This study aimed to develop a model to estimate personal exposure to ultrafine particles (UFPs) and PM2.5, both for total exposure of an individual, and for exposure to particles excluding particles generated by indoor sources. The model was then used in a panel study of patients with chronic lung disease. The practicality of the model was tested, by examining whether ultrafine particle count or particle mass were related to respiratory health effects, and whether there were differences in observed health effects in relation to total exposure and exposure excluding indoor sources. DustTrak and PTrak monitors were used to measure PM2.5 and UFP concentrations in different microenvironments and during different activities. Conclusions: The PM2.5 and UFP personal exposure model represented total personal exposure relatively accurately. This is the first time an attempt has been made to model personal exposure to UFPs and although some improvements are required, both PM2.5 and UFP models were shown to be effective in a panel study with susceptible individuals. Ambient UFP concentrations the previous day were associated with a decrease in PEF the next morning. The UFP personal exposure estimates calculated from the model were not associated with respiratory symptoms or peak flow rate. However, it has recently been suggested that exposure to ultrafine particles might be more important in relation to potential cardiac effects and it is recommended to evaluate the model in a panel study of patients with coronary heart disease.