ASTM F2299

Designation: F 2299 – 03 Standard Test Method for Determining the Initial Efficiency of Materials Used in Medical Face Masks to Penetration by Particulates Using Latex Spheres1 This standard is issued under the fixed designation F 2299; the number immediately following the designation indicates the year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript epsilon (e) indicates an editorial change since the last revision or reapproval. 1. Scope 1.1 This test method establishes procedures for measuring the initial particle filtration efficiency of materials used in medical facemasks using monodispersed aerosols. 1.1.1 This test method utilizes light scattering particle counting in the size range of 0.1 to 5.0 µm and airflow test velocities of 0.5 to 25 cm/s. 1.2 The test procedure measures filtration efficiency by comparing the particle count in the feed stream (upstream) to that in the filtrate (downstream). 1.3 The values stated in SI units or in other units shall be regarded separately as standard. The values stated in each system must be used independently of the other, without combining values in any way. 1.4 The following precautionary caveat pertains only to the test methods portion, Section 10, of this specification. This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limita- tions prior to use. 2. Referenced Documents 2.1 ASTM Standards: D 1356 Terminology Relating to Atmospheric Sampling and Analysis2 D 1777 Method for Measuring Thickness of Textiles3 D 2905 Practice for Statements on Number of Specimens for Textiles3 D 3776 Test Methods for Mass per Unit Area (Weight) of Woven Fabric3 E 691 Practice for Conducting an Interlaboratory Test to Determine the Precision of Test Methods4 F 50 Practice for Continuous Sizing and Counting of Air- borne Particles in Dust-Controlled Areas Using Instru- ments Based Upon Light-Scattering Principles5 F 328 Practice for Determining Counting and Sizing Accu- racy of an Airborne Particle Counter Using Near- Monodispersed Spherical Particulate Materials4 F 778 Methods for Gas Flow Resistance Testing of Filtra- tion Media2 F 1471 Test Method for Air Cleaning Performance of a High-Efficiency Particulate Air-Filter System2 F 1494 Terminology Relating to Protective Clothing2 F 2053 Guide for Documenting the Results of Airborne Particle Penetration Testing of Protective Clothing Materi- als2 3. Terminology 3.1 Definitions: 3.1.1 aerosol, n—a suspension of a liquid or solid particles in a gas with the particles being in the colloidal size range. 3.1.1.1 Discussion—In this test method, aerosols include solid particles having a diameter of 0.1 to 5 µm suspended or dispersed in an airflow at concentrations of less than 102 particles/cm3. 3.1.2 isokinetic sampling, n—a condition where the velocity of the airflow entering the sampling nozzle is the same as the velocity of the airflow passing around the sampling nozzle. 3.1.3 monodispersion, n—scattering of discrete particles in an airflow where the size is centralized about a specific particle size. 3.1.3.1 Discussion—In this test method, the monodispersed particle distribution has a mean diameter size of the aerosol in the 0.1 to 5 µm range, with a coefficient of variation of the mean diameter of 610 % or less, as certified by the manufac- turer. 3.2 For definitions of other protective clothing-related terms used in this test method, refer to Terminology F 1494. 4. Summary of Test Method 4.1 Filtered and dried air is passed through an atomizer to produce an aerosol containing suspended latex spheres. 1 This test method is under the jurisdiction of ASTM Committee F23 on Protective Clothing and is the direct responsibility of Subcommittee F23.40 on Biological Hazards. Current edition approved July 10, 2003. Published September 2003. 2 Annual Book of ASTM Standards, Vol 11.03. 3 Annual Book of ASTM Standards, Vol 07.01. 4 Annual Book of ASTM Standards, Vol 14.02. 5 Annual Book of ASTM Standards, Vol 15.03. 1 Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States. 4.1.1 This aerosol is then passed through a charge neutral- izer. 4.1.2 The aerosol is then mixed and diluted with additional preconditioned air to produce a stable, neutralized, and dried aerosol of latex spheres to be used in the efficiency test. 5. Significance and Use 5.1 This test method measures the initial filtration efficiency of materials used in medical face masks by sampling represen- tative volumes of the upstream and downstream latex aerosol concentrations in a controlled airflow chamber. 5.2 This test method provides specific test techniques for both manufacturers and users to evaluate materials when exposed to aerosol particle sizes between 0.1 and 5.0 µm. 5.2.1 This test method establishes a basis of efficiency comparison between medical face mask materials. 5.2.2 This test method does not establish a comprehensive characterization of the medical face mask material for a specific protective application. 5.3 This test method does not assess the overall effective- ness of medical face masks in preventing the inward leakage of harmful particles. 5.3.1 The design of the medical face mask and the integrity of the seal of the medical face mask to the wearer’s face are not evaluated in this test. 5.4 This test method is not suitable for evaluating materials used in protective clothing for determining their effectiveness against particulate hazards. 5.4.1 In general, clothing design is a significant factor, which must be considered in addition to the penetration of penetration of particulates. 6. Apparatus 6.1 The aerosol test system incorporates the components as shown in Fig. 1. A more detailed diagram of test system components and equipment is found in STP 975.6 6.2 Equipment: 6.2.1 Clean, dry compressed air supply, 6.2.2 HEPA filters (2), 6.2.3 Aerosol generator, 6.2.4 Charge neutralizer, 6.2.5 Humidifier, 6.2.6 Test filter holder and duct assembly, 6.2.7 Pressure drop measuring device, 6.2.8 Air flow rate measuring device, 6.2.9 Temperature and relative humidity detectors, 6.2.10 Air blower (optional for negative pressure system), and 6.2.11 Optical particle counters. 7. System Preparation and Control 7.1 To test in the aerosol particle size range of 0.1 to 5.0 µm, it is necessary to maintain a very clean inlet air supply. Achieve acceptable levels of background aerosol by passing the atom- izing air supply sequentially through a silica-gel dryer (for reduction of moisture), a molecular sieve material (for removal of oil vapor) and an ultra low penetrating aerosol (better than 99.9999 % efficient at 0.6 µm) filter. Then, supply the air to the test chamber of aerosol generator through pressure regulators of 67 kPa [61 psi] accuracy. For throttling of the main airflow as well as other flow splitting requirements, use needle valves to maintain adequate flow stability and back pressure. For recommended flow control measurement, see 7.6. Monitor and record the temperature and relative humidity at the exhaust port of the test chamber. To avoid interference from the test aerosol, take the humidity measurement from the outlet side of the HEPA filter (see 7.6.2) with an in-line probe. 7.1.1 To provide a stable, reproducible aerosol through the test material that remains constant over the sampling time of the efficiency test, maintain the main test duct and filter medium specimen holder in a vertical orientation to minimize aerosol sedimentation losses. 7.2 Aerosol Generation: 7.2.1 The aerosol generator must be capable of a latex sphere count concentrations output of 107 to 108 particles/m3. The suspension reservoir must be large enough to sustain a stabilized output greater than 1 h. Two commercially available atomizing techniques that provide these concentrations of the latex spheres are presented in Figs. 2 and 3. 7.2.2 As viewed in Figs. 2 and 3, these techniques utilize the atomizing of suspended uniform latex spheres from dilute water suspensions. One liter quantities of these suspensions can be made by diluting the 10 % by volume solids of the uniform latex spheres at 1000 to 1 or greater dilution ratios in deionized, filtered distilled water. NOTE 1—The suspensions have a 3 to 6 month usable life. Ideal suspension dilutions are a function of the latex particle size to the aerosol generator droplet size. In order to minimize the atomization of doublets or higher aerosol multiples in the drying process, a recommended latex suspension dilution ratio has been established so that dilution ratios are on the order of 1000:1 to 10 000:1.7 Other aerosols produced from these atomizers can be classified into monodispersed systems but for an industrially recognized standard of particle size and composition the uniform latex spheres are the most reproducible and readily available particles. 7.3 Aerosol Neutralizer—This procedure recommends the use of an aerosol charge neutralizer at the inlet of the test system. This technique generally will ensure aerosol surface charge stability. The aerosol neutralizer can be in the form of a radioactive decay ionizer. The desired Boltzmann’s charge equilibrium for the aerosol has been described.8 Typically, an ionizing flux of 103 mCi/m3/s provides the required aerosol neutralization. NOTE 2—A Krypton 85 source, a Polonium 210 source, or a Corona electrical discharge, A-C source have been found satisfactory for this purpose. 7.4 Aerosol Dilution and Humidity Control—Prior to injec- tion or dispersion of the initial aerosol concentration into the 6 Symposium on Gas and Liquid Filtration, ASTM STP 975, ASTM, Vol 11, 1986, pp. 141-164. 7 Raabe, O., “The Dilution of Monodispersed Suspensions for Aerosolization,” American Industrial Hygiene Association Journal, Vol 29, 1968, pp. 439-443. 8 Liu, B. Y. H. and Piu, D. Y. H., “Electrical Neutralization of Aerosols,” Aerosol Science, Vol 5, 1974, pp. 465-472. F 2299 – 03 2 main test chamber, dry or dilute the aerosol with make-up airflow for the final test aerosol concentration as needed. Conduct material testing in a relative humidity range of 30 to 50 % and hold the relative humidity 65 % during a given test. Complete the aerosol mixing a minimum of 8 duct diameters distance before the inlet sampling probe and the material specimen. 7.5 Material Specimen Holder: FIG. 1 Schematic of Test Method F 2299 – 03 3 7.5.1 The material specimen holder and test section shall be a continuous straight walled vessel, interrupted only by the filter medium sample throughout its length. The material specimen holder must provide an uninterrupted airflow, pas- sage without measurable peripheral air leakage. Use a 50 to 150 mm [2 to 6 in.] cross-sectional diameter for the medium sample size. Choose the specimen size to ensure that the test specimen is representative of the overall material and provides enough rigidity to be self-supporting. NOTE 3—The recommended filter medium cross sections allow face velocities of 0.5 to 25 cm/s [approximately 1 to 50 ft/min] at flow rates of 1 L/min to 1 m3/min [approximately 0.035 to 35 ft3/min] to be developed in testing. 7.5.2 Introduce the latex aerosol a minimum of 10 duct diameters upstream of the material specimen and at a sufficient distance to provide thorough mixing before the upstream sampling probe. 7.6 Airflow Metering: 7.6.1 Use a positive pressure (compressed air) or a negative pressure (exhaust fan or blower) system for the airflow to the main test chamber. For the application of any of these techniques of airflow measurement and calibration, refer to the standards and practices of the American Society of Mechanical Engineers. 7.6.2 Use a High Efficiency Particulate Aerosol (HEPA) type filter (99.97 % efficiency on 0.3 µm aerosol) upstream of the systems airflow measurement. Size the HEPA type filter to provide adequate system collection of the exhausting test aerosol. 7.7 Pressure Drop Measurement: 7.7.1 Use static pressure taps that are flush with the duct walls at a distance of 1 duct diameter upstream and down- stream of the filter medium faces. 7.7.2 With no filter medium in the sample holder, there shall be no measurable pressure loss between the inlet-side and outlet-side pressure taps. Use a pressure-measuring instrument capable of being read to 60.025 cm of water gauge to make this determination. 7.8 Aerosol Sample Extraction and Transport—Use geo- metrically and kinematically identical centerline probes to extract representative aerosols from the inlet and outlet sides of the material specimen test section. Use probes that have a radius of curvature (R) of 12 cm or R/D (Diameter) > 20:1 and present a cross-sectional area of less than 10 % of the cross- sectional area of the test system ducting. Locate the upstream FIG. 2 Atomizer FIG. 3 Collision Atomizer F 2299 – 03 4 probe 8 duct diameters (minimum) downstream of the aerosol injection point and 2 duct diameters ahead of the material specimen. Locate the downstream probe 3 duct diameters downstream of the filter medium specimen. To minimize aerosol sampling transport line losses due to settling, diffusion and inertia for the aerosol particle size range of the test method, use the following characteristics of the sampling. 7.8.1 Maintain the sampling line flow in the laminar flow regime; that is, the Reynolds Number must be less than 1000. Calculate the Reynolds Number in accordance with the follow- ing formula: Re# 5 rgVD1 µg (1) where: rg = gas density (kg/m3), V = gas velocity (m/s), D1 = inside diameter of sampling lines (m), and µg = gas viscosity (kg/m-s). 7.8.2 Limit horizontal sampling line length to less than 100 cm and the total sample transport line to less than 2 m. 7.8.3 Maintain all radius of curvatures to greater than 12 cm. NOTE 4—Isokinetic aerosol sampling is recommended to minimize probe inlet losses. However, in those cases where isokinetic conditions cannot be met, it is recommended that the operation of these probes be 610 % of isokinetic or that the particle Stokes Number at the probe inlet be held to less than 1.0 in order to minimize inertial losses at the probe inlet. It is also recommended that the Reynolds Number of the sample flow lines be held to less than 2000. The Stokes Number is calculated using the following formula: St# 5 Dp 2rpVC 9µgDn (2) where: Dp = particle diameter (m), rp = particle density (kg/m), V = velocity of approach (m/s), µg = gas viscosity (kg/m - s), Dn = diameter of sampling nozzle (m), and C = Cunningham correction factor, which for particles larger than 1.0 3 10-6 m (1 µm) is assumed to be 1.0. NOTE 5—Recommended sampling flow rates for extraction of the mounting volume are to be less than 10 % of the total test system flow rate. 7.9 Aerosol Concentration Counting: 7.9.1 This practice is structured for utilizing automatic, single particle light-scattering counters. For an illustration of the application, calibration, and analyses by these instruments, refer to Practices F 50 and F 328. 7.9.2 Generally, single particle light-scattering counters measure in the range of 0.1 to 15 µm equivalent spherical diameter, with a single particle measurement dynamic range of 50 to 1. These instruments shall be calibrated within the test system, similar to the manufacturer’s standard calibration and with the test aerosol as conditioned for the efficiency testing. For efficiencies approaching 99.9 % and greater, a higher test inlet aerosol concentration is usually required to maintain reasonable sampling times at the outlet. If these conditions exceed the suggested coincidence limits for the single particle counters, an inlet dilution at the optical particle counter of the aerosol is required. Achieve inlet dilution by passing some portion of the conditioned inlet aerosol through a HEPA grade filter and remixing it with the sampled inlet aerosol to the light scattering particle counter. 7.9.3 Establish accurate dilution ratios in order to specify the exact aerosol sample volume extracted from the inlet flow for aerosol particle counting. Recommended sampling times are on the order of 10 to 60 s. If separate particle counters are used for inlet and outlet aerosol concentrations, they must be calibrated for the aerosol particle size and concentration response needed within the test system. NOTE 6—The flow rate of the respective optical particle counter must be measured and recorded. NOTE 7—For test system changes in sampling configuration; that is, alternate upstream and downstream sampling or opening and closing the aerosol flow system, allow a purge time so that 25 sampling line volume changes can occur before counting resumes. (For flow rates of 7 L/min in 6 mm ID samplings, the purge time will be between 10 to 15 s.) 8. Number of Downstream/Upstream Sampling Intervals 8.1 The statistical selection of the number of downstream/ upstream sampling intervals is based on no specimen present in the filter holder. The test apparatus must meet a 100 6 1 % penetration average with a coefficient of variation of 3 %. Use the procedure in 8.2 to obtain this selection. Run this procedure twice and use the number of the two results. 8.2 Procedure: 8.2.1 Obtain 2 consecutive downstream/upstream (100 %) penetration sample observations. Calculate their average. 8.2.2 If the average is between 99 and 101 %, proceed to 8.2.4. If the average is not between 99 and 101 %, run another sample and average it with the previous two samples. 8.2.3 If the new average is between 99 and 101 %, proceed to 8.2.4. If not, continue this process until 100 6 1 % penetration is achieved. If 100 % penetration is not achieved, the test apparatus is biased and must be corrected. 8.2.4 Subtract the highest test sample observation from the lowest sample to give the sample range at that certain number of sample observations. Go to Table 1 for that number of observations and read the adjustment number. 8.2.5 The adjustment number is the greatest range for a coefficient of variation of 3 % at a penetration of 100 6 1 % for the specified number of observations. 8.2.6 If the sample range is equal to or less than the adjustment number, the number of sample observations is the number of downstream/upstream sampling intervals for filter testing. If the sampling range is greater than the adjustment number, then another 100 % penetration sampling observation must be run. 8.2.7 If the new penetration is greater or less than the bounds of the past data, a new range is calculated. If, at the new number of sample observations, the new range is greater than the new adjustment number, this process is repeated until the sample range is less than the adjustment number. That number of sample observations is the number of downstream/upstream sampling intervals used for filter testing. F 2299 – 03 5 8.2.8 If in a suitable number of intervals it is found that the sample range will always be greater than the adjustment number at 20 observations, then the 100 % penetration data is too variable and action must be taken to correct the particle concentration variability or the particle counting methodology. 9. Material Specimen Selection and Conditioning 9.1 Measure material thickness and unit area weight in accordance with Test Methods D 1777 and D 3776, respec- tively. 9.2 Apply a sealing force that does not distort or influence the integrity or continuity of the material specimen. 9.3 Use a total of 5 different material specimens. For statistical-based sampling, choose a number of material speci- mens as indicated in Practice D 2905 or Method F