Impact of time and assisted donning on respirator fit

The goal of this study was to examine the impact of assistance with donning and time on quantitative fit factors (FF) and pass rates for subjects wearing an N95 filtering facepiece respirator (FFR) and a non-certified adhesive mask. Fit factors were measured using two side-by-side TSI Portacount instruments sampling second-by-second simultaneous inside- and outside-facepiece concentrations. Naïve subjects made two visits at least one week apart. At each visit subjects first donned either the respirator or adhesive mask without assistance and performed a five-exercise fast fit test. They then donned a new respirator or mask with assistance in proper donning (e.g., proper forming of the nosepiece, strap placement, etc.) and performed a second five-exercise fast fit test. The same sequence of unassisted and assisted donning was then repeated with the other facepiece. Fifteen subjects participated in the study; time between visits ranged from 7–29 days (average 12 days). On visit 1 the respirator FFs were significantly higher (GM = 88) than for the adhesive mask (GM = 14); with assistance these FFs showed significant improvements to 116 and 124, respectively. At visit 1 almost half of subjects donning the respirator without assistance achieved a passing FF of 100 or greater, while only one-fifth of subjects were able to pass the fit test wearing an adhesive mask without assistance. Pass rates improved for both groups with assistance, but more so for the adhesive mask wearers. On visit 2 the pass rates for both groups had decreased, with the adhesive mask group showing a greater decrease. With assistance, pass rates improved for both groups, but again more so for those wearing the adhesive mask. Results suggest that wearers would benefit from assistance and re-training every time they don a respirator, even if the time between donnings is as short as one or two weeks.     

A new sampling technique that allows quantitative fit testing by using the employee's own respirator.

A new, in-mask sampling technique has been developed that enhances current quantitative respirator fit test methods. This new sampling technique eliminates the need for a permanently modified (probed) respirator for conducting fit tests. With this new technique, a sample was drawn from the breathing zone inside the respirator by means of a sampling adapter, which was temporarily attached between one of the air-purifying filters and the facepiece before a fit test was performed. To ensure that the performance of the respirator was not altered, the sampling adapter was designed so that all major functional aspects of the respirator were unchanged. This new sampling technique was evaluated on human test subjects by simultaneous sampling from a probed respirator by using two quantitative respirator fit test instruments. One instrument sampled through a conventional respirator probe; the other used the new method. Test results show that equivalent fit test data were obtained with both sampling techniques.     

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.     

Release of simulated anthrax particles from disposable respirators

A preliminary study was undertaken to evaluate the potential for a disposable respirator that has been contaminated with anthrax spores to release spores in handling after use. The release of inert particles from disposable respirators was measured for masks dropped 3 feet onto a hard surface. Ten experimental runs were conducted for each of two N95 mask types, the Moldex 2200N95 and the 3M 8210. Anthrax spores were simulated with a test aerosol of single and double 1-micron polystyrene spheres. For the Moldex mask loaded with approximately 20 million spheres on it, an average of 0.16% was released; for the 3M mask an average of 0.29% was released.     

A comparison of total inward leakage measured using sodium chloride (NaCl) and corn oil aerosol methods for air-purifying respirators

The International Organization for Standardization (ISO) standard 16900-1:2014 specifies the use of sodium chloride (NaCl) and corn oil aerosols, and sulfur hexafluoride gas for measuring total inward leakage (TIL). However, a comparison of TIL between different agents is lacking. The objective of this study was to measure and compare TIL for respirators using corn oil and NaCl aerosols. TIL was measured with 10 subjects donning two models of filtering facepiece respirators (FFRs) including FFP1, N95, P100, and elastomeric half-mask respirators (ERs) in NaCl and corn oil aerosol test chambers, using continuous sampling methods. After fit testing with a PortaCount (TSI, Inc., St. Paul, MN) using the Occupational Safety and Health Administration (OSHA) protocol, five subjects were tested in the NaCl chamber first and then in the corn oil chamber, while other subjects tested in the reverse order. TIL was measured as a ratio of mass-based aerosol concentrations in-mask to the test chamber, while the subjects performed ISO 16900-1-defined exercises. The concentration of NaCl aerosol was measured using two flame photometers, and corn oil aerosol was measured with one light scattering photometer. The same instruments were used to measure filter penetration in both chambers using a Plexiglas setup. The size distribution of aerosols was determined using a scanning mobility particle sizer and charge was measured with an electrometer. Filter efficiency was measured using an 8130 Automated Filter Tester (TSI). Results showed the geometric mean TIL for corn oil aerosol for one model each of all respirator categories, except P100, were significantly (p?相似文献   

Determination of known exhalation valve damage using a negative pressure user seal check method on full facepiece respirators

A negative pressure user seal check (NPUSC) method was evaluated for its ability to adequately detect known exhalation valve leakage into a respirator. Three valves with different types of damage were included. Twenty-six test subjects, wearing full facepiece respirators, were asked to perform a NPUSC. Their responses as to whether they passed or failed the user seal check were compared to fit testing results from two quantitative fit test methods: ambient aerosol and controlled negative pressure. In addition, equipment developed at the University of Cincinnati was used to measure in-mask pressures that are generated during the performance of NPUSCs. This technique was employed to assess the ability of respirator wearers to properly conduct user seal checks. The data were analyzed to determine if the user seal check procedure is an effective method for detecting known exhalation valve damage. All test subjects reported passing the user seal check with the undamaged valve. With the warped valve installed, 95 percent of test subjects reported passing the user seal check. With the slit valve installed, 73 percent of test subjects reported passing. With the dirty valve installed, 65 percent reported passing. All fit factors, measured with the damaged valves, were below the Occupational Safety and Health Administration-recognized pass/fail criteria except one fit test with the respirator equipped with the slit valve. Results from the in-mask pressure measurements confirmed whether or not the subject properly conducted a user seal check, but did not detect respirator leakage. In conclusion, the performance of a NPUSC rarely helped to identify damaged exhalation valves. These results support the need for respirator inspection prior to donning with periodic fit testing and the performance of user seal checks as necessary components of an adequate respiratory protection program.     

A Quantitative Assessment of the Total Inward Leakage of NaCl Aerosol Representing Submicron-Size Bioaerosol Through N95 Filtering Facepiece Respirators and Surgical Masks

Respiratory protection provided by a particulate respirator is a function of particle penetration through filter media and through faceseal leakage. Faceseal leakage largely contributes to the penetration of particles through a respirator and compromises protection. When faceseal leaks arise, filter penetration is assumed to be negligible. The contribution of filter penetration and faceseal leakage to total inward leakage (TIL) of submicron-size bioaerosols is not well studied. To address this issue, TIL values for two N95 filtering facepiece respirator (FFR) models and two surgical mask (SM) models sealed to a manikin were measured at 8 L and 40 L breathing minute volumes with different artificial leak sizes. TIL values for different size (20–800 nm, electrical mobility diameter) NaCl particles representing submicron-size bioaerosols were measured using a scanning mobility particle sizer. Efficiency of filtering devices was assessed by measuring the penetration against NaCl aerosol similar to the method used for NIOSH particulate filter certification. Results showed that the most penetrating particle size (MPPS) was ～45 nm for both N95 FFR models and one of the two SM models, and ～350 nm for the other SM model at sealed condition with no leaks as well as with different leak sizes. TIL values increased with increasing leak sizes and breathing minute volumes. Relatively, higher efficiency N95 and SM models showed lower TIL values. Filter efficiency of FFRs and SMs influenced the TIL at different flow rates and leak sizes. Overall, the data indicate that good fitting higher-efficiency FFRs may offer higher protection against submicron-size bioaerosols.     

Respiratory source control using a surgical mask: An in vitro study

Cough etiquette and respiratory hygiene are forms of source control encouraged to prevent the spread of respiratory infection. The use of surgical masks as a means of source control has not been quantified in terms of reducing exposure to others. We designed an in vitro model using various facepieces to assess their contribution to exposure reduction when worn at the infectious source (Source) relative to facepieces worn for primary (Receiver) protection, and the factors that contribute to each. In a chamber with various airflows, radiolabeled aerosols were exhaled via a ventilated soft-face manikin head using tidal breathing and cough (Source). Another manikin, containing a filter, quantified recipient exposure (Receiver). The natural fit surgical mask, fitted (SecureFit) surgical mask and an N95-class filtering facepiece respirator (commonly known as an “N95 respirator”) with and without a Vaseline-seal were tested. With cough, source control (mask or respirator on Source) was statistically superior to mask or unsealed respirator protection on the Receiver (Receiver protection) in all environments. To equal source control during coughing, the N95 respirator must be Vaseline-sealed. During tidal breathing, source control was comparable or superior to mask or respirator protection on the Receiver. Source control via surgical masks may be an important adjunct defense against the spread of respiratory infections. The fit of the mask or respirator, in combination with the airflow patterns in a given setting, are significant contributors to source control efficacy. Future clinical trials should include a surgical mask source control arm to assess the contribution of source control in overall protection against airborne infection.     

Correlation between quantitative fit factors and workplace protection factors measured in actual workplace environments at a steel foundry

Past studies have found little or no correlation between workplace protection factors (WPFs) and quantitative fit factors (FFs). This study investigated the effect of good- and poor-fitting half-facepiece, air-purifying respirators on protection in actual workplace environments at a steel foundry and the correlation between WPFs and FFs. Fifteen burners and welders, who wore respirators voluntarily, and chippers participated in this study. Each subject was fit-tested with two respirator models each with three sizes, for a total of six fit-tests. Models and sizes were assigned this way to provide a wide range of FFs among study participants. Each worker donned the respirator twice per day (at the beginning of the shift and following the lunch break) for 2 days. Quantitative FFs were first obtained for each donning using the PortaCount Plus trade mark in a separate room. Without redonning the respirators, workers performed normal work for 1 to 2 hours, and WPFs were measured by collecting ambient and in-facepiece samples simultaneously. A second fit-test was conducted without disturbing the respirator. FFs were obtained by averaging the results from the first and second fit-tests. The resulting FFs had a geometric mean (GM) of 400 (range=10-6010) and a geometric standard deviation (GSD) of 6.1. Of the 55 valid donnings, 43 were good fitting (FFs> or =100) and 12 were poor fitting (FFs<100). The WPFs had a GM of 920 (range=13-230,000) and a GSD of 17.8. The WPFs were found to be significantly correlated with the FFs (R(2)=.55 and p-value=.0001). Therefore, FF was shown to be a meaningful indicator of respirator performance in actual workplace environments.     

Effect of test exercises and mask donning on measured respirator fit

Quantitative respirator fit test protocols are typically defined by a series of fit test exercises. A rationale for the protocols that have been developed is generally not available. There also is little information available that describes the effect or effectiveness of the fit test exercises currently specified in respiratory protection standards. This study was designed to assess the relative impact of fit test exercises and mask donning on respirator fit as measured by a controlled negative pressure and an ambient aerosol fit test system. Multiple donnings of two different sizes of identical respirator models by each of 14 test subjects showed that donning affects respirator fit to a greater degree than fit test exercises. Currently specified fit test protocols emphasize test exercises, and the determination of fit is based on a single mask donning. A rationale for a modified fit test protocol based on fewer, more targeted test exercises and multiple mask donnings is presented. The modified protocol identified inadequately fitting respirators as effectively as the currently specified Occupational Safety and Health Administration (OSHA) quantitative fit test protocol. The controlled negative pressure system measured significantly (p < 0.0001) more respirator leakage than the ambient aerosol fit test system. The bend over fit test exercise was found to be predictive of poor respirator fit by both fit test systems. For the better fitting respirators, only the talking exercise generated aerosol fit factors that were significantly lower (p < 0.0001) than corresponding donning fit factors.     

一种自动按压呼吸器的研制

目的：研制了一种自动按压呼吸器,实现了呼吸球囊的自动按压。方法：自动按压呼吸器包括弹性气囊、面罩、驱动弹性气囊向面罩循环供气的驱动装置。弹性气囊的出气口与面罩的进气口连通,自动按压呼吸器通过驱动装置驱动弹性气囊向面罩循环供气,驱动装置易与各种控制器连接实现自动控制功能,如接入调速变频器,可根据患者的身体状况,通过调节调速变频器的输出频率来控制驱动装置,改变驱动装置挤压弹性气囊的频率以符合不同患者的需求。结果：该自动按压呼吸器体积小、结构简单、移动灵活、使用方便,与现有的手捏橡皮囊呼吸器相比省时省力,而且供气量均匀、准确。结论：自动按压呼吸器的驱动装置驱动弹性气囊向面罩循环供气,实现了自动按压,提高了危重患者抢救效率和成功率。     

Respirator fit factor performance while sweating

The extent to which sweat accumulation inside respirators affects respirator fit has not been quantified. This study represents an attempt to measure facial sweating and to quantify its effects on fit factors of negative pressure, full-facepiece respirators. Respirator fit factor (FF) data were obtained while 14 subjects completed 30 minutes of treadmill walking at an intensity of 75% of age-predicted maximal heart rate in an aerosol test chamber under ambient environmental conditions. Subject facial and whole body sweat production were also measured. Statistical analysis of the treadmill FF results showed that respirator fit was significantly (p < 0.05) degraded after 14 minutes of exercise. Sweat accumulation inside the respirator facepiece averaged 30.9 +/- 15.5 g. However, no significant correlation of subject facial sweat production with overall FF values measured during exercise was found. The results of this study indicate that respirator FFs degrade significantly over time under moderate exercise and environmental conditions and suggest that facial sweat accumulation alone does not account for the reduced FF levels.     

Physiological and subjective responses to working in disposable protective coveralls and respirators commonly used by the asbestos abatement industry

The physiological and subjective effects of working with different respirators while wearing lightweight disposable (Tyvek 1412 polyolefin) coveralls commonly used by the asbestos abatement industry were studied. Nine healthy men (mean age = 27.3 yr, weight = 76.9 kg) each performed a series of four exercise tests with four different respirator ensembles in counterbalanced order. Treadmill work was performed at a set walking speed of 4 kph (2.5 mph), 0 percent elevation (220 kcal/hr), a controlled environmental temperature of 33.9 degrees C, and 50% relative humidity. Each test continued up to 120 min, with repeated work/rest intervals of 26 min of work and 4 min of rest. Tyvek disposable coveralls and hoods were worn with each of these four different respirator ensembles: (1) control--a lightweight, low resistance mask; (2) HEPA--an air purifying, full facepiece respirator with dual high efficiency filters; (3) SAR--a supplied-air, pressure-demand respirator with escape filter; (4) SCBA--an open circuit, pressure-demand, self-contained breathing apparatus. Physiological measurements obtained every minute during each test included heart rate and skin and rectal temperatures. Subjective evaluations of clothing, respirator, and facepiece comfort, ease of breathing, temperature and perspiration in the mask and clothing, and respirator load also were measured at the end of the test. Data were analyzed using an analysis of variance. Results indicated that heart rate at the end of the test differed by less than 8 BPM between the control condition and the SCBA (heaviest) condition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Development of a new respirator for organic vapors with a breakthrough detector using a semiconductor gas sensor

A method for determining breakthrough of organic vapors in a respirator cartridge was developed. A thick film semiconductor gas sensor was used as a breakthrough detector. Air containing organic vapor was introduced into the cartridge, and an output signal from the sensor inserted in the downstream flow of the cartridge was recorded on an IC card. Simultaneously, the breakthrough curve was obtained by measuring the vapor concentration at downstream from the respirator cartridge with a gas chromatograph (GC) equipped with a flame ionization detector. When the breakthrough was almost completed, the data recorded in the card were transferred to a personal computer and the change in the output signal from the sensor was compared with the breakthrough curve obtained by the GC. Twelve organic solvents including aromatic hydrocarbons, chlorinated hydrocarbons, acetates, alcohols, ketones, and aliphatic hydrocarbons were tested under low (20%-25%) and high (70%-80%) relative humidity ranges. The sensitivity of the sensor for chlorinated hydrocarbons such as 1,1,1-trichloroethane was relatively low, especially when the relative humidity was high, but the rise time of the sensor output signal was almost the same as or earlier than the breakthrough time by the GC. Based on the experimental results, a new respirator for organic vapors that can detect the end of service life was developed.     

The effect of simulated air conditions on N95 filtering facepiece respirators performance

The objective of this study was to determine the effect of several simulated air environmental conditions on the particle penetration and the breathing resistance of two N95 filtering facepiece respirator (FFR) models. The particle penetration and breathing resistance of the respirators were evaluated in a test system developed to mimic inhalation and exhalation breathing while relative humidity and temperature were modified. Breathing resistance was measured over 120 min using a calibrated pressure transducer under four different temperature and relative humidity conditions without aerosol loading. Particle penetration was evaluated before and after the breathing resistance test at room conditions using a sodium chloride aerosol measured with a scanning mobility particle sizer. Results demonstrated that increasing relative humidity and lowering external temperature caused significant increases in breathing resistance (p < 0.001). However, these same conditions did not influence the penetration or most penetrating particle size of the tested FFRs. The increase in breathing resistance varied by FFR model suggesting that some FFR media are less influenced by high relative humidity.     

Exposures from thorium contained in thoriated tungsten welding electrodes.

Information provided in this article can be used for estimating the radiation dose associated with the use of thoriated tungsten electrodes in tungsten inert gas welding. Area and breathing zone concentrations of 232Th generated by welding and electrode sharpening along with particle size information; isotopic composition of electrodes from two domestic manufacturers and one European manufacturer; and process variables and estimates on the number of thoriated tungsten electrodes manufactured are presented. Past literature is reviewed and compared with the results of this study. Isotopic analysis of a nominal 2% thoriated electrode found 0.6 ppm +/- 0.4 ppm 230Th and less than 0.1 ppm 228Th. Analysis of a ceriated tungsten electrode and a lanthanated tungsten electrode for 232Th found 124 ppm and 177 ppm, respectively. Electrode consumption during welding was primarily the result of tip sharpening. Less than 3% of the weight loss was attributable to the welding process. The in-mask concentration of respirable thorium particulate in the welder's breathing zone was 0.002 x 10(-12) microCi 232Th/mL. The concentration of respirable thorium particulate from electrode sharpening was 1.3 x 10(-12) microCi 232Th/mL. The measured sharpening time was 20 sec per electrode. Estimates of the activity median aerodynamic diameters for the respirable fraction of the welding and electrode sharpening aerosols were 3.5 and 5 microns, respectively, when measured in the breathing zone at 0.3 m (12 inches) from the point of operation. The respirable fraction of the total welding and sharpening aerosols was 45 and 60%.     

Evaluation of leakage from a metal machining center using tracer gas methods: a case study

To evaluate the efficacy of engineering controls in reducing worker exposure to metalworking fluids, an evaluation of an enclosure for a machining center during face milling was performed. The enclosure was built around a vertical metal machining center with an attached ventilation system consisting of a 25-cm diameter duct, a fan, and an air-cleaning filter. The evaluation method included using sulfur hexafluoride (SF6) tracer gas to determine the ventilation system's flow rate and capture efficiency, a respirable aerosol monitor (RAM) to identify aerosol leak locations around the enclosure, and smoke tubes and a velometer to evaluate air movement around the outside of the enclosure. Results of the tracer gas evaluation indicated that the control system was approximately 98% efficient at capturing tracer gas released near the spindle of the machining center. This result was not significantly different from 100% efficiency (p = 0.2). The measured SF6 concentration when released directly into the duct had a relative standard deviation of 2.2%; whereas, when releasing SF6 at the spindle, the concentration had a significantly higher relative standard deviation of 7.8% (p = 0.016). This increased variability could be due to a cyclic leakage at a small gap between the upper and lower portion of the enclosure or due to cyclic stagnation. Leakage also was observed with smoke tubes, a velometer, and an aerosol photometer. The tool and fluid motion combined to induce a periodic airflow in and out of the enclosure. These results suggest that tracer gas methods could be used to evaluate enclosure efficiency. However, smoke tubes and aerosol instrumentation such as optical particle counters or aerosol photometers also need to be used to locate leakage from enclosures.     

Study of respirator effect on nasal-oral flow partition

Factors affecting worker tolerance of respiratory personal protective devices are inadequately understood. This study evaluates whether respirator-type loads affected the switch from nasal to oral breathing. Eleven healthy subjects were studied under progressive exercise conditions, using a respirator full-face mask with inspiratory resistance (I), pressure breathing (P) (10 cm H₂O end-expiratory pressure), or no load (N). A rapid-response thermistor was used to determine whether flow was predominantly oral or nasal. Both P and I increased the percentage of time that breathing was predominantly oral. The effect was most pronounced at higher exercise levels. The percentage of mouth breathing appeared to be closely related to the expiratory time. This study suggests that nasal-oral flow partitioning should be considered as a possible determinant of respirator tolerance. Am. J. Ind. Med. 32:408–412, 1997. © 1997 Wiley-Liss, Inc.     

Performance of a novel real-time respirator seal integrity monitor on firefighters: Simulated workplace pilot study

Millions of workers, including firefighters, use respiratory protective device. The key aspect in assuring the intended protection level of a respirator is its fit. However, even if the respirator originally fits well, the faceseal may be breached during its use. Until now, there have been no practically viable, inexpensive means to monitor the performance of a respirator during actual use. A novel Respirator Seal Integrity Monitor (ReSIM) was developed and recently evaluated on manikins by our team. The objective of this study was to evaluate the ReSIM effectiveness on respirator-wearing firefighters exposed to aerosols while performing simulated routine operational activities. Initially, 15 subjects were recruited for the study. Following a preliminary investigation that resulted in modifications in the ReSIM prototype and testing protocol, a subset of nine firefighters was chosen for a full-scale evaluation. The testing was conducted in a 24.3-m³ exposure chamber using NaCl as the challenge aerosol. Controlled faceseal leaks were established by opening a solenoid valve for 10, 15, or 20?sec. Leaks were also established as the tested firefighter slightly repositioned the respirator on his/her face. During the testing, the ReSIM measured particles inside a full-face elastomeric respirator with a 72.7% leak detection sensitivity (probability of correct leak identification) and an 84.2% specificity (probability of correct identification of the intervals which are absent of any leak). After adjusting for false negatives and persistent false positives, sensitivity and specificity increased to 83.6% and 92.2%, respectively. The factors causing minor limitations in leak detection sensitivity and specificity can be attributed to variability among subjects, moisture’s effect on the particle sensor, and some in-mask sampling bias. In conclusion, the ReSIM can promptly detect the breach in a respirator faceseal with high sensitivity and specificity. Due to its capability to alert the wearer of possible overexposure to hazardous aerosols, the ReSIM concept has a remarkable potential to be applied in various working environments, where respirators are used.     

Containment testing of laboratory hoods in the as-used condition

Airborne contaminants generated inside laboratory fume hoods during use leak into the breathing zone of the user. Concentration of the leakage is unknown and variable depending on laboratory design, work practices, arrangement of internal apparatus, face velocity, and sash height. Surrogate tracer gas tests have been developed using sulfur hexafluoride (SF6) and a manikin to estimate leakage. This study presents results of hood leakage tests using SF6 with a manikin and then a live operator performing a phenol:chloroform (P:C) extraction. Four hoods were tested in each of three institutions during normal work hours with the lab occupied. The purpose of the study was to determine leakage concentrations for the SF6-manikin with effects of sash height, hood contents as found and after being cleaned out, face velocity, and the actual P:C and SF6 exposure concentrations of the user during work. Results indicate P:C was not detectable in the breathing zone of the 12 operators (< 0.1 ppm) at their selected operating sash heights (7 to 15 inches). Simultaneous SF6 concentrations were also minimal (average 0.06 ppm). Average leakage was 0.02 percent for SF6 and less than 2 percent based on chloroform concentrations measured in the breathing zone of the operator and inside the hood. SF6 percent leakage was greater when sash height was above the breathing zone of the manikin (average 2.09 percent) and lower leakage (average 0.02 percent) when below the breathing zone (26 inches or less). Average face velocity did not appear to be a predictor of average hood leakage. Cleaning out the hoods did not reduce leakage in most tests. The data from this study shows that when providing training on proper work practices for lab hood use, lowering the sash should be stressed as being the major factor in reducing hood leakage.