It's important to know your normal body temperature so you'll be able to tell when you have a fever.

Until recently, you probably only thought about your body temperature around flu season. But during the pandemic, knowing your body temperature is critical because fever can be one of the key symptoms of COVID-19, the illness caused by the coronavirus.

You might be taking your temperature regularly or having it checked before entering a business or your workplace. When the numbers pop up, you're likely surprised they're not 98.6 degrees. Here's what you need to know about body temperature.

What's a 'normal' body temperature?

You probably grew up believing normal body temperature was 98.6 degrees Fahrenheit (37 degrees Celsius). The number is credited to two different groups of 19th-century researchers, but most widely believed to be based on a study by German physician Carl Wunderlich. He reportedly analyzed more than 1 million temperature readings via armpit from more than 25,000 people and found 98.6 F (37 C) was the average temperature of healthy adults.

Wunderlich also recorded some interesting anomalies with temperature. He found it was lowest between 2 and 8 a.m. and highest between 4 and 9 p.m. He also noted that women tended to have slightly higher temperatures than men, and younger people generally have higher temperatures than older people.

Wunderlich's numbers have been widely accepted ever since. But more recent studies suggest human body temperatures have been falling over time. A 2002 review of 20 studies between 1935 and 1999 found the range for oral temperatures was between 33.2 and 38.2 degrees C (91.76 to 100.76 F). "The ranges of normal body temperature need to be adjusted, especially for the lower values," the researchers wrote in the Scandinavian Journal of Caring Sciences. Similarly, a 2017 study published in The BMJ found the average body temperature now is 36.6 C (97.8 F).

In an even more recent study, Stanford University researchers compared more than 677,000 temperatures from three cohorts including Civil War veterans and people who had participated in two recent studies. Their findings confirmed that men born in the early 19th century had an average body temperature of 98.6 F but since then, it has fallen to an average for men and women of 97.5 F. The results were published in the journal eLife in January 2020.

“We’ve changed in height, weight — and we’re colder,” senior author Julie Parsonnet of Stanford tells Scientific American. “I don’t really know what [the new measurements] mean in terms of health, but they’re telling us something. They’re telling us that we are changing and that what we’ve done in the last 150 years has made us change in ways we haven’t before.”

Why temperatures could be falling

The researchers didn't determine why body temperatures have dropped, but there are several possibilities.

One primary possibility is that today, people have lower metabolic rates. "Like a car engine that’s idling, your body expends energy just keeping things going, and that generates heat," writes Robert H. Shmerling, MD, in Harvard Health. "A lower metabolic rate in modern times could be due to higher body mass (some studies link this with lower metabolic rate), or better medical treatments, preventive measures, and overall health."

Since Wunderlich's time, there has also been a significant decline in infectious diseases and inflammation. As Shmerling points out, diseases like tuberculosis, syphilis, chronic gum disease, and similar conditions that cause inflammation were common centuries ago. They hiked body temperature and there were few treatments.

How to take your temperature

Many businesses are now using non-contact infrared thermometers to do temperature checks.

Because a fever is one of the symptoms of COVID-19, many people are being asked to take their temperature as they enter restaurants and hair salons, offices, and other businesses. You might even check yours regularly at home. It's important that you know your normal temperature so you know when it's elevated.

In most cases, a fever is considered to be a temperature of 100.4 F (38 C) or above.

There are several types of thermometers available. You can take your temperature orally, rectally, in your armpit, or on your forehead. An ear or rectal temperature reading will usually be higher than one taken orally. A temperature taken in the armpit will usually be lower than one taken orally.

Digital: You can use a digital thermometer under your tongue, in your armpit, or rectally. The oral method is most common and is used for most adults and older children. Rectal thermometers are used most often for infants and are the most accurate method. The armpit method is not as accurate as other methods, but used for those who can't sit still for oral or rectal methods.

Ear: Also called tympanic thermometers, these devices measure the temperature inside of the ear and are popular for older babies and children because they're quick. They shouldn't be used if a child has a lot of earwax or an earache.

Forehead: These thermometers are placed on the temporal artery of the forehead to measure the heat coming off the head. They are not as accurate as a digital thermometer.

Non-contact infrared thermometers: These devices are very popular with businesses right now because they involve no contact. The thermometer is placed near a person's forehead, but not touching it.

If you have an old glass thermometer with mercury, don't use it. These thermometers were popular before digital thermometers were available, but they were hard to read and not always accurate.

"The main reason they are no longer recommended is that mercury can poison you. This can happen when the glass breaks and mercury is released. If you do still have one of these thermometers, you should contact your local waste department and find how to dispose of hazardous waste properly," says the Cleveland Clinic. "There are glass thermometers available that do not use mercury, but most people prefer the digital thermometers that do not shatter."