Thermometers and body temperature: Experts answer pressing questions

Published May 11, 2020 7:10 p.m. ET
Updated Oct. 5, 2020 9:53 a.m. ET

Share this story:

Click to Expand

TORONTO -- While not everyone infected with COVID-19 may develop fever, it is still one of the most common symptoms associated with the novel coronavirus.

You can easily check your body’s temperature through the use of a thermometer; experts even recommend keeping one in your medicine cabinet to monitor for this telltale sign of the virus. But with so many different types of thermometers to choose from, it can be hard to determine which is best for you.

According to Natalie Crown, a pharmacist at Women’s College Hospital and assistant professor of teaching at the University of Toronto’s faculty of pharmacy, finding the right thermometer boils down to two things:

“The two main considerations are you want to get an accurate measurement…and ease of use,” Crown told over the phone on Monday.

These factors guide many of the recommendations made by health professionals and associations on which thermometers are best to use for adults and children of different ages, she said. spoke with Crown and Dr. Matthew Oughton, an infectious disease specialist based at the Jewish General Hospital in Montreal, about some frequently asked questions surrounding thermometers and how to use them.


The U.S. Centers for Disease Control and Prevention (CDC) consider a fever to be “a measured temperature of 100.4 degrees Fahrenheit [38 C] or greater.” That being said, the public health institute also acknowledges that this figure may vary depending on the type of thermometer being used and the age of the person being tested. Generally, it defines a fever as a temperature “elevated beyond [a person’s] norm.”

Much debate surrounds what is considered the “normal” body temperature for humans. While Harvard Medical School points out that this figure has long been considered to be about 37 C, or 98.6 F, new research suggests this may be an overestimation, and that the average body temperature for humans today is actually about 36.4 C or 97.5 F.

Oughton also explains that a number of factors can affect a person’s body temperature, including the time of day.

“On average, most people’s core temperatures tend to be a little bit lower in the morning at around 6 a.m. and a little bit higher in the evening, roughly around 6 p.m.,” Oughton told on Monday via telephone. “That normal variation can be as high as half of a degree Celsius.”

But in spite of all these contributing factors, along with existing debate on what is considered “normal,” Oughton said the CDC’s definition of a fever is the standard he recommends going by.

“On average, a temperature of 38 [C] is unquestionably a fever,” said Oughton. “I don’t think any physician would really quibble with 38 [C] as being a reasonable indicator [of fever].”


According to Crown, not all thermometers are likely to offer an identical temperature reading, even when they are used on the same person.

“Normal body temperature depends somewhat on where you’re measuring your temperature,” she said.

The main different types of thermometers include rectal, oral (mouth), tympanic (ear), axillary (armpit) and temporal (forehead). Seeing as different parts of the body maintain slightly different temperatures, readings from thermometers used in the mouth, on the forehead, and in the ear, for example, are not likely to produce the exact same results. Oughton explains that rectal thermometers are seen as the most accurate when measuring a person’s body temperature.

“We usually think of the rectal temperature as being the most closely reflective of the body’s true core temperature,” he said.

This is due to the fact that it is able to get the best reading from inside the body. Ear thermometers are also seen as offering a similar level of accuracy when taking temperatures.

Aside from these body parts, others often looked at to measure temperature will offer slightly varying results. According to a medical review by Healthwise, a non-profit organization specializing in health education, a person’s oral temperature is typically 0.5 C lower than that of their rectal or ear temperature. Additionally, a person’s armpit and forehead temperatures are typically 0.5 C lower than their oral temperature.

Oughton points out that it is important to keep this in mind when taking your temperature to determine if you have a fever.



For the average family with no particular special needs, Oughton said he would most recommend a digital oral thermometer.

“It’s probably the one that is the most useful overall and the best balance between cost [and] benefits,” he said.

While wait times for results can range from a few seconds to about a minute, many come with the ability to beep or offer some other signal to show that the reading is complete, making it easier to use. These thermometers are also available with either flexible or rigid tips, depending on which style is preferred.

Crown recommends that families keep a digital stick thermometer with them at home.

“It has the advantage of being able to be used three ways – you can use it under your tongue … for older children and adults,” she said. “You can also use it in younger children, under the arm to take an armpit measurement or rectally as well.”


The next best option, said Oughton, would be a forehead thermometer. They are incredibly easy to use and one of the most comfortable thermometers available. Through the use of an infrared scanner, these devices measure the temperature of the temporal artery in the forehead.

Forehead thermometers, like many other infrared thermometers, read temperature fairly quickly, usually within just a few seconds. Many also come with a feature to signal that the reading is complete, such as a beep. According to the Mayo Clinic, these thermometers should not be used on infants less than three months old.

“In [older] children and adults, temporal artery [forehead] thermometers may well be the next most accurate measurement tool reflecting the body’s true core temperature,” said Oughton.

There is one important downside – the price. Forehead thermometers are often more expensive than their digital counterparts.


One other slightly less expensive recommendation is the digital ear thermometer. These devices also measure temperature using an infrared ray, but inside the ear canal. They are usually quick to record temperatures and fairly comfortable for both children and adults.

According to the Mayo Clinic, digital ear thermometers are only recommended for infants above six months old as well as children and adults.

Oughton advises people to be cautious of any existing earwax when using these thermometers, as this can affect the accuracy of results. He also recommends that people avoid taking their temperature with digital ear thermometers if they have just come indoors from outside.

“Don’t measure the temperature right away because you can get a falsely low temperature, which is exactly the wrong thing if you’re trying to pick up a fever,” he said.

He suggests waiting about 10 minutes for the temperature in the ear canal to equilibrate to your surroundings before taking your temperature with this thermometer.


For infants, Oughton said he would recommend rectal thermometers for the most accurate reading. He also advises families to clearly label any thermometers they plan to use rectally, and to store them in their own compartment.

Crown also recommends the use of armpit thermometers for children. While she acknowledges that rectal thermometers provide a truer reflection of body temperature, a digital stick thermometer placed directly in the middle of a child’s armpit is also a good option.

“In children, the truly most accurate way of taking a temperature is to take a rectal measurement, but we know that’s hard to do,” she said. “For most young children – babies and children up to the age of five – using a stick thermometer under the armpit is actually a very effective way of taking a child’s temperature.”


Aside from having a thermometer at home that is in good working order and perhaps keeping a few extra batteries, Oughton also recommends not eating anything before taking your temperature. Having something especially hot or cold to eat or drink, for example, could have an impact on your temperature reading, depending on which thermometer is used.

“You’re not supposed to have anything in your mouth for about 15 minutes before checking your temperature if you want it to be as accurate as possible,” he said.

He also reminds people to ensure that they cover the entire sensor or bulb of the thermometer, depending on which type is used, whenever taking their temperature to ensure the most accurate reading.

Finally, Crown advises contacting your local pharmacist with any specific questions you may have about thermometers, including which kind you should be using as well as how to use it.

“We recognize that choosing a thermometer can sometimes be confusing,” she said. “Pharmacists are the most accessible health-care provider and are available to give advice to families on how to interpret the temperatures they’re getting and then also provide some advice on how to treat fevers.

“I would recommend popping into your local pharmacy or even calling them in times like these, where we’re all trying to stay home.”

Read the original version