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As a result of the COVID-19 pandemic, many new materials and masks came onto the market. To determine their suitability, several standards specify which properties to test, including bacterial filtration efficiency (BFE), while none describe how to determine viral filtration efficiency (VFE), a property that is particularly important in times of pandemic. Therefore, we focused our research on evaluating the suitability and efficiency of different systems for determining VFE. Here, we evaluated the VFE of 6 mask types (e.g., a surgical mask, a respirator, material for mask production, and cloth masks) with different filtration efficiencies in four experimental setups and compared the results with BFE results. The study included 17 BFE and 22 VFE experiments with 73 and 81 mask samples tested, respectively. We have shown that the masks tested had high VFE (>99% for surgical masks and respirators, ≥98% for material, and 87–97% for cloth masks) and that all experimental setups provided highly reproducible and reliable VFE results (coefficient of variation < 6%). Therefore, the VFE tests described in this study can be integrated into existing standards for mask testing.

The respiratory system undergoes various physiological and immunological changes with age. After birth, alveolization is one of the main factor associated with progressive growth of lung, which is relatively poorly analysed in respect of lung function. In this study we considered growth of lung caused by progressive increment in number of alveoli and simultaneously calculated its effect on airflow dynamics and filtration efficiency of human lung from childhood to age of 30. Incorporating the idea of filtration through lung with respect to age biofilter model is used by assuming porosity of lung varied with respect to number of alveoli and their surface area. Transportation and deposition properties of nanoparticles of various shapes during inhalation is taken under consideration by using particle shape factor. Generalized Navier Stokes equation is used for flow dynamics and finite difference scheme is used to solve the problem numerically. Simulation is done by using user defined code on MATLAB at very fine grid. Results indicate that the filtration efficiency of lung decreases as age increases from childhood to age of 30; additionally, nonspherical nanoparticles with high aspect ratio’s will take a longer time to be filterd from lung as compared to spherical nanoparticles of the same diameter.

In this paper, a novel electrostatic-assisted melt blown process was reported to produce polypropylene (PP) microfibers with a diameter as fine as 600 nm. The morphology, web structure, pore size distribution, filtration efficiency, and the stress and strain behavior of the PP nonwoven fabric thus prepared were characterized. By introducing an electrostatic field into the conventional melt-blown apparatus, the average diameter of the melt-blown fibers was reduced from 1.69 to 0.96 μm with the experimental setup, and the distribution of fiber diameters was narrower, which resulted in a filter medium with smaller average pore size and improved filtration efficiency. The polymer microfibers prepared by this electrostatic-assisted melt blown method may be adapted in a continuous melt blown process for the production of filtration media used in air filters, dust masks, and so on.

Masks are essential and effective small protective devices used to protect the general public against infections such as COVID-19. However, available systematic reviews and summaries on the filtration performance of masks are lacking. Therefore, in order to investigate the filtration performance of masks, filtration mechanisms, mask characteristics, and the relationships between influencing factors and protective performance were first analyzed through mask evaluations. The summary of filtration mechanisms and mask characteristics provides readers with a clear and easy-to-understand theoretical cognition. Then, a detailed analysis of influencing factors and the relationships between the influencing factors and filtration performance is presented in. The influence of the aerosol size and type on filtration performance is nonlinear and nonconstant, and filtration efficiency decreases with an increase in the gas flow rate; moreover, fitness plays a decisive role in the protective effects of masks. It is recommended that the public should wear surgical masks to prevent COVID-19 infection in low-risk and non-densely populated areas. Future research should focus on fitness tests, and the formulation of standards should also be accelerated. This paper provides a systematic review that will be helpful for the design of masks and public health in the future.

Electret melt-blown nonwovens are widely used for air purification due to their low pressure drop and high filtration efficiency. However, the charge stability could be affected by the ambient temperature and humidity, reducing the filtration efficiency, resulting in the electret melt blown filter not providing effective protection. Herein, we used corona charge to prepare electret melt-blown nonwovens and systematically studied the effects of different temperature and humidity on the structure, morphology, filtration performance, and surface potential within 24 h. The effect of treatment temperature and humidity on pressure drop was minimal because the fiber morphology and web structure of melt-blown nonwovens were not damaged. When the treatment temperature was lower than 70 °C, the effect on the filtration efficiency of the sample was small, but when the temperature increased to 90 or 110 °C, the filtration efficiency decreased significantly with the increase of the treatment time, and the surface potential also declined similarly. In conclusion, high temperatures will lead to charge escape and reduce the electrostatic adsorption effect. Furthermore, at the same temperature, increasing relative humidity can accelerate the charge release and make the filtration efficiency drop more. After the sample was treated at 110 °C and 90% relative humidity for 24 h, the filtration efficiency decreased from 95.49% to 38.16% at a flow rate of 14.16 cm s−1, and the surface potential dropped to the lowest value of −1.01 kV. This result shows that all links of electret melt-blown filter material from raw material to final use should be avoided in high temperature and high humidity conditions to ensure the protection effect.

The particulate matters (PMs) and toxic gases in air have resulted in serious impacts on public health. The development of “green” air filtering materials for isolating these pollutants is of vital importance. Here, we prepared a multi-functional cellulose-based air filter (CFs@ZIF-8 filter) by in situ growth of ZIF-8 nanocrystals on the surface of cellulose fibers. The incorporation of ZIF-8 nanocrystals increased the specific surface area of filter, strengthened the interactions between filter and PMs, and provided abundant cavities and gas adsorption sites for filter. The filtration efficiency of CFs@ZIF-8 filter for PM 0.3 could reach to an ultrahigh level of 99.9%. The gas (nitrogen) adsorption capacity of CFs@ZIF-8 filter was 200 times higher than that of original cellulose-based filter (CFs-filter). The contributions of ZIF-8 on these surpassing properties of CFs@ZIF-8 filter were deeply analyzed. This study provided an effective strategy for developing “green” and multi-functional cellulose-based air filter.

In this study, filtration efficiency of different respirators, face masks, and a 2-ply cotton handkerchief bandana was compared for particles in the size range of 60 nm–4 µm under a “perfect fit” condition. The filtration efficiency at the most penetrating particle size of 0.3 µm on average ranged from 83–99% for N95 and KN95 respirators, 42–88% for surgical masks, 16–23% for cloth masks, and 9% for bandana. We also investigated the effects of using double surgical masks or layering a cloth mask over various surgical masks in terms of their filtration characteristics. In most of these combinations, the filtration efficiency improved by ~25% for particles 0.3–1 µm in diameter without any substantial change in the filter quality factor when compared to the highest of the individual mask results. To investigate the reusability of cotton cloth masks, 2-layer cotton fabric sample coupons were machine washed and dried for 52 cycles leading to an increase in inhalation resistance (~20 Pa) without affecting size-resolved filtration efficiency. Scanning electron microscopy revealed that washing and drying led to the gradual deconstruction of cotton fibers at the scale of several micrometers to hundreds of nanometers in the form of delamination of the fiber wall and fibrillation of the nanofiber constituents. Results indicate that cloth masks may be layered over surgical masks for additional benefits, and that cloth masks made out of cotton fabric can be washed and reused numerous times without a significant loss in filtration efficiency.

Filters serve as core equipment to ensure the normal operation of micro-irrigation systems, with head loss and filtration efficiency serving as the two key indicators for evaluating performance. In this study, we used a pre-pump filter-pontoon mesh rotary filter as the research object and conducted physical model tests under the flow rate (798–1050 L 1  h −1 ), sand content (0.5–2.5 g 1  L −1 ), and aperture of the filter screen (0.125–0.180 mm). We then adopted range analysis, variance analysis (ANOVA), dimensional analysis, and the multiple linear regression (MLR) method to analyze the results. The results showed that the order of factors affecting the head loss of the assessment indices, from large to small, was as follows: flow rate, sand content, and aperture of the filter screen. The order of factors affecting the filtration efficiency of the assessment indices from large to small was as follows: sand content, flow rate, and aperture of the filter screen. Predictive models for head loss and filtration efficiency were developed, with coefficients of determination R 2 of 0.969 and 0.954, and root mean square error (RMSE) values of 0.1041 and 0.0183. The model exhibited high accuracy and could be used to predict the head loss and filtration efficiency of the pontoon mesh rotary filter. In the test range, the optimal working condition of this filter was a flow rate of 930 L 1  h −1 , sand content of 2.0 g 1  L −1 , and 0.150 mm aperture of the filter screen. In addition, the head loss under this condition was 0.281 m, and the filtration efficiency was 84.01%. These results could serve as a reference for the further optimization and application of the pontoon mesh rotary filter, while also enriching the hydraulic performance and filtration performance of the pre-pump filter.

The dynamics of filtration efficiency and pressure drop during the simultaneous filtration of soot and oil aerosols on single- and multi-layer filters were investigated, and the determined filtration efficiency was compared with the theoretical efficiency obtained via the classical filtration theory. Additionally, the influence of liquid-phase aerosols on the morphology of the formed deposits was investigated. It was concluded that the addition of oil aerosols decreased the filtration efficiency and lowered the pressure drop increase rate for multi-layer filters. Additionally, for the filtration of aerosols containing soot and high oil concentrations, once maximum filtration efficiency was reached, an efficiency decrease occurred. The system imperfection factor was proposed as a mean to predict the efficiency of multi-layer filters. The modified version of the single fibre efficiency method was used to calculate filter mass change with reasonable accuracy.

Face masking proved essential to reduce transmission of COVID-19 and other respiratory infections in indoor environments, but standards and literature do not provide simple quantitative methods for quantifying air leakage at the face seal. This study reports an original method to quantify outward leakage and how wearing style impacts on leaks and filtration efficiency. The amount of air leakage was evaluated on four medical masks and four barrier face coverings, exploiting a theoretical model and an instrumented dummy head in a range of airflows between 30 and 160 L/min. The fraction of air leaking at the face seal of the medical masks and barrier face coverings ranged from 43% to 95% of exhaled air at 30 L/min and reduced to 10–85% at 160 L/min. Filter breathability was the main driver affecting both leak fraction and total filtration efficiency that varied from 5% to 53% and from 15% to 84% at 30 and 160 L/min, respectively. Minor changes were related to wearing style, supporting indications on the correct mask use. The fraction of air leaking from medical masks and barrier face coverings during exhalation is relevant and varies according to design and wearing style. The use of highly breathable filter materials reduces air leaks and improve total filtration efficiency.