What Defines a Fever?
Normal body temperature isn't a single, fixed number but a range that varies by person, age, and time of day. Fever is the body's natural defense mechanism, raising its temperature to fight off infection. It is important to know the specific temperatures that signify a fever, depending on where the measurement is taken.
Oral Temperature
An oral temperature is taken with a digital thermometer placed under the tongue. For adults and older children, a reading of 100°F (37.8°C) or higher indicates a fever. It is important to wait at least 20–30 minutes after eating or drinking anything hot or cold to ensure an accurate reading.
Rectal Temperature
This is the most accurate method, especially for infants and young children. A rectal temperature of 100.4°F (38°C) or higher is considered a fever. A rectal reading typically runs slightly higher than an oral reading.
Armpit (Axillary) Temperature
Taking a temperature under the arm is convenient but the least accurate method. An armpit temperature of 99°F (37.2°C) or higher is a sign of fever. If an armpit reading suggests a fever, it is often recommended to follow up with a more accurate method, like an oral or rectal reading.
Forehead (Temporal) Temperature
Temporal artery thermometers use an infrared sensor to measure the temperature of the temporal artery on the forehead. A reading of 100.4°F (38°C) or higher is typically considered a fever. This method is less accurate than a rectal reading but offers a quick, non-invasive option.
Fahrenheit vs. Celsius: A Conversion Table
Understanding both temperature scales is helpful, especially when consulting with a healthcare provider or referring to medical resources from different parts of the world.
| Fahrenheit (°F) | Celsius (°C) | Condition (Oral Reading) |
|---|---|---|
| 97.5°F - 99.5°F | 36.4°C - 37.5°C | Normal range |
| 100°F | 37.8°C | Low-grade fever |
| 100.4°F | 38.0°C | Fever (common threshold) |
| 102°F | 38.9°C | Moderate fever |
| 104°F | 40.0°C | High fever |
| 105°F+ | 40.6°C+ | Very high fever, seek medical attention |
What Causes a Fever?
Fever is not an illness itself but a symptom indicating that your immune system is responding to a threat. Common causes of fever include:
- Infections: Viral infections like the flu, COVID-19, and the common cold, as well as bacterial infections such as strep throat, pneumonia, and urinary tract infections, are common culprits.
- Immunizations: Many people, especially children, experience a low-grade fever for a day or two after receiving a vaccine.
- Inflammatory Conditions: Conditions like rheumatoid arthritis and lupus can cause fever as a result of the body's overactive immune response.
- Heat Exhaustion: Overheating due to intense physical activity or exposure to hot weather can also cause an elevated body temperature.
How to Manage a Fever at Home
For most mild fevers, the focus is on managing discomfort rather than immediately eliminating the fever, which serves a purpose in fighting infection.
- Stay Hydrated: Drink plenty of fluids like water, electrolyte solutions, and clear broths to prevent dehydration caused by sweating.
- Rest: Allow your body to rest and recover. Avoid strenuous activity.
- Use Over-the-Counter Medication: Acetaminophen or ibuprofen can be used to reduce fever and discomfort, following package directions carefully. Always follow product instructions and consult a healthcare professional before administering medication, especially to children.
- Stay Cool: Wear light clothing and use a thin blanket. Take a lukewarm bath or use a cool, damp washcloth on your forehead and neck.
When to Seek Medical Attention
While many fevers resolve on their own, certain signs warrant a call to a doctor or a trip to the emergency room.
- Infants (under 3 months): Any fever in an infant should be evaluated by a healthcare provider.
- High Fever: A temperature of 105°F (40.6°C) or higher is cause for immediate concern, especially if it doesn't come down with treatment.
- Accompanying Symptoms: Seek urgent care if a fever is accompanied by a severe headache, stiff neck, rash, sensitivity to light, confusion, or seizures.
- Persistent Fever: If a fever lasts for more than 48–72 hours despite home treatment, or if fevers come and go for a week or more, it's time to see a doctor.
Types of Thermometers
Different types of thermometers are available, each with pros and cons.
- Digital Thermometers: Use heat sensors and can be used for oral, rectal, or armpit readings. They provide quick, accurate readings.
- Infrared Thermometers: Use infrared technology to measure temperature from the forehead (temporal) or ear (tympanic). They offer a fast, non-contact option but may be less accurate than rectal readings.
- Mercury Thermometers: Once common, these are no longer recommended due to the potential toxicity of mercury if the glass breaks.
- Forehead Strips: These change color based on temperature but are not accurate enough to reliably diagnose a fever.
For a detailed comparison of thermometers and their accuracy, consult the Centers for Disease Control and Prevention's guidance on taking your temperature.
Conclusion
Understanding how many degrees is a fever on a thermometer and the various factors involved is crucial for accurate self-monitoring and knowing when to seek professional help. A fever is a sign that your body is fighting back, and proper management can aid in a quicker recovery. While home care is often sufficient, recognizing the signs of a more serious issue ensures you get the right care at the right time.
