A few days ago, I started feeling a little lousy, ‘under the weather’ as they say in the UK, and decided to take my temperature and sure enough I had a low fever of about 100.6F. In these pandemic days, even though I am vaccinated and take precautions, wear masks in public places and as much as possible only associate with vaccinated people, there is always the chance of breakthrough infections so even though I had none of the other symptoms of covid such as loss of taste and smell, I decided to take a home test to see if I had contracted covid. The test came beck negative, which was a relief, but these rapid tests are not that reliable so one can never be sure, so I decided to self-isolate until my fever went back to normal.
And this was the. problem: what I was not sure what the ‘normal’ temperature is. The conventional normal is 98.6F (37C) but I knew that my body temperature when I am not sick is much lower, around 97F. So if my temperature dropped to around 98.6F, which it did after about 12 hours, was my fever gone? Or did I still have a low-grade fever?
It turns out that the conventional measure of what constitutes normal body temperature is way overdue for a re-evaluation. For one thing, normal temperatures vary quite a bit from individual and even for any given individual, their body temperature varies with many factors, including the time of day, with temperatures being higher later in the day. So what is normal in the evening might not be so the next morning. Furthermore “Women tend to have higher body temperature than men, and younger people tend to have higher temperatures than older folks.”
But what I found most interesting is that average body temperatures have been dropping over time.
An analysis of 20 studies between 1935 and 1999 found that the average oral temperature was 97.5˚ F. And a 2017 study of more than 35,000 people found an average body temperature of 97.9˚ F.
On this last point, a remarkable new study is among the best to make a case that normal body temperature has been drifting down over the last two centuries.
In this study, researchers analyzed temperature recordings from three periods of time over 157 years:
- 1860–1940: A mix of armpit and oral temperatures of nearly 24,000 veterans of the Civil War were measured.
- 1971–1975: Oral temperatures of more than 15,000 people from a large population study (the National Health and Nutrition Examination Survey) were analyzed.
- 2007–2017: Oral temperatures of more than 150,000 people in another large research project (the Stanford Translational Research Integrated Database Environment) were reviewed.
During the nearly 160 years covered by the analysis, the average oral temperature gradually fell by more than one degree. As a result, the “new normal” seems closer to 97.5˚ F.
This observation held up even after accounting for age, gender, body size, and time of day.
So what might be the cause for this drop? It turns out that changes in the way body temperature is measured is not likely to be a factor. More likely is a change in metabolic rates, which is one of the biggest determinants of body temperature and 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.”
Having a better measure for what constitutes normal body temperature is important because it is a key measure of one’s health. Physicians might miss early warning signs of illness if they think a patient’s temperature of around 99.5F is close enough to normal to not worry about, when. in reality that person’s normal temperature may be 97F or less.
So what to do?
Fever is typically any temperature above 100˚ F. The most common cause of fever is any infection in the body, but there are other causes, including heat stroke or a drug reaction. Although you can be sick with a normal temperature, body temperature is clearly an important and useful indicator of health.
While news that the normal body temperature may be drifting down over time is intriguing, it is not cause for alarm — and it doesn’t mean the definition of fever should change. We’ll need to rely on additional research to tell us how important these findings may be. In the meantime, it’s probably time to abandon the assumption that 98.6˚ is a normal temperature. Something closer to 97.5˚ may be more accurate.
After I felt back to ‘normal’, I took my temperature multiple times and it was around 96F in the morning, so clearly I am at the lower end of whatever can be considered the normal range of body temperatures.