Due to the increasing use of nanomaterials in research and product development, it is probable that the number of situations of occupational exposure to them is also rising. The same is true for the number of workers. Although current research in nanotoxicology is far from conclusive, it is clear that relying on mass concentration and chemical composition alone is not appropriate in all cases and alternative measurement methods and approaches need to be developed. In this work, we propose a method based on simultaneous size-integrated measurements of two particle concentrations (number and lung-deposited surface area, CNC/NSAM), and on the estimation of the average size of potentially inhaled particles from the combination of these measurements. The proposed method could be part of a measurement strategy that is practical as it would use field-portable, commercially available aerosol instruments. In the absence of instruments providing real-time sizeresolved measurements, this original approach can be carried out as considering that the ratio of these concentrations is a monotonous function of particle size. Indeed, the latter function depends only on the geometric standard deviation of airborne particle number size distribution, assumed to be lognormal. Compared to SMPS data for polydisperse aerosols having three chemical natures with count median diameters ranging from 64 to 177 nm, experimental results were obtained with acceptable relative discrepancies of ±30%. Though the method proposed is less accurate than traditional instruments like SMPS, it can be used for workplace air monitoring or as a screening tool to detect the presence of airborne nanoparticles.