There is provided a method for use in determining whether a subject is allergic to a substance. The method comprises: receiving a first set of spatially distributed light intensity values covering a skin region of the subject including a location at which the substance has been applied； wherein the light intensity values in the first set are intensities of visible light； receiving a second set of spatially distributed light intensity values covering the skin region, wherein the light intensity values in the second set are intensities of infrared, IR, light； generating a first spatial distribution of photoplethysmogram, PPG, pulse amplitudes based on the first set of light intensity values； generating a second spatial distribution of PPG pulse amplitudes based on the second set of light intensity values； comparing the first spatial distribution to the second spatial distribution, and to the location at which the substance has been applied； and outputting an indication of whether the subject is experiencing an allergic reaction to the substance based on the comparing.
