Temperature Values are Changing: After 150 Years, the Norm is No Longer 98.6° F

Pat Stricker, RN, MEd
Former SVP, Clinical Services
TCS Healthcare Technologies

Everyone knows a normal temperature is 98.60 F, right?  Well, that consistent standard that we all knew to be true is no longer correct. The recent results of a study conducted at Stanford University found that the average, normal temperature has been decreasing about 0.050 F every decade for the past 150 years. That means, instead of the normal temperature being 98.60 F, the new normal body temperature, compared to temperatures in the early 1900s, has dropped to 97.50 F for men (1.060 F less) and 98.0 F for women. This seems unreal. What has caused this? Could it be the more accurate thermometers that are being used today? Let’s start by looking at the development of the thermometer over the years.   

History of the Thermometer

The precursor of the thermometer invented in 1593 by Galileo Galilei was known as a thermoscope, not a thermometer, because it had no scale and could not measure differences in temperatures or record results for future reference. However, it was easy to develop a thermometer measuring device by simply adding a scale. That was done in 1612 by Santorio Santorio, the inventor of the thermometer. The first liquid glass thermometer that we are familiar with today was developed by Grand Duke Tuscany, Ferdinand II in 1654, but it was inaccurate and had no standardized scale.  In order for thermometers to be accurate, they had to find a way to include a standardized scale.  

In 1714, Daniel Gabriel Fahrenheit invented the first modern, accurate mercury thermometer with a standardized (Fahrenheit) scale, which is still used today. In 1742, Anders Celsius developed a thermometer scale using 0° for the boiling point of water to 100° for the freezing point, and a year later, Jean Pierre Cristin inverted the scale, creating the Centigrade scale. (NOTE: In 1948, Cristin’s adapted scale officially became the Celsius scale, which is used today). The liquid-filled glass (alcohol and mercury) thermometers were basically accurate and remained unchanged for about a hundred years.

The first real medical thermometer was invented in 1867 by Sir Thomas Allbut, but it was six inches long and took about five minutes to take a person’s temperature. Over the years, these liquid-filled alcohol and mercury glass thermometers were made smaller and quicker to obtain a temperature. Infrared and electrically-sensored thermometers developed after World War II were made even quicker and more accurate, but they still use the same numerical scales developed in the 1700s by Fahrenheit and Celsius.

Temperature Research Studies

Since the development of the thermometer, research studies have been conducted to determine norms and variations. As the years passed, large datasets were analyzed to determine if different types of thermometers or the way temperatures were taken made a difference in determining the standard normal body temperature. The theory was that these would not make a difference since physicians in each historical period were basically using the same types of thermometers and temperature-taking procedures, so the value for the normal temperature should be consistent and accurate.

One of the first studies was conducted by a German physician, Carl Reinhold August Wunderlich in 1851. He collected millions of underarm temperatures from about 25,000 patients, studying temperature variations with illnesses and between people of different ages, genders, weights and heights. His study identified 37°C (98.6°F) as the average, normal body temperature. Other U.S. and European studies in that timeframe found the same results, so 98.6°F became the average, normal body temperature.

In more recent times, the results of a review of studies analyzing temperatures conducted between 1935 and 1999 identified normal temperature ranges based on the type of temperature taken. The oral temperature showed a large range of 91.8-100.7° F.  Another study conducted from 2009-2014 in the United Kingdom of more than 35,488 people with over 243,506 temperature measurements found the average oral temperature to be 97.9° F. The men’s temperatures were about 1.1 degree lower and women’s were about 0.6 degrees lower than the norm of 98.6° F. The study also determined that variations were not due to measurement error or environmental factors.

In 2017, another study using data from the Feverprints iPhone app analyzed 5,038 temperatures from 329 healthy adults from 2016-2017 and found the normal temperature may be closer to 97.70 F. The study recommended that body temperatures should be considered "flexible” based on the patient's weight, height, age, gender and time of day, as well as other pertinent factors.  

In February 2020, the results of the largest study to date was released. It was conducted at Stanford University and included analyses of patterns from three large data sets that include 677,423 temperature measurements from nearly 190,000 patients between 1862-2017:

• 1862-1930:  Union Army veterans of the Civil War
• 1971-1975:  United States National Health and Nutrition Examination Survey I
• 2007-2017:  Stanford Translational Research Integrated Database Environment

Some of the researchers' significant findings include:

• On average, the temperatures dropped by 0.05°F per decade over the 150-year span.
• The mean body temperature is 1.6% lower than in the pre-industrial era.
• The body temperature of men born in the 2000s are, on average, 1.06° F lower than men born in the early 1900s; Women’s temperatures are about 0.58° F lower than women born in the 1890s.
• Temperatures for Civil War veterans were higher than the other two groups. Some experts feel the Civil War data is “inherently suspect," since it is too old, may have had different types of thermometers, it may have included soldiers with wounds or inflammation, and some variables may not have been controlled for.
• To minimize bias, the study compared temperatures within a single population to eliminate problems related to inaccurate thermometers in the early years and the way physicians took temperatures. The trends remained strong, showing temperatures were lower in those born later.
• Temperatures differ from person to person; by age, sex, and time of day (half a degree higher in afternoon than in the morning); and are lower in the elderly.
• Normal body temperature ranges from 97 – 99° F according to the National Institutes of Health.
• The Stanford study predicts that temperatures will decline more slowly now as the increase in life expectancy starts to level off.

Reasons for Decreased Temperature

Even though 98.6° F has been an accepted norm since 1851, it is obvious that it is just a general temperature guideline. It is impossible for every living person to have the same temperature for a variety of reasons. The following are just a few of the reasons for this change.

• Human physiology has changed over the past 200 years, making us physiologically different.
• Metabolism, which plays a big factor in maintaining body temperature and energy production, has changed over the years.
• People have grown in height and weight and become less active.
• Lifestyle also plays a factor in the decreased temperature values. Today’s sedentary lifestyle leads to less energy production, decreased metabolism, and a lower temperature compared to previous generations, who were much more physically active.
• The environment has also changed. Heating and air conditioning keep homes at a more constant temperature, which results in expending less energy to maintain body temperature.
• The presence of infectious diseases and inflammation have decreased over the years, eliminating the need for the body to fight them off.
• Nutrition has improved.
• There is better medical care today, as well as more public health services. 

Research Study Recommendations

The studies found that temperature changes were not due to improved thermometers, since the temperature declines occurred during years when thermometers were the same or similar. Some of the research studies recommended moving away from the concept of an average normal temperature to an individualized or flexible body temperature based on height, weight, age, time of day, and outside temperature. However, in general, most researchers are not recommending that temperature standards and guidelines be updated at this time. They feel they should continue to be used by physicians as a general assessment point, but that physicians should continue to treat patients based on how they look, feel, and act, not on an actual temperature value. And they agree that the current fever temperature (100.4° F) should not be changed.  

The data collected in these studies should allow us to estimate what a normal temperature is for an individual person. However the algorithm needed to determine that normal temperature using all variables (gender, age, weight, height, time of day, and other specifics pertinent to the individual) would be much more complicated.  

So for now, the normal temperature is still officially 98.6° F, but normal temperature ranges should be considered to be 97 – 99° F, according to the National Institutes of Health.