1. A device for determining blood oxygen saturation of a subject comprising: an interface that receives a data stream derived from detected electromagnetic radiation emitted or reflected from one or more skin portions of the subject, said data stream comprising a data signal per skin pixel for a plurality of skin pixels of said one or more skin portions, a data signal representing the detected electromagnetic radiation emitted or reflected from the respective skin pixel over time, an analyzer that determines the change in blood oxygen saturation of said plurality of skin pixels based on the data signals of said plurality of skin pixels, a selector that selects a group of skin pixels comprising either the skin pixels showing the fastest change in blood oxygen saturation or said plurality of skin pixels except for skin pixels showing the slowest change in blood oxygen saturation, and a processor that determines the blood oxygen saturation of the subject based on the data signals of the selected group of skin pixels.