1. A device for determining blood oxygen saturation of a subject comprising: an interface configured to receive 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 configured to determine a change in blood oxygen saturation of said plurality of skin pixels based on the data signals of said plurality of skin pixels; a selector configured to select a group of the skin pixels, the group including (a) the skin pixels showing a fastest change in blood oxygen saturation or (b) said plurality of skin pixels except for skin pixels showing a slowest change in blood oxygen saturation, said selector being configured to select said group of skin pixels by use of an upper or lower threshold for the blood oxygen saturation or by use of a threshold for a percentage of skin pixels of said plurality of skin pixels to be selected as said group; and a processor configured to determine the blood oxygen saturation of the subject based on the data signals of the selected group of skin pixels.