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I often bike to work at a place that screens all who enter for fever. Lately this has been done with a no-touch forehead thing, presumably an IR gadget. When I bike, even if it is cold enough that I wish I'd worn full-finger gloves, the final uphill makes me sweat, yet even so I consistently arrive with my forehead so cool that the gadget gives a number in the 92° Fahrenheit (33°C) range, or more often just refuses to give any number at all. When I make the trip by car, by contrast, it consistently gives more normal readings of 94.7° fahrenheit (35°C)

As a 65-year-old male on a beta blocker, I expect my body to run a little cooler than average, but 92°F (33°C) or lower for a forehead reading seems extreme. It's as if the biking itself is somehow an additional cooling factor. Could this be? Have others found this, now that "temping in" is grown so common?

I get that evaporative cooling with sweat is peculiarly effective for the cyclist, thanks to the air flow, and that the forehead is a prime site for it; but it hardly seems like that should make the forehead peculiarly cool, since (with apologies to Shakespeare) the blood is hot that must be cooled by this. (By analogy, the fins on a motorcycle engine also exploit air flow for cooling; but one hardly expects them to be cool to the touch after a long uphill has put their cooling capacity to the test.)

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    I'm interested too - I used a general purpose IR thermometer rather than a medical one, but after a hot ride on a hot day, wearing a helmet, I was surprised to see it cooler than just sitting around at home. It's presumably to do with the depth that emits IR of the relevant wavelength, compared to the depth debate the nerve endings are that sense heat (in itself an oversimplification). Probably the thermometer reads the very surface, which is effectively cooled by sweat, but the sensation of heat is deeper. Probably. – Chris H Sep 29 '20 at 19:50
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    @VladimirF, on the contrary, I do wear a helmet. As I always say: if you don't think your brain is worth protecting, you're probably right. – Brian Donovan Sep 29 '20 at 20:37
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    if sweat is actively evaporating from your forehead, i would expect the IR thermometer to read lower. – Paul H Sep 29 '20 at 21:06
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    This is precisely what the receptionist at my dentist's office said when I cautioned that I'd arrived via the well known "bike trail" (left out the "on a unicycle" part) and was worried I might initially read high; she claimed that they'd seen people typically read low. While "the plural of anecdote is not data", given the character of the town and the office location at the junction of three great rail trails connecting many of its neighbourhoods I don't doubt that patients riding to appointments happened fairly frequently. – Chris Stratton Sep 29 '20 at 21:51
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    Don't temperatures in that range imply it may not be working at all? – Nathan Knutson Sep 30 '20 at 0:43
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The body controls its core temperature, not the skin temperature. Skin temperatures will vary significantly, to the point they are generally considered an unreliable way to measure core temperature, especially for people under heat stress (Riding a bicycle). (Refer : here)

Screening using such a thermometer is more about the "Security theater" of being seen to do something, but the high incidence of false positives and false negatives make the concept unreliable and falling into the realm of "Theater". (Here)

  • I have an IR thermometer with 3 emissivity settings. The readings differs by 1°C from setting to setting. So much for precision. – Carel Sep 30 '20 at 8:04
  • Readings from a non-contact IR thermometers are generally well-correlated with other temperature measurements, although they do tend to be a bit lower, and there are a few things that can screw up the reading. But still, studies have shown that they can detect fevers with high sensitivity/specificity. Where this becomes security theater is not in the IRT's ability to detect a fever, but whether the fever is diagnostic of COVID. Regardless of how temperature is measured (even if done perfectly), temperature checks are still not sufficiently sensitive/specific to be a good screen for COVID. – Nuclear Hoagie Sep 30 '20 at 16:26
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    IR thermometers can be used in controlled conditions - indoors with sedentary patients to infer core temperature based on experimental observations made of the last decades. Using an IR thermometer in field conditions came from the Military need to quickly identify the living from the dead. The Covid response to deploy these has combined the two use cases with little science to back it up. Its theater that gets lucky very occasionally with little concern for the many negative side effects of its deployment. – mattnz Sep 30 '20 at 19:38
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    This is merely anecdotal, but I find that if you point an IR thermometer at the inside of someone's mouth, you get very accurate and consistent readings. Whether this is sanitary or not is a different issue altogether. – Robert Harvey Sep 30 '20 at 22:51
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    @NuclearWang So the IR thermometer can measure core temperatures and not just surface temps? How is that supposed to work? Obviously core and surface temps can be in sync under given circumstances but "person straight from street" is not one of those I'd think. – Voo Oct 1 '20 at 10:25
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One factor could be the lower emissivity of sweat compared to human skin. At a given temperature, different materials emit different amounts of infrared radiation for the thermometer to detect. Skin has an emissivity of about 0.97. (1.0 is the maximum possible value). At the same temperature, pure water will emit slightly less because it has 0.96 emissivity. Sweat might emit even less than water because of the salt and everything else mixed in. Hospitals need to reconfigure IR thermometers if a patient's skin has ultrasound gel, disinfectant, etc... because all of those things change the skin's emissivity. I wouldn't count on the IR thermometer at the front door being smart enough to automatically configure its emissivity setting when pointed at a sweaty face.

IR thermometers are used because they are fast and contactless. They are accurate when measuring external temperature and the material's emissivity is known and constant. But in the scenario described in the question, there are too many factors at play for an IR thermometer to reliably detect fever in every person.

If you JUST stepped off your bike after a ride in the cold, I doubt it's possible for an IR thermometer to grab a useful reading on your internal temperature no matter where they point it.

  • How do they compensate for different body parts having different surface temperature? – ojs Sep 30 '20 at 8:15
  • @ojs There is no reason to compensate for that, that is what you are trying to measure. – Vladimir F Sep 30 '20 at 8:52
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    You are certainly meaning that it has an emissivity of 97 %, not a reflectance of 97 %. With a reflectance of 97 % the measurement wouldn't work out at all. – Arsenal Oct 1 '20 at 9:34
  • @Arsenal Right, totally mix up the verbiage in my head. Updating answer... – Henry A. Kissinger Oct 1 '20 at 14:13
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Due to Covid I have to measure my body temperature when I enter the office. Since I happen to bike to work, I was concerned my temperature would be measured to be "hot".

Well, it turned out that in the morning, even though I am sweaty from a 40 minutes ride, my forehead barely reaches 29 Celsius, regardless if I am cycling bare head or wearing a head cover.

Don't underestimate the cooling effect on extremities.

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    And, do they sent you to ER urgently to be treated for severe hypothermia? :-) – gerrit Sep 30 '20 at 7:50
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    @gerrit, I am glad they are after covid symptoms and not zombie apocalypse's one :D – L.Dutch - Reinstate Monica Sep 30 '20 at 8:43
  • I too measure my temperature after cycling to work. Short ride, just over 5 minutes. Yet my temperature is usually measured as "Lo", i.e < 34 C. A guy behind me in the queue (who I assume drove to work) measured 36.5, so the thermometer isn't broken. – slingeraap Oct 1 '20 at 13:48

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