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Is Your Forehead Good Enough for a Fever Check?

No-contact thermometers that pick up the heat coming off your skin have the potential to help screen masses of people for fever, but the body of evidence on their accuracy is dissatisfying.

Your forehead is about to get a lot of action. Is it greasy? You may want to wipe that sweat off. In the age of COVID, our forehead will become the target of fever spot checks. Thermal cameras and non-contact infrared thermometers will get deployed in an effort to prevent people with a fever from entering into an area where they may infect others. The only question is, do these temperature-sensing technologies actually work?

Thermometers go way back to the early 1600s, but even before their existence, physicians understood that there were degrees of heat and cold. Johannis Hasler of Berne, a theologian and physician, thought body temperature went up in tropical latitudes and published a table with presumed body temperatures around the world to guide his mixing of “medicines.” Thermometers were initially based on the expansion of water as it warmed up; this water was later replaced by mercury, which responded much more quickly to changes in temperature, then to alcohol due to mercury’s toxicity. More recently, liquid was done away with in electronic thermometers, many working under the principle that electric resistance changes due to temperature. And now, contact with the body is no longer needed. There is a class of devices called thermal imagers which includes both thermal cameras seen in some airports as well as non-contact infrared thermometers (NCITs). All of these thermal imagers function under the principle that heat is emitted by any object above absolute zero (-273.15°C or -459.67°F). Our forehead emits heat in the form of infrared radiation. An NCIT has a lens that focuses this heat onto a detector, which transforms it into electricity. And because forehead temperature is lower than the temperature underneath our tongue, it has to be converted by the device into an “oral temperature equivalent.” The laser you see on some devices does not detect temperature; it exists to make sure the user is pointing the device correctly.

Thermal imagers have their advantages, especially for the mass screening of people during a pandemic. They are relatively easy to use. The lack of contact should lower the spread of disease. And they are quick to take and retake a temperature.

But if we focus for a minute on forehead infrared thermometers--also sometimes referred to as “thermometer guns”--much can theoretically affect their accuracy. They are not meant to be used in the direct sunlight, because the Sun will warm up your forehead and bias the reading. Sweat on your brow can artificially lower the temperature that is measured, essentially disguising a fever. There is actually a significant list of reasons why these infrared thermometers can miss a fever: circulatory problems, previous injuries, heavy makeup, certain drugs. On the other hand, the device might tell you you are feverish even though you’re not because of stress, physical activity, nicotine, a hot beverage, hormonal treatments, pregnancy, hot air currents or even a sunburn. And we can easily imagine a scenario in which someone is contagious with COVID and is taking anti-fever medicine like acetaminophen, which will bring their temperature down.

These are the built-in pitfalls of forehead infrared thermometers. So how do they measure up in real life compared to other types of thermometers?

We need to cool down our expectations

The gold standard to assess body temperature is not rectal temperature but core temperature. This can mean sticking a catheter in the artery in someone’s lung to measure the temperature of their blood, because this is what the brain senses as it adjusts the temperature of your body. Suffice to say that, in a pandemic, doctors can’t go around sticking catheters in people’s pulmonary arteries to screen them for a fever. Rectal temperature is a decent proxy but its status as such has been criticized by medical scientists as essentially unearned. Still, a recent meta-analysis of studies that compared a wide variety of thermometers to core temperature (the gold standard) concluded that rectal and oral electronic thermometers were the best, with in-ear thermometers (commonly known as tympanic thermometers) dead last.

Non-contact infrared thermometers, which include the tympanic and the forehead ones, have been studied, and the scant body of knowledge on these devices is a microcosm of the problems hurting scientific clarity. There’s a study over there but they only tested one model, so how representative are their results? There’s a study here, but it was done in infants by trained healthcare workers: will the results hold in an adult population screened by non-medical personnel? And there’s a review over there of many studies that highlights the fact that the studies may have been biased. The point is that the accuracy of these infrared thermometers is hard to pin down because the studies are deeply imperfect. I’m tempted to give the numbers that came out of a meta-analysis in 2009, but since NCITs were compared to tympanic temperature, and since tympanic temperature is not reliable, I’m not sure we can trust these numbers.

And finally, an extra kink in the armour: a group of English scientists tested nine common NCITs on a number of measures and revealed that five of them were outside their manufacturer’s stated range of accuracy and the medical standard to which they were supposed to adhere.

As for thermal cameras (many of which skip the forehead and measure the temperature at the inner corner of the eye), a recent summary of the evidence published by The Conversation pointed to a similar lack of evidence while also raising the spectre of privacy issues. These cameras are often capable of identifying faces and the authors wondered if the facilities capturing this personal information are securely storing it.

What to make of all this? A team from the Food and Drug Administration has argued that infrared thermometers represent the only viable way of mass fever screening the public for infectious disease. But there is no one temperature threshold that everyone agrees defines a fever, as normal body temperature exists on a range. Also, you don’t need to have COVID-19 to get a fever and you don’t need a fever to have COVID-19. And because of the lack of good data on the accuracy of infrared thermometers, it’s easy to ask on what basis can we justify their use? The subpar studies may convince us these devices can effectively rule out a fever, but these studies mostly involve healthcare workers taking the temperature on infants. What happens when poorly trained individuals, worried about catching the coronavirus, start using these devices much further away from the 3-to-15 cm recommended by the manufacturers?

The question that remains is the following: thermal imagers like non-contact infrared thermometers and thermal cameras may not be great but are they good enough? Will their use be a net good in the middle of a pandemic? The evidence base is unfortunately mum on the subject. We need better studies.

Take-home message:
- Infrared thermometers capture the heat coming off of your forehead to determine if you have a fever or not
- The scientific literature on their accuracy is dissatisfying, as studies are often done in controlled environments or they compare these thermometers to other thermometers that are themselves not so reliable
-A team from England tested 9 infrared thermometers for accuracy in the lab and concluded that 5 of them did not meet the criteria they were claimed to adhere to


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