A portable system, using electrostatic collection, for the measurement of exhaled thoron activity in humans is described, together with details of the basic theory, equipment, calibration procedures, measurement and preliminary use. The portable system built on experience at the Argonne National Laboratory (ANL) to achieve a reduction in measurement time from 30 h to 200 min, and to increase the total efficiency of the system from 50% (at ANL) to 70%. The limit of detection of the system (2sigma above zero) is 0.007 Bq of thoron activity outside the body. The total standard error of this system is 47% for a thorium lung burden of 0.22 Bq. The average background of this scintillation detector was 0.003 counts min(-1).