An infrared (IR) thermometer is a non-contact device that measures temperature by detecting the thermal radiation, or infrared energy, emitted by an object’s surface. These devices are popular for their speed and ease of use, particularly for temperature screening. The short answer is no: IR thermometers are not designed or calibrated for the unique environment of the oral cavity. Attempting to use a non-contact thermometer inside the mouth will provide a reading that is inaccurate and meaningless for determining core body temperature.
How Infrared Thermometers Measure Temperature
Infrared thermometers operate on the principle of black-body radiation, where all objects above absolute zero emit thermal energy as infrared light. The device uses a lens to focus this light onto a detector, called a thermopile, which converts the thermal energy into an electrical signal. This signal is processed to calculate a temperature reading based on the intensity of the detected infrared energy.
A factor in this calculation is emissivity, which is a material’s effectiveness in emitting thermal radiation. Human skin, particularly on the forehead, has a consistently high emissivity value, typically around 0.98, making it an ideal surface for calibration. Medical IR thermometers are programmed to assume this high emissivity value and use an algorithm to estimate core body temperature from the surface reading. This process fundamentally measures only the temperature of the targeted surface, not the internal temperature of the body.
The Limitations of Oral Application
The oral cavity presents multiple environmental challenges that negate the accuracy of an IR thermometer. Unlike the skin, the interior of the mouth is a highly variable and moist environment due to saliva and constant airflow from breathing. These factors interfere with the detection of infrared radiation, as moisture can scatter or absorb the emitted energy before it reaches the sensor.
The device’s factory calibration for human skin emissivity does not apply to the mucous membranes and soft tissues inside the mouth. This calibration is designed to interpret the forehead’s surface temperature as a proxy for core temperature, a relationship that fails inside the mouth. Pointing the device into the mouth also introduces issues with the distance-to-spot ratio, causing the thermometer to measure a mixture of temperatures from the tongue, palate, and surrounding air.
This combination of moisture, air movement, and incorrect surface calibration results in inconsistent readings significantly lower than the actual core body temperature. The thermometer measures the temperature of a rapidly changing, damp, internal surface, which is not what the device was engineered to do. For accurate oral measurement, a contact device is required to maintain a consistent thermal connection for a sustained period.
Recommended Methods for Accurate Temperature Reading
For a quick, non-contact temperature reading, the proper use of an infrared thermometer involves targeting the forehead or the ear canal. Forehead or temporal artery thermometers scan the skin over the temporal artery, providing a reading that correlates well with core body temperature due to the artery’s proximity to central circulation. Tympanic thermometers use infrared technology to measure the heat radiating from the eardrum, which shares the same blood supply as the brain’s temperature control center.
To ensure accuracy with a non-contact device, the area must be clean and dry, and the thermometer should be held at the exact distance specified by the manufacturer, typically three to five centimeters from the skin. If an oral temperature reading is desired, the most reliable tool is a digital stick thermometer. When using a contact thermometer orally, wait at least 15 minutes after eating, drinking, or smoking to allow the mouth temperature to stabilize and achieve an accurate result.