A portable device for analyzing the composition of the respired gasses of a subject, wherein the device comprises: (a) a body adapted to be held in a hand of the subject; (b) at least one air flow conduit through which the subject can inspire or expire air through the body of the device; (c) a sample analysis chamber; (d) at least one sampling portal through which air moves into or out of the sample analysis chamber, (e) wherein the design of the air flow conduit and the sampling portal is adapted to provide passive sampling of expired air into the sample analysis chamber by creating a diodicity by which inspired air flowing through the air flow conduit from an inhalation passes by the sampling portal with only a negligible amount entering the sample analysis chamber, while the expired air flowing through the air flow conduit from an exhalation is subject to forces that cause a portion of the expired air to move through the sampling portal and into the sample analysis chamber, thereby favoring net inflow of expired air into the sample analysis chamber by the diodicity generated by the design of the air flow conduit and the sampling portal, characterised in that the sampling portal has a cross-sectional shape of an isosceles triangle the basis of which widens in the direction of a connector attached to the body in order to support contact of the subject's nose and/or mouth and perpendicular to the direction of a portal for allowing the expired air out of and the inspired air into the air flow conduit, and wherein the tip and the base of the isosceles triangle are open for the airflow; (f) an oxygen sensor for measuring the oxygen concentration of the air inside the sample analysis chamber; and (g) at least one flow sensor for measuring the flow of inspired or expired air (13, 12) through the device. (a) a body adapted to be held in a hand of the subject; (b) at least one air flow conduit through which the subject can inspire or expire air through the body of the device; (c) a sample analysis chamber; (d) at least one sampling portal through which air moves into or out of the sample analysis chamber, (e) wherein the design of the air flow conduit and the sampling portal is adapted to provide passive sampling of expired air into the sample analysis chamber by creating a diodicity by which inspired air flowing through the air flow conduit from an inhalation passes by the sampling portal with only a negligible amount entering the sample analysis chamber, while the expired air flowing through the air flow conduit from an exhalation is subject to forces that cause a portion of the expired air to move through the sampling portal and into the sample analysis chamber, thereby favoring net inflow of expired air into the sample analysis chamber by the diodicity generated by the design of the air flow conduit and the sampling portal, characterised in that the sampling portal has a cross-sectional shape of an isosceles triangle the basis of which widens in the direction of a connector attached to the body in order to support contact of the subject's nose and/or mouth and perpendicular to the direction of a portal for allowing the expired air out of and the inspired air into the air flow conduit, and wherein the tip and the base of the isosceles triangle are open for the airflow; (f) an oxygen sensor for measuring the oxygen concentration of the air inside the sample analysis chamber; and (g) at least one flow sensor for measuring the flow of inspired or expired air (13, 12) through the device.