During the pandemic, the chances are that as you enter an airport or doctor’s surgery, someone will point an infrared thermometer at your forehead in order to assess your temperature. Your skin temperature is being measured to try to identify whether or not you have an increased deep body temperature, which is indicative of fever, one of the leading signs of COVID-19.
The good thing about using infrared thermometers is that they are quick, simple and non-invasive. You can rapidly screen many people without inconveniencing them, for instance travelers moving through an airport or people entering a sports stadium. But to be useful mass-screening tools, infrared thermometers also need to be accurate—and this is where problems arise.
Although fever is a leading symptom of COVID-19, many infected people don’t have any symptoms, or they develop a fever after they have become infectious, got ill and entered hospital. At least 11% of those with COVID-19 do not have a fever, and only 43% of patients ill enough to be admitted to hospital have one. So, looking for fever is not a foolproof approach.
Plus, while an infrared thermometer can measure skin temperature accurately, the real question is: does forehead temperature tell us anything about deep body temperature, the true sign of fever? In certain, highly controlled circumstances, an increased forehead temperature can indicate a raised deep body temperature—this is why people put their hand on your forehead when you say you don’t feel very well.
But the problem is, forehead or skin temperature can be increased or decreased independently of deep body temperature, for many reasons. Having just been in a cold or hot environment, sunburn, having just exercised, wearing too many clothes, having drunk alcohol, having just eaten, having a variety of skin conditions—these can all influence skin