Summary There is an increasing concern about the potential health hazards posed to workers exposed to inhalation of nanoparticles (NPs). Common sources of nanoparticles in working environments include fumes and exhausts from different processes like laser ablation and milling. Nanoparticles have potential toxic properties: a high particle surface area, number concentration, and surface reactivity. Inhalation, the most common route of nanoparticle exposure, has been shown to cause adverse effects on pulmonary functions, and the deposited particles in the lung can be translocated to the blood system by passing through the pulmonary protection barriers. Filtration is the simplest and most common method of aerosol control. It is widely used in mechanical ventilation and respiratory protection. However, concerns have been raised regarding the effectiveness of filters for capturing nanoparticles.In order to reach a certified level of health protection from exposure to NPs, filtering face-piece respirators are widely used by workers. N, R and P Series are three classes of such respirators approved and certified by the National Institute of Occupational Safety and Health (NIOSH). The N95 face-piece respirator is one of the most commonly used masks. Serving a broad range of industries, N95 face piece respirators are known for their disposability, low cost, suitability, etc. According to NIOSH standards (42 CFR 84, NIOSH, 1997), N95 respirators are approved to remove at least 95% of 300 nm particles under an airflow rate of 85 liters/min. NIOSH uses the average particle size of 300 nm for the approval tests, because they correspond to the most penetrating particle size (MPPS) on mechanical filters. However, previous studies demonstrated that the MPPS shifts to smaller particle sizes for electrostatic charged filters. However, a lot of information is lacking to characterize the performance of respiratory filters for nanoparticles in the different situations encountered in the working environment. Examples include the effect of temperature, humidity and respiratory flow rates.In this study, the performance of one model of N95 NIOSH approved filtering face-piece respirator (FFR) was characterized against poly-dispersed and mono-dispersed NPs using two different experimental set-ups. With poly-dispersed NPs, a methodology was developed to measure the performance of the N95 respirators against NaCl aerosols in the size range of 15 to 200 nm in three scenarios. First, the initial particle penetration through N95 respirator was examined at four constant airflow rates: 85, 135, 270 and 360 liters/min. Second, the effect of time on the particle loading was investigated for duration of five hours. Third, the effect of the relative humidity (RH) (10, 30 and 70%) on the particle penetration was assessed at 85 liters/min. In addition, the FFR performance was also characterized at 85 liters/min against twelve mono-sized NaCl aerosols with sizes ranging from 20 to 200 nm. The results were compared with the initial penetrations of the corresponding particle size on the FFR tested against poly-dispersed aerosols.Using the poly-dispersed aerosols test (PAT) method, the results demonstrated that the initial particle penetration was significantly enhanced with the increased airflows and a shift toward smaller particle size was observed for the most penetrating particle. The particle penetration decreased with further loading, while a gradual increase in penetration was observed for the larger particle sizes. The MPPS was also found to shift toward the larger sized particles; from 41 to 66 nm. In addition, for the particles below 100 nm, the particle penetration augmented slightly as the RH increased. However, for the larger size particles, penetration was similar at RH of 10 and 30%; and subsequently increased as RH elevated to 70%.The mono-dispersed aerosol test (MAT) method was performed at 85 liters/min constant flow rate; the initial particle penetration at the MPPS was below 5% NIOSH certification criterion. Moreover, the initial particle penetration value, measured with MAT method was higher than the one measured with PAT method at each corresponding particle size.