If your forehead feels even slightly warm these days, it’s easy to assume the worst-case scenario. Fever is a hallmark symptom of COVID-19. And although it’s not the only indicator of the coronavirus, or even a surefire one (many illnesses cause fever), temperature checks are a relatively quick and inexpensive way to identify a possible infection and help spread the curb of the virus.
You’d think this would be a straightforward process, but there are actually lots of factors that go into determining what’s a normal temperature for someone and what’s an unusual spike. Furthermore, regular temperature checks — at office buildings, stores, all sorts of appointments — will be part of the new, post-COVID normal. So get used to fever screenings and make sure you understand the importance of how, when and even where you measure your body heat.
Here’s what to know about taking and interpreting your temperature.
What is a normal body temperature range?
The long-established average body temperature of 98.6 degrees Fahrenheit is just that — an average, and it may not even be accurate. In line with other recent research, a January 2020 study found that the average body temperature for men and women in the US has fallen to 98.5 degrees. Regardless, that’s not a rule your own body will necessarily follow. Your baseline body temperature could be higher or lower — anywhere in the range of 97.2 to 99.9 degrees Fahrenheit — based on factors including gender, body weight, age and even the weather.
Thermometer readings also vary depending on the time of day you take your temperature. For one thing, your body temperature is lower in the morning and higher later in the day, changing throughout the day by as much as 1 degree Fahrenheit. A recent study even suggests that taking temperatures in the morning as part of COVID-19 screening could miss potential carriers. So “normal” isn’t something that a single reading can establish.
Are all thermometers equally trustworthy?
If you have a digital stick thermometer, you can take your temperature under your tongue (orally), in your armpit or rectally. Each of these methods can produce slightly different readings in the same person. Temperatures measured rectally are typically one-half to a full degree Fahrenheit higher than those taken orally, with a similar increment found between armpit and oral measurements. Of these three methods, armpit readings are the least accurate; consider them a last-resort option.
Tympanic (ear) thermometers and temporal (forehead) thermometers, both types of infrared thermometers, are quick and easy to use, especially for taking the temperatures of babies and toddlers. They measure thermal radiation emitted from the ear and forehead to infer core body temperature. While the most accurate infrared thermometers need to touch the skin (e.g., a quick swipe across the forehead), others have scanners that enable temp checks without direct contact. These no-touch thermometers still need to be held within millimeters of a person’s body, though. And both sweat and air temperature can influence their readings.
Overall, “rectal and tympanic temperatures are the most accurate, but not the most practical,” says Seattle-based Dr. Chunbai Zhang, incoming medical director of VA Puget Sound Health Care System. “Taking a forehead temperature is more convenient, but this slightly underestimates the core temperature.”
Exercise, eating, drinking a hot or cold beverage, or bathing in hot water can also affect temperature readings. “Wait at least 30 minutes after you drink cold or hot liquid, eat or smoke before taking a temperature by mouth,” says Dr. Tammy Penhollow, an anesthesiologist in Phoenix. “And wait at least 15 minutes after activity or exercise, or if coming in from the cold or the heat.” You can use an ear thermometer if you wear hearing aids, but you should remove your hearing aid and wait 20 minutes before taking a reading.
There are also times when it makes sense to avoid specific temp-check methods altogether. For instance, if your forehead is covered, or you have a lot of ear wax, use a different part of your body.
For obvious reasons, no-touch thermometers and scanners are gaining popularity as COVID-19 screening tools at places including offices, daycare centers and restaurants. Companies such as Amazon use infrared cameras to screen workers for fever; airlines such as Frontier are screening crew and passengers for fever with touchless thermometers, prohibiting anyone with a temperature of 100.4 degrees Fahrenheit or higher from flying. (Frontier will allow passengers to be retested and potentially fly following a short rest, as time permits.)
While it would be frustrating and disappointing to be turned away from your office or a flight for a questionable fever — especially since readings can fluctuate by as much as one degree — erring on the side of caution is the right thing to do. Similarly, no one should enter an enclosed space with other people if they are sick or have COVID-19 symptoms, whether or not they have a fever.
What’s considered a “high” temperature?
The latest CDC definition of a fever, as it pertains to COVID-19, is at least 100.0 degrees Fahrenheit, recently lowered from a minimum of 100.4 degrees Fahrenheit. “Generally, a temperature over 100.4 degrees Fahrenheit most often indicates the presence of fever caused by infection or illness,” Penhollow says. “But the CDC changed the recommendation, so a fever is either a measured temperature of 100.0 degrees Fahrenheit or higher or a subjective fever. For healthcare professionals, 100.0 degrees Fahrenheit is the threshold to screen and discuss COVID-19 symptoms with patients.”
That said, a fever of 100.0 degrees Fahrenheit or higher isn’t proof that COVID-19 is present.
What patterns should patients look out for?
“A diagnosis of any disease, let alone COVID-19, cannot be made based on a single temperature,” Penhollow says. “An endoscopy center I know is having patients keep temperature logs for 14 days before their elective colonoscopy or endoscopy procedures. The admitting nurse reviews a patient’s log before bringing them back for the procedure. Doing this, at least 14 data points are available to review for a trend, but even that is only one symptom: fever.”
Penhollow points out that additional symptoms should be considered when assessing someone for potential COVID-19. According to the CDC, these symptoms include cough, shortness of breath or difficulty breathing, sore throat, chills, muscle pain and loss of taste or smell.
Conversely, temperatures in the normal range don’t necessarily mean COVID-19 isn’t present. Not everyone who’s ill runs a fever, and experts say asymptomatic carriers (with normal-range temperatures) can still be contagious.
Regardless, taking your temperature and establishing your own personal baseline is an easy way to assess whether you’re at risk of being sick and potentially spreading the coronavirus to others. If you or the person whose temperature you’re taking have a fever — anything higher than 99.9 degrees Fahrenheit — the best course of action, until a doctor tells you otherwise, is to stay home.