What Exactly is 'Normal' Human Body Temperature?

In 2020, researchers found that the average body temperature had decreased slightly over time.

Either from a healthcare provider or your parent, you may have learned that the standard average human body temperature is 98.6 degrees Fahrenheit. 

But that number is more than just a reading on a thermometer. While it varies from person to person, your body temperature is essential to your health. Your body temperature can signal if something is slightly off or is majorly wrong.

Here's what you should know about the average body temperature, what it means if the reading on the thermometer is slightly higher or lower than 98.6 degrees Fahrenheit and whether research has found that it's decreasing.

What Is Normal Body Temperature?

Human body temperature can vary by age, activity, and time of day. But the generally accepted reading is 98.6 degrees Fahrenheit, which is 37 degrees Celcius.

That number has been the standard reading since 1871, after German physician Carl Reinhold August Wunderlich conducted readings on millions of patients.

Still, it's essential to consider a healthy body temperature as a range rather than a specific number. Anywhere between 97 to 99 degrees Fahrenheit is a healthy body temperature.

How Does Body Temperature Change?

Everyone has a unique body temperature that's healthy for them. One study published in 2017 in the BMJ found that body temperature can differ based on demographic factors. For instance, older adults tend to have cooler body temperatures than younger adults.

Additionally, the researchers found that specific medical conditions link to lower or higher temperatures. For example, people with hypothyroidism (an underactive thyroid) may have lower body temperatures than usual. Also, people with cancer may have higher body temperatures than average.

You may also notice that your body temperature shifts as your body adjust to environmental conditions. For example, your body temperature rises while exercising. Or, a chilly day may cause your reading to decrease.

The part of your brain called the hypothalamus controls those changes. The hypothalamus checks your current body temperature and compares it with the base body temperature of 98.6 degrees Fahrenheit.

If your body temperature is too low, the hypothalamus sends signals to your body to generate and maintain heat. Likewise, when your body temperature is too high, it signals the body to give off heat or sweat to cool the skin.

Additionally, body temperatures range depending on the time of day and place where the reading is taken. The lowest body temperature usually occurs around 4 a.m., and the peak temperature occurs at about 6 p.m.

Also, the average rectal temperature is slightly higher than the oral temperature. In contrast, the armpit temperature is typically lower than the oral temperature. Also, body temperature usually rises 0.5 to one degree after ovulation. During the menstrual cycle, ovulation happens when one of the ovaries matures and releases an egg cell.

What Is Abnormal Body Temperature?

Body temperature becomes abnormal when it dips below or rises above that range of readings. 

A fever means a temperature between 99 to 99.5 degrees Fahrenheit in adults, depending on factors such as the time of day or physical activity.

A low-grade fever is between 99.1–100.4 degrees Fahrenheit, while a high-grade fever is between 102.4–105.8 degrees Fahrenheit. Brain damage may occur if the fever increases to 107.6 degrees Fahrenheit or more.

Using a thermometer to check body temperature is essential, especially in children. Some evidence suggests that judging body temperature with touch, like the back of your hand, is inaccurate. Also, remember that you should closely monitor body temperatures and fevers in babies, especially up to 3 months old. 

However, a fever is not necessarily a diagnosis or disease but rather a symptom. In other words, a fever is a sign that your body may be trying to fight off an illness or infection.

What Causes a Fever?

People get a fever when their brain sets the body temperature higher than average. A fever may happen as a reaction to viruses or bacteria.

Most microorganisms thrive at your average body temperature. But when that temperature increases, it's difficult for them to survive.

Fevers can also happen as a reaction to substances made by the body, such as prostaglandins. Prostaglandins are chemicals your body produces in response to tissue injury, infection, or illness. Prostaglandins are also involved in the menstrual cycle. The chemicals cause the uterine muscles to contract and expel tissue during menstrual bleeding.

What Is Hyperthermia?

Hyperthermia is an abnormally high body temperature. Hyperthermia usually happens if your body's heat-regulating mechanism fails to deal with excess heat. Health-related factors that may lead to hyperthermia include:

  • Dehydration
  • Age-related changes to the skin, such as impaired blood circulation and inefficient sweat glands
  • Heart, lung, and kidney diseases, as well as any illness that causes general weakness or fever
  • High blood pressure or other conditions that require changes in diet
  • Reduced sweating caused by medications such as diuretics, sedatives, tranquilizers, and certain heart and blood pressure drugs
  • Taking several drugs for various conditions
  • Being substantially overweight or underweight
  • Drinking alcoholic beverages

What Is Hypothermia?

Just as body temperature can rise too high, it can also dip too low. Readings below 95 degrees Fahrenheit are known as hypothermia

Hypothermia happens after prolonged exposure to frigid temperatures. Your body loses heat faster than it can produce, eventually using up its stored energy.

Eventually, low body temperature will impact your brain. For example, you might have trouble thinking clearly or moving. Often, someone experiencing hypothermia won't even know that it's happening and won't be able to react.

Other conditions that can lead to hypothermia include:

  • Infection
  • Drug or ethanol ingestion
  • A central nervous system event
  • Malnutrition
  • Endocrine deficiencies

Is Normal Human Body Temperature on the Rise?

According to one study published in 2020 in Elife, what we consider "normal" body temperature may have dropped by nearly one degree over 150 years since the study's publication.

The researchers from Stanford University analyzed over 650,000 readings taken from over 190,000 people spanning over two centuries, concluding that the "norm" had dropped to 97.9 degrees Fahrenheit.

"Our temperature's not what people think it is," Julie Parsonnet, MD, the study's lead author and a professor of medicine and health research and policy at Stanford University, said in a statement. "What everybody grew up learning, which is that our normal temperature is 98.6 [degrees Fahrenheit], is wrong."

While analyzing the data, the researchers observed that the average body temperature had between 0.03 and 0.29 degrees Celcius per decade for men and women, respectively.

The decreasing average body temperature could be due to various factors. For example, the adult population has grown in average height and weight over time. Those factors may impact the average body temperature.

Additionally, compared to the 19th and 20th centuries, when the researchers acquired the readings, people have better access to nutrition and healthcare services.

Then, there is the fact that many infectious diseases that would have caused a temperature spike previously have become treatable. Public health campaigns have also lessened the burden of those diseases, like tuberculosis and syphilis, with medicines and vaccines.

However, some medical experts are skeptical about the findings of the 2020 Elife study. 

"While intriguing, there are so many methodological issues with the study. It's hard to know how valid these results are," Matthew Mintz, MD, an internist based in Bethesda, Md., told Health.

Some of those problems with the study may include a lack of knowledge about what ailed the people studied, where people took their body temperatures—for example, oral, rectal, or armpit—and what instrument recorded the temperature.

Does It Matter if Normal Body Temperature Has Dropped?

Dr. Mintz pointed out that even if body temperature has dropped, it is unclear if it even means anything. 

"The body temperature is fairly constant because it is at this temperature where the body's chemical processes function the best," said Dr. Mintz. "Even if you are in the hot sun or freezing cold, the body maintains its temperature for optimal functioning."

So, experts may have differing views on whether the average human body temperature is declining. But they agree on one thing: The temperature currently considered a fever is still a fever and should not be changed. 

A Quick Review

While people typically consider 98.6 degrees Fahrenheit to be the average body temperature, that may fluctuate slightly for individuals. Some experts believe that the average normal body temperature is decreasing. 

In any case, knowing your average body temperature is helpful. Healthcare providers regularly check your body temperature at regular visits or if you are in the hospital. If you're feeling ill, manually checking your body temperature can indicate a low- or high-grade fever. Knowing how your fever responds to medicines like Tylenol (acetaminophen) or Motrin (ibuprofen) is also helpful.

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