Measurement of inward leakage of full-face masks in EN and ISO standards: comparison of gas and aerosol test agents.

In Europe, respiratory protective devices must be certified before they can be marketed. Among the parameters of interest, inward leakage (IL) characterizes the tightness between the face seal and the face, to verify that the device is well-designed. European standard EN 13274-1 (2001) and International Organization for Standardization (ISO) standard ISO 16900-1 (2019) specify that IL should be measured using sodium chloride (NaCl) aerosol or sulfur hexafluoride (SF6) gas. For reusable masks made of nonporous materials, both test agents are considered equally acceptable. However, the few studies that have compared IL values measured with various aerosols and gases have come to divergent conclusions. This work then aimed to measure IL with the test agents recommended by the standards to determine whether they are really equivalent. Since krypton (Kr) is an interesting candidate for replacing SF6 in standard tests, IL was assessed with SF6 and Kr simultaneously, and with NaCl aerosol using various calculation methods. Tests were carried out on 5 models of full-face masks donned on a headform connected to a breathing machine simulating 3 sinusoidal breathing rates of various intensities. The respirator fit on the headform was evaluated using a controlled negative pressure method to determine a manikin fit factor. Four scenarios were then tested to represent very poor, bad, good, and excellent fit. Gas concentration was measured using a mass spectrometer, and IL was calculated for SF6 and Kr. A combination of 3 devices allowed the determination of the number-based concentration of particles with diameters between 20 nm and 2 µm, and IL was calculated for each of the 33 channels, as well as using a cumulative number concentration. In addition, to comply with standards, a conversion was carried out to calculate IL using a cumulative mass concentration. The results of this work evidenced that the IL values measured with NaCl were systematically lower than those determined with gases. IL was also shown to vary with particle size, with a maximum value exceeding that calculated with cumulative concentrations (in number or mass). As part of the revision of the standards, protocols for measuring inward leakage should be redefined. On the one hand, acceptability thresholds should be re-evaluated according to the nature of the test agent (gas or aerosol), as it is clear that the 2 options do not give the same results for a given configuration. On the other hand, the aerosol leakage measurement protocol needs to be reworked to enable the measurement of a well-defined, robust, and reproducible inward leakage value.

Validation of krypton as a new tracer gas for the standardization tests of collective and individual protection systems.

Sulfur hexafluoride (SF6) is the reference tracer gas in many international standards for characterizing respiratory protective devices (RPD), fume cupboards, building ventilations, and other installations. However, due to its significant impact on global warming, its use is becoming increasingly restrictive. Krypton 84 (Kr) was chosen to be a possible replacement based on theoretical and practical criteria for the properties that a substitute gas should possess. While compliance with these criteria is generally sufficient to guarantee the reliability of the choice, it is essential in the case of widespread use such as a standard to validate experimentally that this tracer has the same behavior as SF6. In this regard, numerous tests have been carried out to characterize the face leakage of RPD and the rupture of containment of fume cupboards performance tests under different operating conditions. The results obtained are identical with both tracers and lead us to propose the use of Kr as a new reference gas in standards for which SF6 was used.

Respiratory protective device: One size to fit them all?

When exposed to hazardous or toxic substances, workers may be required to wear respiratory protective devices, selected in accordance with the pollutant, required protection level, individual characteristics, and work conditions. To emphasize the importance of the selection procedure, this study aimed to investigate the effects of the facial dimensions and breathing rate on the fit and the protection efficiency provided by full-face respirators. Manikin total efficiency measurements (mTEs) were then conducted on five head forms of various facial dimensions equipped with nine respirators of different models and sizes. A breathing machine simulating sinusoidal breathing rates was used to represent seven work rates, from rest to maximal intensity. For each experiment, the manikin fit factor (mFF), characterizing the respirator fit on the head form, was measured by a controlled negative pressure method. By varying the head form, respirator, breathing rate, and mFF, a total of 485 values of mTE were measured. Findings indicate that even if the respirator was equipped with a high-efficiency filter, mTE strongly decreases if the respirator does not fit the face of the wearer. In particular, it was highlighted that one given respirator cannot fit all facial dimensions and that the best match between the respirator size and facial dimensions is difficult to predict because respirator sizes are not standardized. Moreover, although the total efficiency of a well-fitted respirator naturally decreases when increasing the breathing rate due to filtration mechanisms, the reduction is more significant if the respirator does not fit well. To consider both the mTE and the breathing resistance, a quality factor value was determined for each tested combination of head form, respirator, and breathing rate. The maximum manikin fit factor mFFmax measured for each combination of head form and respirator was compared to that measured on nine human subjects with similar facial dimensions, providing encouraging results concerning the use of head forms during respirator testing.

Course of explosion behaviour of metallic powders - From micron to nanosize.

This work presents an overview about the explosion behaviour of metallic powders from micron to nanosize. Aluminium, magnesium, titanium, iron and zinc were considered and their explosion safety parameters were analysed as a function of their mean primary particle size either determined by BET measurements, particle size distribution. To depict the course of explosion behaviour for these metals, extensive literature review has been performed and additional experimental tests were also performed. Generally, decreasing the particle size in a metallic powder leads to a higher explosion severity. It appears that this statement is true till a critical diameter below which the explosion severity (pmax, dp/dtmax) decreases for all the considered powders. This critical size can be explained by theoretical considerations on the nature of thermal transfer in the flame, namely by analysing the Cassel model. Finally, semi-empirical models were also developed for aluminium to highlight the specific micrometre and nanometre behaviour and the influence of turbulence, particle burning time, diameter and concentration. The influence of these key parameters needs to be further assessed in a future work in order to better understand the mechanisms involved and to extend the scope to other powdered materials.