The pandemic has made everyone aware of the need to monitor their body temperature to see if they might be carrying the virus. I wrote earlier about how my ‘normal’ body temperature when I have no symptoms of anything is much lower than the canonical value of 98.6F, which makes it hard to determine how to use my body temperature to determine if I am sick or if I have returned to normal after being sick. This article explains that there is a huge amount of variation because a multiplicity of factors affect the temperature.
Many trace this number back to an 1871 book on medical thermometry by a German physician named Carl Reinhold August Wunderlich who measured the underarm temperatures of thousands of people. Readers seem to have taken away the monolithic 98.6 number, but Wunderlich in fact wrote that based on his measurement “healthy persons under varying conditions” had temperatures that ranged from 97.16 F to 100.4 F.
Even accounting for that variation, a recent study suggested that, across the population, core temperatures have declined about 0.6 F (0.3 C) below the traditionally accepted levels and about 0.85 F (0.5 C) compared to the end of the 19th century and beginning of the 20th century.
The researchers acknowledge these observed reductions could be the result of changes in equipment and measurement technique. But they suggest the reduction could also reflect physiological shifts in metabolic rate and reductions in inflammation related to underlying medical conditions that people manage more effectively now than in the past.
Your body’s temperature is based on the heat from your energy production. Digesting food, physical activity and infection increase the metabolic rate and can all raise your temperature.
Babies may have slightly elevated normal body temperatures and caloric needs as a result of increased metabolic rate to support growth and development, while seniors may have slightly lower body temperatures due to lower rates of cellular growth and repair.
In addition to changes that occur as the result of age, everyone has variation throughout the day based on their circadian rhythms. Both the 1871 book by Wunderlich and more recent research have demonstrated that people typically experience their lowest body temperatures overnight. Similarly, body temperature will fluctuate through the varying phases of females’ menstrual cycle.
So basically, it seems like each of us has our own ‘normal’ temperature that we can use as a baseline, that is obtained by measuring at the same part of the body using the same device at the same time of day.