A method of assessing a blood concentration level, in a human or animal subject, of ethanol, the method comprising the steps of: positioning a sensor means (21,22) within an expiratory gas-flow of the subject, the sensor means (21,22) being configured to detect the presence of ethanol and provide a first output signal (C extEtOH) representative of the concentration of the ethanol in air, and to detect the presence of carbon dioxide and provide a second output signal (C ext CO 2) representative of the concentration of carbon dioxide in air; sampling a flow of expiratory gases from the subject using the sensor (21,22) to provide the first output signal (C extEtOH) and the second output signal (C ext CO 2) for the expiratory gases simultaneously;: comparing the variation of the first output signal (C extEtOH) for the expiratory gases over time with the variation of the second output signal (C ext CO 2) for the expiratory gases over time by plotting the time-variation of the first output signal (C extEtOH) and the second output signal (C ext CO 2) and matching a curve portion of the first output signal (C extEtOH) with a curve portion the second output signal (C ext CO 2) and, depending on the result of the comparison if a curve portion of the first output signal (C extEtOH) matches a curve portion of the second output signal (C ext CO 2): making the second output signal (C ext CO 2) for the expiratory gases representative of a degree of dilution of the expiratory airflow to adjust the concentration of ethanol represented by the first output signal (C extEtOH); sampling ambient air and using the sensor means (21,22) to provide the first and second output signals representative of ethanol concentration (C extEtOH) and carbon-dioxide concentration (C ext CO 2) in the ambient air at the location of the sensor means (21,22); correcting the first and second output signals for the expiratory gases by subtracting the first and second output signals representative of ambient air from the first and second output signals for the expiratory gases to yield corrected first and second output signals (C extEtOH, C ext CO 2) for the expiratory gases, dividing the corrected first signal (C extEtOH) representative of the concentration of ethanol in the expiratory gases by the corrected second signal (C ext CO 2) representative of the concentration of carbon dioxide in the expiratory gases to yield a ratio (C extEtOH /C ext CO 2); multiplying the ratio (C extEtOH /C ext CO 2) by a factor representative of the subject's alveolar carbon dioxide concentration (C alv CO 2) to define the adjusted concentration of ethanol (C alvEtOH); and determining the blood concentration level of ethanol wherein the degree of dilution is defined by the concentration of carbon dioxide in the expiratory gases divided by the subject's alveolar carbon dioxide concentration defining (C ext CO 2)/(C alv CO 2). positioning a sensor means (21,22) within an expiratory gas-flow of the subject, the sensor means (21,22) being configured to detect the presence of ethanol and provide a first output signal (C extEtOH) representative of the concentration of the ethanol in air, and to detect the presence of carbon dioxide and provide a second output signal (C ext CO 2) representative of the concentration of carbon dioxide in air; sampling a flow of expiratory gases from the subject using the sensor (21,22) to provide the first output signal (C extEtOH) and the second output signal (C ext CO 2) for the expiratory gases simultaneously;: comparing the variation of the first output signal (C extEtOH) for the expiratory gases over time with the variation of the second output signal (C ext CO 2) for the expiratory gases over time by plotting the time-variation of the first output signal (C extEtOH) and the second output signal (C ext CO 2) and matching a curve portion of the first output signal (C extEtOH) with a curve portion the second output signal (C ext CO 2) and, depending on the result of the comparison if a curve portion of the first output signal (C extEtOH) matches a curve portion of the second output signal (C ext CO 2): making the second output signal (C ext CO 2) for the expiratory gases representative of a degree of dilution of the expiratory airflow to adjust the concentration of ethanol represented by the first output signal (C extEtOH); sampling ambient air and using the sensor means (21,22) to provide the first and second output signals representative of ethanol concentration (C extEtOH) and carbon-dioxide concentration (C ext CO 2) in the ambient air at the location of the sensor means (21,22); correcting the first and second output signals for the expiratory gases by subtracting the first and second output signals representative of ambient air from the first and second output signals for the expiratory gases to yield corrected first and second output signals (C extEtOH, C ext CO 2) for the expiratory gases, dividing the corrected first signal (C extEtOH) representative of the concentration of ethanol in the expiratory gases by the corrected second signal (C ext CO 2) representative of the concentration of carbon dioxide in the expiratory gases to yield a ratio (C extEtOH /C ext CO 2); multiplying the ratio (C extEtOH /C ext CO 2) by a factor representative of the subject's alveolar carbon dioxide concentration (C alv CO 2) to define the adjusted concentration of ethanol (C alvEtOH); and determining the blood concentration level of ethanol wherein the degree of dilution is defined by the concentration of carbon dioxide in the expiratory gases divided by the subject's alveolar carbon dioxide concentration defining (C ext CO 2)/(C alv CO 2).