A face mask for delivering oxygen to, and sampling carbon dioxide exhaled from, a subject includes a partition wall that divides the mask into a subject respiratory space that primarily contains carbon dioxide exhaled by the subject, and an oxygen reservoir space that primarily contain oxygen. The partition wall includes one or two holes to which naris conduits are respectively connected. The naris conduit are positioned in proximity to the subjects nares to closely obtain carbon dioxide samples. The naris conduits enable oxygen to flow from the oxygen reservoir space to the subject respiratory space during inhalation, while quickly expelling traces of CO2, and they configured such that exhaled CO2 quickly fills them up, during exhalation, and while expelling oxygen traces back to the oxygen reservoir space. During exhalation.
