Aerosol penetration through respirator exhalation valves

Exhalation valves are a critical component of industrial respirators. They are designed to permit minimal inward leakage of air contaminants during inhalation and provide low resistance during exhalation. Under normal conditions, penetration of aerosol through exhalation valves is minimal. The exhalation valve is, however, a vulnerable component of a respirator and under actual working conditions may become dirty or damaged to the point of causing significant leakage. Aerosol penetration was measured for normal exhalation valves and valves compromised by paint or fine copper wires on the valve seat. Penetration increased with increasing wire diameter. A wire 250 microns in diameter allowed greater than 1% penetration into the mask cavity. Dirt or paint accumulated on the exhalation valve allowed a similar level of penetration. Work rate had little effect on observed penetration. Penetration decreased significantly with increasing aerosol particle size. The amount of material on the valve or valve seat necessary for significant (greater than 0.5%) inward leakage in a half-mask respirator could be readily observed by careful inspection of the exhalation valve and its seat in good lighting conditions.     

Measurements of the efficiency of respirator filters and filtering facepieces against radon daughter aerosols.

The filtration efficiencies of respirator filters and filtering facepieces have been tested against radon daughters in a fluorspar mine. The test method involved the use of sampling filters exposed to natural radon daughters in air filtered by the test respirators. Respirators with a filtration efficiency high enough for them to be considered suitable for use against toxic dusts generally reduced radon daughter levels by 90% or more, though nuisance dust masks were ineffective. The measured penetration of radon daughters through the former types of filter correlated reasonably well with the penetration of 0.1 microns neutralized monodisperse aerosols, and with that of the BS 4400 sodium chloride aerosol, measured in the laboratory. To simulate exposure in working conditions mine air containing radon daughters was drawn through the test filters for 8 h, but their performance was not affected and they were not measurably radioactive as a result.     

Variation in penetration of submicrometric particles through electrostatic filtering facepieces during exposure to paraffin oil aerosol

Several studies show the increase of penetration through electrostatic filters during exposure to an aerosol flow, because of particle deposition on filter fibers. We studied the effect of increasing loads of paraffin oil aerosol on the penetration of selected particle sizes through an electrostatic filtering facepiece. FFP2 facepieces were exposed for 8 hr to a flow rate of 95.0 ± 0.5 L/min of polydisperse paraffin aerosol at 20.0?± 0.5?mg/m(3). The penetration of bis(2-ethylhexyl)sebacate (DEHS) monodisperse neutralized aerosols, with selected particle size in the 0.03-0.40?μm range, was measured immediately prior to the start of the paraffin aerosol loading and at 1, 4, and 8 hr after the start of paraffin aerosol loading. Penetration through isopropanol-treated facepieces not oil paraffin loaded was also measured to evaluate facepiece behavior when electrostatic capture mechanisms are practically absent. During exposure to paraffin aerosol, DEHS penetration gradually increased for all aerosol sizes, and the most penetrating particle size (0.05?μm at the beginning of exposure) shifted slightly to larger diameters. After the isopropanol treatment, the higher penetration value was 0.30?μm. In addition to an increased penetration during paraffin loading at a given particle size, the relative degree of increase was greater as the particle size increased. Penetration value measured after 8 hr for 0.03-μm particles was on average 1.6?times the initial value, whereas it was about 8?times for 0.40-μm particles. This behavior, as well evidenced in the measurements of isopropanol-treated facepieces, can be attributed to the increasing action in particle capture of the electrostatic forces (Coulomb and polarization), which depend strictly on the diameter and electrical charge of neutralized aerosol particles. With reference to electrostatic filtering facepieces as personal protective equipment, results suggest the importance of complying with the manufacturer instructions when it is specified that their use has to be restricted to a single shift.     

Breakthrough time and adsorption capacity of respirator cartridges.

A model developed in previous studies was applied to investigate the respirator cartridge contaminant breakthrough of 121 different chemical compounds. Included in this set of 121 contaminants are representative examples of various types of organic compounds, (e.g., acetates, ketones, aromatics, alcohols, amines, alkanes, chlorinated hydrocarbons). Intrinsic to the model are two important parameters, k' (a rate constant) and tau (50% breakthrough time). Values of the two parameters pertinent to an assault concentration of 1000 ppm were determined for each of the 121 organic compounds. These values were used to calculate the breakthrough time at specified fractional breakthrough as well as the respirator cartridge loading capacity under various conditions. An approach was presented that permits the determination of contaminant loading capacity (on carbon) as a function of breakthrough time at a specified assault concentration. The effect of contaminant assault concentration on the saturation capacity was investigated for various combinations of nine different compounds and three different types of respirator cartridges. Calculated saturation capacity data derived from application of the model compare favorably (+/- 5%) with corresponding experimental data. Saturation capacities calculated by using the model addressed in this study were compared with corresponding results previously calculated by others using the Dubinin adsorption isotherm.     

Penetration of methyl isocyanate through organic vapor and acid gas respirator cartridges

Methyl isocyanate (MIC) is a volatile, toxic chemical [Threshold Limit Value (TLV) = 0.02 ppm] used to manufacture carbamate pesticides. The principal manufacturer of MIC is Union Carbide, and the site of production is Institute, West Virginia. In light of the December 1984 Bhopal, India disaster and possible safety problems at the Institute facility, NIOSH conducted this research as a basis upon which to recommend protective equipment that might be used in an emergency situation where extremely high MIC concentrations might be encountered. Both protective clothing and respirators were evaluated. In particular, NIOSH studied air-purifying respirators in order to assess their effectiveness against MIC vapor penetration. NIOSH does not recommend any air purifying respirator for MIC because of its high toxicity and lack of warning properties and because no effective end of service life indicator currently is available for MIC. This report addresses only MIC penetration through air-purifying cartridges at challenge concentrations designed to simulate emergency escape conditions. Another report addresses the protective clothing issue. The results presented are for two different manufacturers' organic vapor (OV) and acid gas cartridges. Penetration tests were conducted at three or four MIC challenge concentrations and at three different humidity conditions. In general, breakthrough times (1% of challenge concentration) were very short (less than 20 min). Also, high relative humidity was found to decrease the breakthrough time of MIC.     

Characteristics of face seal leakage in filtering facepieces.

Several studies have found that aerosol size, testing method, leak size, leak position, sampling probe location, and the mixing condition inside the respirator affect the results of fit factor measurements. This study focuses on the effect of leak shape and filter resistance because leaks have been reported to vary in shape from circular to slit-like. Four leaks of different shape but the same cross-sectional area were used to study their effect on aerosol penetration. Dust-mist and high-efficiency particulate air filtering facepieces provided different filter resistances. An aerodynamic particle sizer and a laser aerosol spectrometer were used to measure the particle size-dependent aerosol concentrations inside and outside the respirators. The filtering facepieces were sealed to a mannequin and artificial leaks were inserted near the right cheek. Aerosol penetration was measured for five flow rates ranging from 5 to 100 L/min. The pressure drop across the mask was monitored with an inclined manometer. At a given pressure differential, a slit-like leak and multiple circular leaks have been found to pass less aerosols than a single circular leak of equal cross-sectional area because the leak flow decreases with an increase in leak shape complexity. If there is substantial lack of face seal fit and the breathing rate is low, a HEPA respirator may provide less protection than a dust-mist respirator because the pressure drop is considerably higher for a HEPA respirator, resulting in more aerosol flow through the leak.     

Estimating reusability of organic air-purifying respirator cartridges

Reuse of organic vapor air-purifying respirator cartridges after a job or shift can provide economy and energy savings. However, standards and manufacturers' guidance discourage reuse, presumably due to a lack of quantitative objective exposure and use information. Storage and simulated reuse laboratory studies and modeling have been done to provide such information. Two important parameters of breakthrough curves, midpoint time (related to adsorption capacity) and midpoint slope (related to adsorption rate), have been shown to be unchanged during storage for reuse. Extrapolations to smaller breakthrough concentrations and times can be made from this reference breakthrough and time. Significant step increases in breakthrough concentration upon cartridge reuse have been observed in some cases. Values of immediate breakthrough concentrations upon reuse (IBURs) have been measured and correlated. The Dubinin/Radushkevich adsorption isotherm equation has been used to estimate maximum IBURs, which depend on many factors, including conditions and duration of first use. An empirical equation describing rate of approach to maximum IBUR as a function of storage time has been developed to provide intermediate IBUR estimates, which are also very dependent on the vapor identity and extent of first-use loading. Using these equations, IBUR estimates with appropriate safety factors can be compared with the allowable breakthrough concentration to help the Industrial Hygienist make reusability decisions.     

Determination of pressure drop across activated carbon fiber respirator cartridges

Activated carbon fiber (ACF) is considered as an alternative adsorbent to granular activated carbon (GAC) for the development of thinner, lighter, and efficient respirators because of their larger surface area and adsorption capacities, thinner critical bed depth, lighter weight, and fabric form. This study aims to measure the pressure drop across different types of commercially available ACFs in respirator cartridges to determine the ACF composition and density that will result in acceptably breathable respirators. Seven ACF types in cloth (ACFC) and felt (ACFF) forms were tested. ACFs in cartridges were challenged with pre-conditioned constant air flow (43 LPM, 23°C, 50% RH) at different compositions (single- or combination-ACF type) in a test chamber. Pressure drop across ACF cartridges were obtained using a micromanometer, and compared among different cartridge configurations, to those of the GAC cartridge, and to the NIOSH breathing resistance requirements for respirator cartridges. Single-ACF type cartridges filled with any ACFF had pressure drop measurements (23.71–39.93 mmH₂O) within the NIOSH inhalation resistance requirement of 40 mmH₂O, while those of the ACFC cartridges (85.47±3.67 mmH₂O) exceeded twice the limit due possibly to the denser weaving of ACFC fibers. All single ACFF-type cartridges had higher pressure drop compared to the GAC cartridge (23.13±1.14 mmH₂O). Certain ACF combinations (2 ACFF or ACFC/ACFF types) resulted to pressure drop (26.39–32.81 mmH₂O) below the NIOSH limit. All single-ACFF type and all combination-ACF type cartridges with acceptable pressure drop had much lower adsorbent weights than GAC (≤15.2% of GAC weight), showing potential for light-weight respirator cartridges. 100% ACFC in cartridges may result to respirators with high breathing resistance and, thus, is not recommended. The more dense ACFF and ACFC types may still be possibly used in respirators by combining them with less dense ACFF materials and/or by reducing cartridge bed depth to reduce pressure drop to acceptable levels. ACFF by itself may be more appropriate as adsorbent materials in ACF respirator cartridges in terms of acceptable breathing resistance.     

Estimating service lives of air-purifying respirator cartridges for reactive gas removal

A mathematical model has been developed to estimate service lives of air-purifying respirator cartridges that remove gases reactively from flowing air. Most gases, because of their high volatility and low polarizability, are not effectively removed by physical adsorption on activated carbon. Models previously developed for toxic organic vapors cannot estimate service lives of cartridges for toxic gases. Often, an activated carbon is impregnated with a chemical to enhance gas removal by chemical reaction(s). The kinds of reactions, types and amounts of impregnants, and effects of the presence of water vary; therefore, the model requires user inputs of gas capacity and water effect parameters. Ideally, these should be available from manufacturers of the cartridges. If they are not, they can be extracted from measured breakthrough times using this model. The key to this model is the observation that adsorption rates of gases can be adequately quantified by the same correlations that have been reported for organic vapors. The resulting model has been used to correlate and predict breakthrough times for several common toxic gases.     

N95 filtering facepiece respirator deadspace temperature and humidity

The objective of this study was to determine the levels of heat and humidity that develop within the deadspace of N95 filtering facepiece respirators (N95 FFR). Seventeen subjects wore two models each of N95 FFR and N95 FFR with an exhalation valve (N95 FFR/EV) while exercising on a treadmill at a low-moderate work rate for 1 and 2 hr in a temperate ambient environment. FFR deadspace temperature and relative humidity were monitored by a wireless sensor housed within the FFR. Each FFR was weighed pre- and post-testing to determine moisture retention. After 1 hr, FFR deadspace temperature and humidity were markedly elevated above ambient levels, and the FFR deadspace mean apparent heat index was 54°C. N95 FFR/EV use resulted in significantly lower deadspace temperatures than N95 FFR (p = 0.01), but FFR deadspace humidity levels were not significantly different (p = 0.32). Compared with the first hour of use, no significant increase in FFR deadspace heat and humidity occurred over the second hour. FFR mean moisture retention was < 0.3 grams over 2 hr. N95 FFR/EV offer a significant advantage in deadspace heat dissipation over N95 FFR at a low-moderate work rate over 1 hr of continuous use but offered no additional benefit in humidity amelioration. Moisture retention in N95 FFR and N95 FFR/EV is minimal after 2 hr of use. [Supplementary materials are available for this article. Go to the publisher's online edition of Journal of Occupational and Environmental Hygiene for the following free supplemental resource: a file containing N95 filtering facepiece respirator deadspace mean RH and temperature recordings for 17 subjects treadmill exercising at 5.6 Km/H over 1 hour.].     

Nanoparticle penetration through filter media and leakage through face seal interface of N95 filtering facepiece respirators

National Institute for Occupational Safety and Health recommends the use of particulate respirators for protection against nanoparticles (<100 nm size). Protection afforded by a filtering facepiece particulate respirator is a function of the filter efficiency and the leakage through the face-to-facepiece seal. The combination of particle penetration through filter media and particle leakage through face seal and any component interfaces is considered as total inward leakage (TIL). Although the mechanisms and extent of nanoparticle penetration through filter media have been well documented, information concerning nanoparticle leakage through face seal is lacking. A previous study in our laboratory measured filter penetration and TIL for specific size particles. The results showed higher filter penetration and TIL for 50 nm size particles, i.e. the most penetrating particle size (MPPS) than for 8 and 400 nm size particles. To better understand the significance of particle penetration through filter media and through face seal leakage, this study was expanded to measure filter penetration at sealed condition and TIL with artificially introduced leaks for 20-800 nm particles at 8-40 l minute volumes for four N95 models of filtering facepiece respirators (FFRs) using a breathing manikin. Results showed that the MPPS was ~45 nm for all four respirator models. Filter penetration for 45 nm size particles was significantly (P < 0.05) higher than the values for 400 nm size particles. A consistent increase in filter penetrations for 45 and 400 nm size particles was obtained with increasing breathing minute volumes. Artificial leakage of test aerosols (mode size ~75 nm) through increasing size holes near the sealing area of FFRs showed higher TIL values for 45 nm size particles at different minute volumes, indicating that the induced leakage allows the test aerosols, regardless of particle size, inside the FFR, while filter penetration determines the TIL for different size particles. TIL values obtained for 45 nm size particles were significantly (P < 0.05) higher than the values obtained for 400 nm size particles for all four models. Models with relatively small filter penetration values showed lower TIL values than the models with higher filter penetrations at smaller leak sizes indicating the dependence of TIL values on filter penetration. When the electrostatic charge was removed, the FFRs showed a shift in the MPPS to ~150 nm with the same test aerosols (mode size ~75 nm) at different hole sizes and breathing minute volumes, confirming the interaction between filter penetration and face seal leakage processes. The shift in the MPPS from 45 to 150 nm for the charge removed filters indicates that mechanical filters may perform better against nanoparticles than electrostatic filters rated for the same filter efficiency. The results suggest that among the different size particles that enter inside the N95 respirators, relatively high concentration of the MPPS particles in the breathing zone of respirators can be expected in workplaces with high concentration of nanoparticles. Overall, the data obtained in the study suggest that good fitting respirators with lower filter penetration values would provide better protection against nanoparticles.     

Correlations between workplace protection factors and fit factors for filtering facepieces in the welding workplace

Workplace protection factor (WPF) means a measure of the actual protection of respirator provided in the workplace when correctly worn. While fit factor (FF) represents a quantitative measure of the fit of a particular respirator to an individual and it is determined in the laboratory. To evaluate the relationship between WPF and FF is very important since FF may or may not be taken advantage of estimating WPF. Outside and inside Fe concentrations for three brand N95 filtering facepieces were collected on 14 workers/three respirator combinations in the welding workplace. The WPF measurements on the samples of the three respirator brands worn by 14 workers were observed to range from 2.2 to 132.9 with a geometric mean of 15.9 and a geometric standard deviation of 2.63. Respirator performances as measured by the WPF differed significantly among different respirator brands (p<0.05). In this study, correlations were found between the WPF measurements and the FF data for all samples of the three respirators (R2=0.38). The percentage of Fe particles having a smaller fraction than 1.1 microm diameter was observed as 71.6% of the total.     

Workplace protection factors for an N95 filtering facepiece respirator

This study evaluated the workplace performance of an N95 filtering facepiece, air-purifying respirator in a steel foundry. Air samples were collected inside and outside respirators worn by workers who were properly trained and qualitatively fit tested. For most workers, three or four pairs of air samples were collected on each of 2 days. The 49 valid sample sets were analyzed for iron, silicon, and zirconium. Only iron was present in sufficient concentrations to perform workplace protection factor (WPF) calculations. Individual WPF measurements ranged from 5 to 753. The geometric mean of the distribution was 119 with a lower 5th percentile value of 19. Time-weighted average WPFs (WPF(TWA)) were also calculated for each day for each worker as an estimate of the protection an individual might receive with daily respirator use. The WPF(TWA) values ranged from 15 for the worker with the single WPF value of 5, to a high of 684. The distribution of WPF(TWA) had a geometric mean of 120 and a lower 5th percentile of 22. Both data treatments indicate this respirator's performance was consistent with the assigned protection factor of 10 typically used for half facepiece respirators. The respirator provided adequate protection as used in this study. All contaminant concentrations inside the respirator were well below the relevant occupational exposure limits. Data collected also illustrate the dynamic nature of faceseal leakage in the workplace.