Systematic experimental comparison of particle filtration efficiency test methods for commercial respirators and face masks

Interlaboratory comparison of particle filtration efficiency testing equipment

Particle filtration efficiency (PFE) is a critical property of face masks, with the most common test methods using sodium chloride as a challenge aerosol. In the absence of bottom-up uncertainty budgets for PFE, interlaboratory comparisons provide an alternative route to robustly quantify the precision and bias of the method. This work presents the results of several interlaboratory comparisons of particle filtration efficiency performed across a network of laboratories. Using log-penetration as a surrogate for PFE, it is shown that expanded reproducibility intervals were consistent across most samples, at around 26% of the nominal value of log-penetration. Between-laboratory contributions to this reproducibility were significant, nearly doubling the lab-reported uncertainties in most instances and emphasizing the need for ongoing interlaboratory studies to be performed for particle filtration. More work is required to identify the causes of these between-laboratory differences, requiring dedicated testing. Alongside uncertainty quantification, testing materials across a range of variables (such as the number of layers, amount of charge on the material, and basis weight) affirm that constant quality is a good approximation when layering or changing the basis weight on an otherwise identical material.

Evaluation of Two Particle Number (PN) Counters with Different Test Protocols for the Periodic Technical Inspection (PTI) of Gasoline Vehicles

Thousands of particle number (PN) counters have been introduced to the European market, following the implementation of PN tests during the periodic technical inspection (PTI) of diesel vehicles equipped with particulate filters. Expanding the PN-PTI test to gasoline vehicles may face several challenges due to the different exhaust aerosol characteristics. In this study, two PN-PTI instruments, type-examined for diesel vehicles, measured fifteen petrol passenger cars with different test protocols: low and high idling, with or without additional load, and sharp accelerations. The instruments, one based on diffusion charging and the other on condensation particle counting, demonstrated good linearity compared to the reference instrumentation with R-squared values of 0.93 and 0.92, respectively. However, in a considerable number of tests, they registered higher particle concentrations due to the presence of high concentrations below their theoretical 23 nm cut-off size. The evaluation of the different test protocols showed that gasoline direct injection engine vehicles without particulate filters (GPFs) generally emitted an order of magnitude or higher PN compared to those with GPFs. However, high variations in concentration levels were observed for each vehicle. Port-fuel injection vehicles without GPFs mostly emitted PN concentrations near the lower detection limit of the PN-PTI instruments.

Particle filtration efficiency measured using sodium chloride and polystyrene latex sphere test methods

Standards governing face masks differ in the test methods used to determine sub-micron particle filtration efficiency (PFE), such that the meaning of PFE is not universal. Unifying the meaning of PFE requires data using these different test methods to drive improvements in standards. This simple data set provides the equivalence between two major test methods used to assess PFE: (1) a test method using a neutralized, polydisperse sodium chloride (NaCl) and (2) a test method using an unneutralized, "monodisperse" polystyrene latex sphere (PSL) aerosols. Measurements are made on over 5800 real-world medical masks, leading to the establishment of a relationship between these two kinds of PFE for these products.

Comparison of measurement systems for assessing number- and mass-based particle filtration efficiency

The particle filtration efficiency (PFE) of a respirator or face mask is one of its key properties. While the physics of particle filtration results in the PFE being size-dependent, measurement standards are specified using a single, integrated PFE, for simplicity. This integrated PFE is commonly defined concerning either the number (NPFE) or mass (MPFE) distribution of particles as a function of size. This relationship is non-trivial; it is influenced by both the shape of the particle distribution and the fact that multiple practical definitions of particle size are used. This manuscript discusses the relationship between NPFE and MPFE in detail, providing a guide to practitioners. Our discussion begins with a description of the theory underlying different variants of PFE. We then present experimental results for a database of size-resolved PFE (SPFE) measurements for several thousand candidate respirators and filter media, including filter media with systematically varied properties and commercial samples that span 20%-99.8% MPFE. The observed relationships between NPFE and MPFE are discussed in terms of the most-penetrating particle size (MPPS) and charge state of the media. For the sodium chloride particles used here, we observed that the MPFE was greater than NPFE for charged materials and vice versa for uncharged materials. This relationship is observed because a shift from NPFE to MPFE weights the distribution toward larger sizes, while charged materials shift the MPPS to smaller sizes. Results are validated by comparing the output of a pair of automated filter testers, which are used in gauging standards compliance, to that of MPFE computed from a system capable of measuring SPFE over the 20 nm-500 nm range.

COVID-19 Pandemic: New Prevention and Protection Measures

As of the end of February 2021, more than 420,000,000 confirmed cases of COVID-19 have been reported worldwide, with 5,856,224 deaths. Transmission of the different genetically engineered variants of SARS-CoV-2, which have been isolated since the beginning of the pandemic, occurs from one infected person to another by the same means: the airborne route, indirect contact, and occasionally the fecal–oral route. Infection is asymptomatic or may present with flulike symptoms such as fever, cough, and mild to moderate and severe respiratory distress, requiring hospitalization and assisted ventilation support. To control the spread of COVID-19, the World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC) have indicated that the appropriate use of personal protective equipment (PPE), as well as the adoption of effective hygiene systems, is one of the primary prevention measures for the entire population. Companies and institutions around the world are therefore trying to find the best ways to reorganize their operations, minimizing the risk of infection among their employees, in order to protect their health and prevent internal outbreaks of SARS-CoV-2, including through the development of new technologies that could also be an innovative and driving factor for the relaunch of companies in a more sustainable, ethically correct, and safe for the health of employees perspective. On the basis of the above premises, in view of the coexistence with SARS-CoV-2 that will most likely accompany us in the coming years, and in view of the vaccination campaign adopted worldwide, the purpose of our narrative review is to update the previous operational protocols with the latest scientific knowledge to be adopted in the workplace even when the emergency crisis is over.