Since a common symptom of Covid-19 is a fever, some concerned folks may be taking their temperatures more often these days. If you feel panic when your thermometer beeps and reads 0.2 degree higher than 98.6 degrees Fahrenheit (37 degrees Celsius), you might have the wrong idea, experts have said.

Any measurement over 98.6 F may seem concerning, but it’s not that simple. There are many personal, environmental and medical factors that influence body temperature and determine whether a person has a fever.

The concept of a standard body temperature is generally credited to late German doctor Carl Wunderlich, who, in the mid-1800s, analyzed over 1 million armpit temperatures of about 25,000 patients. From those readings, he came up with the average of 98.6 F and his standard has since prevailed — at least in the minds of many people who aren’t doctors.

Humans vary in their individual normal temperature ranges. Checking one's temperature by mouth is one of the best ways to do so.

“Anecdotally, I would say that there are a lot of people who may not be aware that normal temperature is a range, not a fixed number,” said Dr. Donald Ford, a family medicine physician at Cleveland Clinic in Ohio, via email. “More recent large studies show normal temperatures ranging from 97 to 99 degrees, and many individuals may fall outside of this range and be perfectly fine.”

You may know your normal blood pressure, heart rate or blood sugar level, “but not a lot of people in this world right now know what their normal body temperature is or how much it varies normally,” said Dr. Waleed Javaid, the director of infection prevention and control at Mount Sinai Downtown in New York state.

“This the time that we should start thinking about normal body temperature: What it is, what does it mean for me and so on.”

As flu season coincides with rising Covid-19 cases, understanding what a fever actually is and your personal temperature range is important. Read on to get familiar with those things, influential factors and how to accurately check your temperature.

What a fever really is and why it happens

The US Centers for Disease Control and Prevention considers a person to have a fever when his or her temperature is 100.4 F (38 C) or greater. People who feel warm or provide a history of feeling feverish may also be determined to have a fever.

When a pathogen “enters our body and our body recognizes it, it produces a reaction in our body with antibodies and other responses,” Javaid said. “Those antibodies cause other cells in our body to release some chemicals,” which enter our brain and change our temperature set point.

“The majority of times with infection, that set point goes a little higher,” he added. “Shivering causes our body to metabolize more excessively” and results in fever.

The variance of temperature ranges and how to know yours

Our body temperatures tend to be lower in the morning but peak by one or two degrees in the evening. For some people, Covid-19 fevers have tended to heighten in the evenings, just like how our temperatures normally fluctuate — so how can one distinguish this natural pattern from Covid-19 symptoms?

A Covid-19 fever “will stay high, so it won’t go away,” Javaid said. “Fever versus body temperature fluctuation is a totally different thing. … Usually, you don’t feel any difference between your higher temperature time and low temperature time.”

Additionally, fevers are accompanied by other symptoms, including shivering.

Learning your baseline temperature range is achievable by checking it three times daily at times you’ll remember, such as morning, around noon and evening. Over several weeks, chart those results. By the end of that experiment, you should be able to average your inputs and know what is normal for you at different times of the day. If you visit your doctor frequently, you could also ask him or her your average temperature.

“When we know our body temperature, then when it varies, we can safely say that, ‘OK, this is persistently high temperature,’” Javaid said. “That’s just like, ‘I have persistently high blood pressure.’ It needs to be cured.”

Other factors can influence your temperature. Medications like acetaminophen and ibuprofen are commonly taken for pain, but they lower fevers as well. Anti-inflammatory drugs — like those often used for cancer, rheumatoid arthritis and lupus — can also affect your temperature.

People should look at the “package inserts of the medication or look online to some reputable websites about the medication they’re taking and see how it affects their temperature,” Javaid said.

The most effective thermometers and methods

Hospital-grade or thermal thermometers from reputable companies such as drugstores are best, Javaid said. Evaluate the accuracy by checking your temperature three times consecutively, with 10 minutes in between each test. If the results vary by a degree or more while you still feel the same, you probably need a better instrument.

Temperatures can be taken via the skin, mouth, rectum, armpit or ear. “Skin or ear thermometers are easy to use but may be less accurate than oral or rectal thermometers,” Ford said. “Armpit (also called axillary) are somewhere in between. The difference is that fever is defined as an elevation of core body temperature, and skin and ear membranes may be more subject to outside temperatures or exercise and may not reflect the core temperature.”

Accuracy can also depend on your environment. “If I just came from outside from freezing cold,” Javaid said, “my forehead, my ears and everything will be very low temperature for (the) next maybe 30, 40 minutes, and I should wait for an hour or two before I check my temperature on my forehead.

“You can always do the oral temperature or putting the thermometer in your mouth. That usually gives you a very good idea.”

Although assessing temperature is the most accurate method to confirm fever, taking a temperature isn’t always feasible if thermometers aren’t available. Or, if the sick person has taken medication, that would lower the measured temperature. In those cases, you could maybe detect fever by noting the appearance of a flushed face, glassy eyes or chills.

How to accurately take your temperature

In addition to extreme weather, consuming a “very hot or very cold food or drink within minutes before checking an oral temperature” can throw it off,” Ford said. Wait 30 to 40 minutes before taking your temperature.

If your reading is a degree or two higher than usual and you have no other symptoms, monitor your temperature for the next day, Javaid said. “If there’s nothing else going on, it usually will go away and go back normal overnight.” If it doesn’t, you should be concerned and get tested for coronavirus — especially if you start experiencing other symptoms such as a cough, shortness of breath, fatigue or loss of taste or smell.

Unless restricted by your doctor, taking acetaminophen or ibuprofen can alleviate your fever, which can be harmful “in terms of dehydration and exhaustion,” Ford said. “It’s important to push fluids as much as possible because that helps our body clear infections.”

How often we should check our temperature depends on a few factors, Javaid said. “One factor is what is the requirement of your job or the locality you’re in,” he said. “For example, some health care workers, we advise them to actually check their temperature daily.”

For people who aren’t following mandates, feeling unwell or warmer than usual and being unsure of why, and exposure to someone with Covid-19, are other good reasons to take your temperature.

“If a person is feeling well there is no need to check daily temperatures outside of screening,” Ford said.

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Although your body temperature can be an indicator of a Covid-19 fever, fever is not the only important factor in assessing whether you have Covid-19. Many people with Covid-19 haven’t shown any symptoms, and some may have a cough or trouble breathing but no fever.

“It can vary in different people,” Javaid said. “Checking your temperature weekly, just to see if you have Covid-19, is probably not the best way of doing it.”

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