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Might biking lower forehead temp readings at destination?

Bicycles Asked by Brian Donovan on October 16, 2020

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.)

3 Answers

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)

Correct answer by mattnz on October 16, 2020

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.

Answered by L.Dutch - Reinstate Monica on October 16, 2020

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.

Answered by Henry A. Kissinger on October 16, 2020

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