A portable monitoring system for assessing dynamic hyperinflation in a monitored subject comprising a wearable item (71, 75, 83) comprising one or more sensors that provide data comprising sizes at one or more selected positions on the torso of the subject, in-cluding one or more positions on the vib cages and one or more positions on the abdomen, the positions being selected so that the torso sizes change with respiration; a portable data unit (73, 81, 87) operably linked to the sensors, the portable data unit comprising a processing device and a computer memory operably linked to the processing device and configured with computer instructions that cause the processing device to perform the steps of receiving sensor data monitored during a current period of monitoring; determining tidal volumes in dependence on a difference of a moment-by-moment lung volume parameter at end of inspiration and the lung volume parameter at end of the following expiration, the moment-by-moment lung volume parameter being determined by linearly combining the RC sizes and the AB sizes; determining from the received data at one or more times during the monitoring period a M%RC parameter that reflects the relative contributions of expansions and contractions of the rib cage of the subject to tidal volumes of the subject, wherein determination of the M%RC parameter comprises dividing the respiratory volumes due to RC expansion and contraction by VT; determining from the received data at one or more times during the monitoring period a MqdEELV parameter that reflects the accumulated changes in end-expiratory lung volume, wherein determination of the MqdEELV parameter comprises determining a breath-by-breath change in end-expiratory lung volume (EELV); determining a cumulative change in EELV in dependence on a plurality of consecutive breath-by-breath changes in EELV; and determining the MqdEELV parameter in dependence on the absolute values of a plurality of cumulative changes in EELV; and assessing DH in the subject during the monitoring period in dependence on both the determined MqdEELV parameters and the determined M%RC parameters. receiving sensor data monitored during a current period of monitoring; determining tidal volumes in dependence on a difference of a moment-by-moment lung volume parameter at end of inspiration and the lung volume parameter at end of the following expiration, the moment-by-moment lung volume parameter being determined by linearly combining the RC sizes and the AB sizes; determining from the received data at one or more times during the monitoring period a M%RC parameter that reflects the relative contributions of expansions and contractions of the rib cage of the subject to tidal volumes of the subject, wherein determination of the M%RC parameter comprises dividing the respiratory volumes due to RC expansion and contraction by VT; determining from the received data at one or more times during the monitoring period a MqdEELV parameter that reflects the accumulated changes in end-expiratory lung volume, wherein determination of the MqdEELV parameter comprises determining a breath-by-breath change in end-expiratory lung volume (EELV); determining a cumulative change in EELV in dependence on a plurality of consecutive breath-by-breath changes in EELV; and determining the MqdEELV parameter in dependence on the absolute values of a plurality of cumulative changes in EELV; and assessing DH in the subject during the monitoring period in dependence on both the determined MqdEELV parameters and the determined M%RC parameters.