Results obtained in a study of the microscopic distribution of plutonium in the lungs of deceased Pu workers from the Mayak Production Association showed that the long-term retention of Pu was greater than predicted by the current ICRP 66 respiratory tract dosimetry model (HRTM). These data were therefore applied to the HRTM by modifying selected parameters, namely the transfer rate of Pu from the transformed state compartment and the fraction of Pu that transfers to the bound state compartment. Invoking the latter compartment into the modelling allowed a better representation of the long-term Pu retention as well as providing a convenient means of describing the workplace-specific characteristics of the different Pu aerosols found in the Mayak plant. In particular, the present model describes a significantly greater long-term retention of Pu nitrate aerosols in the lung compared with the Type M default.