In plutonium (Pu) handling facilities, there is a potential for internal exposure of radiation workers along with external exposure. Nasal swabs (NS) taken rapidly, on site, help in not only providing an early estimate of internal dose due to inhalation of Pu but also in decision making for medical intervention. However, the Committed Effective Dose (CED) computed based on the observed NS activity and that estimated from bioassay measurements of individuals may vary significantly. One of the reasons would be the use of default particle size (5 μm) for computing the CED based on the activity on NS. Other parameters like breathing pattern and levels of exercise would also influence the NS reference activity level and hence their impact needs to be assessed. This study indicated that observed NS reference levels had a direct dependence on all these parameters and use of default parameters for evaluation of internal dose based on NS activity would result in large uncertainties in the dose assessment.