Do surface temperatures measure core temperature? No.

Working from home during the COVID19 pandemic has proven a challenge for many of us. Our students are not allowed to pursue their research, and yet most of us are working as hard from home as we would be on campus.

Anyhow, at the beginning of the lockdown, I gathered what equipment I could from the lab and set up to research at home. Hardly a serious pursuit, but I was designing some training material for an overseas student and needed the equipment at home anyway.

What kind of research can you do on yourself on lockdown you might ask? A little bit of thermal imaging!

Since COVID19 is/was on everyone’s mind, I knew it was only a matter of time before the fever scanning would start up again (remembering SARS 2003 and all the thermal cameras in airports).

But thermoregulatory physiologists know that surface temperature rarely, if ever, is equal to core temperature so then, how do we reconcile this with the return to use of superficial fever scanning?

Maybe it is because imaging is appealing to people and it is compelling that a region of the face or head close to the eye always appears warm, so naturally people assume it might represent or correlate to core measurements of body temperature. Indeed, even in animal thermal biology, this is one of more common questions people ask me: “Can eye temperature be an estimate of core temperature?”. To which I quip, “No”, with caveats.

I am not aware of systematic studies demonstrating that these warm eye/head surface temperatures are really good at estimating core temperature, but we previously measured core and surface temperatures in a previous study of ours in ducklings across different times of day and during a period of fasting:

Duckling thermal images – captures at two air temperatures and under different conditions.

Core temperature rises and falls quite substantially (ranging from ~39 to ~42C) across these different states and time periods:

Core (crop) temperature in ducklings measured over a course of 12 days, including a fasting period between days 5-8.

But the correlation between core temperature and the maximum eye region temperature is not that great. Indeed, you would expect the values to fall along or at least be parallel to the dotted line of equality below, but in the case of low air temperatures, the relationship is quite poor and the surface temperatures are much cooler than core.

Maximum eye region temperature correlates with core temperature, but not very strongly and is quite heavily dependent on surrounding air temperature.

So, it is easy to conclude that max eye temperature is not always a reliable indicator of core temperature in these ducks. Maybe we could derive an empirical calibration curve, taking into account air temperature, but the point is that this requires accurate temperature data and stable environmental conditions rarely present in the field.

So, what about the lockdown research mentioned earlier?

Having plenty of time to myself, I set up a thermal camera to capture a thermal image of my face at various times throughout the day to capture the natural variation in body temperature (no fever, per se, but my daily oral temperature measurements range from 35.6 to 36.9C). Here is a sample image, outlining the typical regions of skin surface measured:

Red arrow (inner canthus of the eye, typical hottest part of the face, ~35.2C here). Blue arrow (tongue, equivalent to measurement with oral thermomemeter, ~36.7C here). Green question mark (forehead, location of scanners seen on a lot of websites, ~32.4C here).

It got too scary the longer I was in lockdown as my hair grew too long and disheveled, so I ceased the experiment after only a short period.

But the results (next 3 graphs) below show how poorly forehead assesses normal oral temperature, and even how maximum eye temperature is ~1C cooler than oral temperature and influenced by nearby ambient conditions (my garage was cold back in April so I set up there for a few measurements).

Conclusions: Ignoring the N=1 subject (due to the pandemic this seems justified), forehead is a poor measure of oral temperature (3.85C too low), maximum eye temperature a bit better (but still 1C too low and affected by air temperature), while simply pointing the camera in your mouth and getting maximum temperature yields a temperature ~0.5C higher than an oral thermometer.

So, why if you look at any images of companies and airports doing fever scanning they point devices at people’s foreheads or relying on a single pixel value from possibly the eye region?

The simple answer is that it is easy to do, but from a target accuracy perspective, it is terrible, especially if low accuracy devices are being employed inappropriately. In the one image above, the device is pointing at someone’s hair, which shields the skin and thus produces a cooler value. Cooler values will not trigger a fever detection even if it is there!

So, wherefore is the future of fever scanning? Intuitively, it seems it should work, but are we measuring the wrong thing? Why don’t we measure inside the mouth where normal oral thermometers do? At least this is better than crappy forehead measurements. Is this a privacy issue? Is it feasible to do in rapid scanning processes? Will it be feasible if we are all wearing masks?

I am not the first to write on this. This blog project was mostly a distraction in the early days of lockdown to keep my mind off the situation. I attach a few key articles and opinion pieces on the subject below that have commented more clearly on the connection (or lack) between fever and infection and why fever screening is not a panacea.

Links to further reading:

Scientific studies demonstrating reasonable predictive power for fever scanning:

Other articles discussing why using fever scanning does not equate to infection and misses asymptomatic cases of COVID19:

WHO Recommendations on temperature screening:

Idea for a Review Paper Anyone?

I think this situation really needs another look by the physiology community. My anecdote here is simply based on self reflection/measurement but also based on years of experience with thermal imaging.

The first rule of thermal imaging in biological systems: “Surface Temperature is not equal to Core Temperature.” We can’t forget that. If you want to use surface temperature, you have to do a lot of calibration checks or have very good control over your subject.

In case a grad student locked down at home wants a writing project, here are a few key points that I know should impact the predictive power of max surface temperature measurements in the context of rapid fever scanning in public places:

  • Air temperature near skin
  • Air flow (convective heat exchange) over skin
  • Blood flow relationship with the skin surface
  • Camera user skills and training
  • Quality and accuracy of the thermal scanner (some scanners I see people using have accuracies of +-2 to 4C).
  • Pre-symptomatic people lack fever
  • Masking of fever with antipyretic drugs undetected by scanning
  • What is the precise correspondence of eye canthus temperature with core temperature measurement?

2 thoughts on “Do surface temperatures measure core temperature? No.

  1. Nice one.

    On Fri, Jun 26, 2020 at 4:24 PM Tattersall Lab (T.E.M.P.) wrote:

    > gtattersall posted: ” Working from home during the COVID19 pandemic has > proven a challenge for many of us. Our students are not allowed to pursue > their research, and yet most of us are working as hard from home as we > would be on campus. Anyhow, at the beginning of the lo” >

    Liked by 1 person

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