A substance concentration monitoring method includes inducing a periodic change in a body surface temperature over time using a heating and/or cooling element and yielding a function of temperature that varies with time so as to exhibit a temperature derivative with respect to time that oscillates periodically. Mid-infrared radiation absorbed or emitted from the body is measured while the surface of the body exhibits the oscillating, periodic derivative. The method includes determining a measured value based on the MIR radiation measurements and determining a concentration of a substance in the body based on a correlation with the measured value. A substance concentration monitoring apparatus includes a processor that initiates operations continuously monitoring whether the concentration or the measured value are within a tolerance. The operations further include generating a warning output when the concentration or the measured value is outside the tolerance.
