Study Questions Efficacy of Popular Forehead Thermometer

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According to Crandall, there are several reasons why forehead temperature is not a reliable indicator of core body temperature. For one thing, forehead temperature is based on the temperature of the temporal artery, which lies between the skull and the skin of the forehead. But that artery is not in exactly the same location in every individual, nor is it found at precisely the same depth.

More important, there are any number of factors that can change skin temperature without affecting internal temperature. During periods of temperature transition, such as when an individual is developing a fever or when a fever "breaks," blood flow to the skin changes, raising or lowering skin temperature relative to the core body temperature.

Sweat, especially, can affect skin temperature, Crandall noted. "Sweating cools the skin even if the internal temperature is high. I can get someone's temperature elevated, and if I put water on the face and use a fan, the skin temperature will be cool."

For athletes, "this is a serious problem, especially in the summer, because people get very hot and can develop life-threatening hyperthermia if you don't measure body temperature accurately," Levine added.

The research team wiped sweat off each subject before taking temperatures, but according to manufacturer instructions for the consumer product, "wiping the forehead is not recommended, since the sweating immediately begins again." Instead, Exergen recommends reading the temperature of a sweating individual behind the ear.

"We proved many years ago that perspiration does not stop by wiping, and the skin never dries, even though the tiny droplets of water immediately forming at the pores may not be visible. Unless the skin is dry, readings will always be low. As an educated guess, this is probably the most significant cause of the low [temporal] temperatures reported in the study," Pompei said.

Pompei also noted that the use of the temporal thermometry on sweaty athletes such as those running in the Boston Marathon has been largely investigative to this point.