A portal (1) has a millimetric wave imager (10, 11) arranged to detect articles concealed on the person. A pump (21) supplies jets of air through gas outlets (20) in the lower part of the portal (1), which are directed onto the feet of the person being screened. A second pump (25) pumps air with substances released by the jets via inlets (22) to an ion mobility spectrometer (24) arranged to detect the presence of explosives or narcotics. A screen (12) indicates the presence of any suspect article or substance on the person being screened.