous or life-threatening.

Body temperature readings - celsius to fahrenheit conversions

When taking body temperature readings, these may be in celsius. To convert celsius to fahrenheit you can use the following equations: 

T(°F) = T(°C) × 9/5 + 32


T(°F) = T(°C) × 1.8 + 32

Or you can refer to the table below which details celsius to fahrenheit conversions for easy reference.

Celsius (°C) Fahrenheit (°F)
35 °C 95 °F
35.1 °C 95.18 °F
35.2 °C 95.36 °F
35.3 °C 95.54 °F
35.4 °C 95.72 °F
35.5 °C 95.9 °F
35.6 °C 96.08 °F
35.7°C 96.26 °F
35.8 °C 96.44 °F
35.9 °C 96.62 °F
36 °C 96.8 °F
36.1 °C 96.98 °F
36.2 °C 97.16 °F
36.3 °C 97.34 °F
36.4 °C 97.52 °F
36.5 °C 97.7 °F
36.6 °C 97.88 °F
36.7 °C 98.06 °F
36.8 °C 98.24 °F
36.9 °C 98.42 °F
37 °C 98.6 °F
37.1 °C 98.78 °F
37.2 °C 98.96 °F
37.3 °C 99.14 °F
37.4 °C 99.32 °F
37.5 °C 99.5 °F
37.6 °C 99.68 °F
37.7 °C 99.86 °F
37.8 °C 100.04 °F
37.9 °C 100.22 °F
38 °C 100.4 °F
38.1 °C 100.58 °F
38.2 °C 100.76 °F
38.3 °C 100.94 °F
38.4 °C 101.12 °F
38.5 °C 101.3 °F
38.6 °C 101.48 °F
38.7 °C 101.66 °F
38.8 °C 101.84 °F
38.9 °C 102.02 °F
39 °C 102.2 °F
39.1°C 102.38 °F
39.2 °C 102.56 °F
39.3 °C 102.74 °F
39.4 °C 102.92 °F
39.5 °C 103.1 °F
39.6 °C 103.28 °F
39.7 °C 103.46 °F
39.8 °C 103.6 °F
NEXT Common causes and types of fever

Other Articles of Interest