It’s 98.6 degrees Fahrenheit, right? Not necessarily, and here’s why that matters.
If you’ve ever taken your temperature only to wonder if you’re cold-blooded after getting a low reading, it could be because the standard 98.6 degrees F is based on some very old research.
“A German physician came up with 98.6 degrees F, or 37 degrees Celsius, as the average human temperature, after obtaining millions of axillary (armpit) temperatures from 25,000 patients in 1851,” says Rose Taroyan, MD, a family medicine physician at Keck Medicine of USC and clinical assistant professor of family medicine at the Keck School of Medicine of USC.
Recently, the thinking on body temperature has changed a bit based on more current data. But in the age of COVID-19, in which fever is a main symptom, how does this shift affect how doctors determine a high body temperature? Let’s examine the evidence.
How normal temperatures for adults have changed
New data seem to point to temperatures trending downward in the last century and a half. One study that looked at body temperatures since 1862 found a steady decline (a fraction of a degree) every decade, even when accounting for different methods of measuring temperatures. A recent large study found adults’ average temperature today to be 97.8 degrees F.
It seems like people’s bodies are actually less warm than they used to be. Doctors are now recognizing this variation in temperatures.
“Normal body temperature may be slightly higher or lower than 98.6 degrees F,” Taroyan says. “For a typical adult, body temperature can be anywhere from 97 to 99 degrees F.”
What can influence your body temperature
Taroyan says that many factors can influence the average temperature range. For example, does body temperature change during the day? Yes, and it’s usually higher in the evening. “It can also vary based on other factors like heavy clothing, physical activity, hot weather, warm foods and recent immunizations,” she explains. In addition, a woman’s menstrual cycle may raise body temperature by 1 degree or more after ovulation.
Why a fever occurs in the body
When you’re sick, your body fights off infection, which raises your body temperature.
“Fever is an inflammatory response — inflammatory proteins, called cytokines, trigger the hypothalamus in the brain to increase body temperature,” Taroyan says. “But fever is not synonymous with infection, because it can be triggered by any inflammatory response.”
This is why some autoimmune diseases, which cause inflammation, also sometimes cause fever.
Many conditions can cause a fever, such as:
- Viruses, like the flu or COVID-19
- Bacterial infections, such as urinary tract infections
- Autoimmune conditions, like rheumatoid arthritis and lupus
- Some cancers
- Some medicines or recent vaccines
So, what is considered a fever?
If body temperatures vary from person to person, do fevers, as well? The standard criteria for a fever, Taroyan says, is 100.4 degrees F or 38 degrees C. The Centers for Disease Control and Prevention does note, though, that number may not be the same for everyone and that a fever really means a person’s temperature is elevated beyond their norm.
Some research has suggested that the threshold for a fever should actually be lower; the United States National Library of Medicine says an adult probably has a fever when the temperature is 99 to 99.5 degrees F or above. The starting point for fever in a child may also range from 99 degrees F (measured under the armpit) to 100.4 degrees F (measured rectally).
Given the wide range, how can you tell if you actually have a fever or not? You may have other symptoms of a fever, such as feeling warm to the touch, feeling flushed or having the chills. If you’re familiar with your usual temperature (such as, if you take it daily at the same time, or have a fitness tracker that records your temperature), you may also be able to see your personal temperature baseline.
How to take your temperature
“There are many thermometers on the market today, each with pros and cons,” according to Taroyan. “Oral, under-the-tongue thermometers take several minutes and may be altered by recently eating or drinking but otherwise can be fairly accurate,” she says. Armpit thermometers also require several minutes and can be less accurate. Forehead thermometers are fast and easy but are expensive and error-prone, as are ear thermometers, which can even be affected by earwax.
Your choice may come down to personal preference, but Taroyan does recommend going digital. “Digital thermometers are the most accurate and fastest to use, however, they’re expensive,” she says. “If you’re unable to get a digital thermometer, the next recommendation would be a regular oral thermometer.”
When to call the doctor
Taroyan says a fever is not a reason to rush to the doctor, generally speaking.
“Fever by itself is not dangerous, until it reaches 106 degrees F,” she says. “But, in the context of other neurological findings like lethargy, it requires an emergency visit.”
She recommends scheduling an appointment with your doctor or visiting urgent care if:
- Your temperature is greater than or equal to 103 degrees F.
- You’ve had a fever for more than three days.
- You have a fever with symptoms like throat-swelling, vomiting, headache, chest pain, stiff neck or rash.
If you are concerned you might have COVID-19, you can call your doctor to see if you should be tested. Although the range of coronavirus symptoms is wide, they include cough, shortness of breath, loss of taste or smell, congestion, nausea or vomiting. If you have emergency warning signs of COVID-19, such as trouble breathing, pressure in the chest, inability to stay awake, confusion or bluish lips or face, seek emergency care.
Virtual visits, also called telehealth, can be an easy way for your doctor to do an initial assessment of your fever and accompanying symptoms to see what further action you should take.