Abstract: A device for use in trapping and assaying 14CO2 in air exhaled by a person to which a 14CO labelled compound has been administered, in particular 14CO labelled urea, comprises first and second substantially flat oblong duct forming elements joined to each other at their edges except for one of their short edges and forming a duct between them. The first duct forming element includes a gas permeable matrix element for absorption of CO2. The device is provided with indicator means for visual detection of CO2 absorption and a film element of low &bgr;-radiation absorptivity interposed or interposable between the matrix element and a &bgr;-radiation measuring instrument. Also disclosed is a corresponding method.