Experimental examination of factors that affect dust generation by using Heubach and MRI testers.

Four factors that affect dust generation were investigated--type of test material, particle size distribution of the test material, moisture content of the test material, and apparatus used to generate dust. Dust generated from silicon carbide and aluminum oxide was measured by using MRI and Heubach dustiness testers modified to allow the measurement of dust particle size distribution with an Andersen impactor. The two materials investigated generated similar dusts. The size distribution of the test material slightly influenced the amount but strongly influenced the size distribution of the dust generated. Increased moisture content decreased the amount of dust generated; moisture content had little influence on dust size distribution. The two testers generated different amounts of dust; however, the dust particle size distributions generated were similar. These results help explain factors that affect dust generation and the relative importance of alternative methods for dust control.     

An investigation of dust generation by free falling powders.

To identify the dust generation processes, aluminum oxide powder was dropped as a free falling slug in a test chamber. The effect of the slug's mass, diameter, and drop height upon the aerosol concentration and size distribution was measured with an aerodynamic particle sizer. To differentiate between aerosol generated during the free fall and at the end of the fall, the slug was dropped either onto a flat surface or into a container of water that suppressed dust generation associated with the impact at the end of the fall. Aerosol generation occurred during the slug's free fall as well as at the end of the fall. The falling solid induced an airflow that followed the falling solid to the end of the fall. This induced airflow contained the aerosol generated during the free fall. At the end of the free fall, the induced airflow, combined with air jets created on impact, dispersed the aerosol throughout the test chamber. Additional measurements were made by using "neutral buoyancy" helium-filled bubbles to visualize the airflow in the test chamber. The airflow and ensuing turbulence were sufficient to keep large, inspirable particles suspended throughout the test chamber for periods greater than 10 min. During experimental work, the effect of drop height, mass, and slug diameter upon aerosol generation by a single slug of powder was studied. The results indicated that the manner in which a powder is handled may be as important as material dustiness as measured by a dustiness tester. Aerosol generation can be reduced by minimizing the contact between the falling powder and the air.     

Factors affecting the Heubach and MRI dustiness tests

The effect of test parameters upon material dustiness measured by the Heubach dust measurement appliance and the MRI dustiness tester was studied. The users of these tests can alter test parameters such as flow rate, sampling time, mass of material tested, bulk density, and vibrator setting. The effect of these parameters upon the aerosol produced in the dustiness tester was experimentally studied. All of the parameters affected in a complicated manner, the amount of dust and the size distribution of the dust generated during these tests. Therefore, dustiness test results should not be adjusted for variations in test parameters. The users of dustiness tests need to carefully control dustiness test parameters in order to have reproducible dustiness tests.     

More on the dynamics of dust generation: the effects of mixing and sanding chrysotile, calcium carbonate, and other components on the characteristics of joint-compound dusts

An ongoing research effort designed to reconstruct the character of historical exposures associated with use of chrysotile-containing joint compounds naturally raised questions concerning how the character (e.g. particle size distributions) of dusts generated from use of recreated materials compares to dusts from similar materials manufactured historically. This also provided an opportunity to further explore the relative degree that the characteristics of dusts generated from a bulk material are mediated by the properties of the bulk material versus the mechanical processes applied to the bulk material by which the dust is generated. In the current study, the characteristics of dusts generated from a recreated ready mix and recreated dry mix were compared to each other, to dusts from a historical dry mix, and to dusts from the commercial chrysotile fiber (JM 7RF3) used in the recreated materials. The effect of sanding on the character of dusts generated from these materials was also explored. Dusts from the dry materials studied were generated and captured for analysis in a dust generator-elutriator. The recreated and historical joint compounds were also prepared, applied to drywall, and sanded inside sealed bags so that the particles produced from sanding could be introduced into the elutriator and captured for analysis. Comparisons of fiber size distributions in dusts from these materials suggest that dust from commercial fiber is different from dusts generated from the joint compounds, which are mixtures, and the differences persist whether the materials are sanded or not. Differences were also observed between sanded recreated ready mix and either the recreated dry mix or a historical dry mix, again whether sanded or not. In all cases, however, such differences disappeared when variances obtained from surrogate data were used to better represent the 'irreducible variation' of these materials. Even using the smaller study-specific variances, no differences were observed between the recreated dry mix and the historical dry mix, indicating that chrysotile-containing joint compounds can be recreated using historical formulations such that the characteristics of the modern material reasonably mimic those of a corresponding historical material. Similarly, no significant differences were observed between dusts from sanded and unsanded versions of similar materials, suggesting (as in previous studies) that the characteristics of asbestos-containing dusts are mediated primarily by the properties of the bulk material from which they are derived.     

Combined single-drop and rotating drum dustiness test of fine to nanosize powders using a small drum

A dustiness test has been developed that performs both a single-drop and a continuous rotation test using a 6-g sample. Tests were completed on pigment-grade and ultrafine TiO2, two grades of corundum (Aloxite), yttrium-stabilized zirconia (Y-zirconia) granules, fumed silica, goethite, talc and bentonite. The generated particles were quantified by counting and sizing at 1-s time resolution using the TSI Fast Mobility Particle Sizer and the TSI Aerodynamic Particle Sizer and by collecting the particles on a filter for weighing. The method generated reproducible amounts and size distributions of particles. The size distributions had two more or less separated size modes >0.9 microm and in addition all materials except TiO2 pigment-grade and Aloxite F1200 generated a size mode in the range from approximately 100 to approximately 220 nm. Pigment-grade TiO2 had the lowest dustiness and ultrafine TiO2 the highest dustiness as measured by particle number for both the single-drop and rotation test and as measured by mass for both tests combined. The difference was a factor of approximately 300. Three types of dust generation rate time profiles were observed; brief initial burst (talc, both grades of corundum), decaying rate during rotation period (fumed silica, TiO2 ultrafine and pigment grade, bentonite) and constant rate (Y-zirconia, goethite). These profile types were in agreement with the differences in the ratio of amount of particles generated during the single drop to the amount generated during the single-drop and rotation test combined. The ratio ranged a factor approximately 40. The new test method enables a characterization of dustiness with relevance to different user scenarios.     

Comparison of coarse coal dust sampling techniques in a laboratory-simulated longwall section

Airborne coal dust generated during mining can deposit and accumulate on mine surfaces, presenting a dust explosion hazard. When assessing dust hazard mitigation strategies for airborne dust reduction, sampling is done in high-velocity ventilation air, which is used to purge the mining face and gallery tunnel. In this environment, the sampler inlet velocity should be matched to the air stream velocity (isokinetic sampling) to prevent oversampling of coarse dust at low sampler-to-air velocity ratios. Low velocity ratios are often encountered when using low flow rate, personal sampling pumps commonly used in underground mines. In this study, with a goal of employing mine-ready equipment, a personal sampler was adapted for area sampling of coarse coal dust in high-velocity ventilation air. This was done by adapting an isokinetic nozzle to the inlet of an Institute of Occupational Medicine (Edinburgh, Scotland) sampling cassette (IOM). Collected dust masses were compared for the modified IOM isokinetic sampler (IOM-MOD), the IOM without the isokinetic nozzle, and a conventional dust sampling cassette without the cyclone on the inlet. All samplers were operated at a flow rate typical of personal sampling pumps: 2 L/min. To ensure differences between collected masses that could be attributed to sampler design and were not influenced by artifacts from dust concentration gradients, relatively uniform and repeatable dust concentrations were demonstrated in the sampling zone of the National Institute for Occupational Safety and Health experimental mine gallery. Consistent with isokinetic theory, greater differences between isokinetic and non-isokinetic sampled masses were found for larger dust volume-size distributions and higher ventilation air velocities. Since isokinetic sampling is conventionally used to determine total dust concentration, and isokinetic sampling made a difference in collected masses, the results suggest when sampling for coarse coal dust the IOM-MOD may improve airborne coarse dust assessments over “off-the-shelf” sampling cassettes.     

The effectiveness of handheld ventilated sanders in reducing inhalable dust concentrations

Ventilated sanders are commonly used during aircraft surface abrasion but there is limited data on their effectiveness in reducing worker exposures. This study compared two handheld ventilated sander brands, DCM and Dynabrade, in a laboratory glovebox. Both sanders collect particulates by drawing air through holes in the sanding pads; the dust subsequently passes into a vacuum collection system. Aluminum panels coated with aircraft epoxy primer and polyurethane paint were abraded and inhalable dust concentrations were measured inside the glovebox with IOM samplers. The results indicate that both sanders effectively control inhalable dust, with the DCM sander reducing mass concentrations by 93 percent, and the Dynabrade by 98 percent, when the ventilation system is used. The Dynabrade unit, however, was more aggressive and produced over four times as much dust per unit time as the DCM unit. In spite of this, the Dynabrade sander adequately collected this additional dust. Varying abrasive grit size did not significantly affect dust generation, although the differences between the grit sizes used (180 and 240 grit) were not great and may have influenced the results.     

Experimental study on effects of drilling parameters on respirable dust production during roof bolting operations

Underground coalmine roof bolting operators exhibit a continued risk for overexposure to airborne levels of respirable coal and crystalline silica dust from the roof drilling operation. Inhaling these dusts can cause coal worker's pneumoconiosis and silicosis. This research explores the effect of drilling control parameters, specifically drilling bite depth, on the reduction of respirable dust generated during the drilling process. Laboratory drilling experiments were conducted and results demonstrated the feasibility of this dust control approach. Both the weight and size distribution of the dust particles collected from drilling tests with different bite depths were analyzed. The results showed that the amount of total inhalable and respirable dust was inversely proportional to the drilling bite depth. Therefore, control of the drilling process to achieve proper high-bite depth for the rock can be an important approach to reducing the generation of harmful dust. Different from conventional passive engineering controls, such as mist drilling and ventilation approaches, this approach is proactive and can cut down the generation of respirable dust from the source. These findings can be used to develop an integrated drilling control algorithm to achieve the best drilling efficiency as well as reducing respirable dust and noise.     

A dust generator for laboratory use.

A dust generator has been developed to produce dry dust from powderous materials for instrument calibration and laboratory studies. The powder is fed at a constant rate via a chain conveyor into a fluidized bed where the particles are deagglomerated and aerosolized. The powder feed rate is raviable from 1.2 to 36 mm3/min and the air flow rate from 9 to 30 Lpm. Aerosols of coal, silica, potash, rock, and Arizona road dust have been successfully generated for calibrating optical particle counters and aerosol mass monitors.     

Successful experiences with a handy equipment to measure dust concentrations in various workplaces

Performance of a direct-reading handy equipment for dust counting in workroom air, PDR (MIE, USA), was field-tested in comparison with the results of low-volume air sampling-gravimetry. Application in 64 workplaces of various type dust work with dust concentrations of up to 1.6 mg/m3 showed that the agreement was generally good with a correlation coefficient (r) >0.64. The calculated regression line had a slope close to one, and the intercept on the axis was next to nil, suggesting that the measurement with PDR is essentially the same with that by low-volume air sampling. Further analysis by types of dust work disclosed that the correlation was best when examined in 12 foundries with r as large as 0.95. In contrast, r was small (0.52) in 11 welding workplaces, possibly due to smaller particle size of dust generated during welding.     

Examination of dust generated from a workplace where steel wire coil are unwound

Analyses of constituents, measurement of concentrations, research of size distributions and scanning electron microscopic examinations were carried out to determine the type and amount of dust generated in a working environment where steel wire coils are unwound. In about a 700 m2 work area unit, stationary samplings, including A-sampling (24 points) and B-sampling (1 point), which are defined by the law, were carried out. The dust concentration was measured by a digital dust indicator. A low volume air sampler was used to determine the conversion factor of the values obtained by the digital dust indicator. Because the result of the dust level by A-sampling and B-sampling were 0.64 mg/m3 and 0.82 mg/m3 respectively, they can both be classified as Control Class 1. A proportion of less than 7 microns of the airborne dust made up about 15 to 20% (volume %) of the total dust in the working environments. Metallic dust and white dust were identified in the accumulated dust. By measuring with a X-ray diffractometry, it was determined that the metal dust constituent was Zn and the white dust constituent was Zn4CO3(OH)6.H2O. By a scanning electron microscopic examination, the scope of the particles was proven to be non-fibrous.     

A study on dust emission, particle size distribution and formaldehyde concentration during machining of medium density fibreboard

A study to characterise the quantity, particle size distribution and morphology of dust created during the machining of MDF was carried out. Four different types of MDF boards were included in this study, including a 'zero-formaldehyde' board that contains isocyanate-based resin, rather than urea-formaldehyde resin. In addition, natural softwood (pine) and natural hardwood (oak) were included in the study, for comparison with MDF. The results show that in general, the dust generated by machining MDF is comparable in terms of particle size distribution and morphology with the dust generated by similarly machining hardwood or softwood. The quantity of dust generated during sanding is higher for sanding MDF compared with sanding either hardwood or softwood. However, for sawing there is no significant difference between MDF and natural woods, in terms of the quantity of dust generated. Free formaldehyde in the air was less than 0.17mg m(-3) during machining of the Class B (higher formaldehyde potential) MDF board. There was no measurable isocyanate in the dust generated from the boards.     

Chamber for testing asbestos-containing products: validation and testing of a re-created chrysotile-containing joint compound

Joint compound products containing chrysotile asbestos were commonly used for building construction from the late 1940s through the mid-1970s. Few relevant data exist to support reconstructing historical worker exposures to fibers generated by working with this material. Therefore, we re-created 1960s-era chrysotile-containing joint compound (JCC) and compared its characteristics to a current-day asbestos-free joint compound (JCN). Validation studies showed that a bench-scale chamber with controlled flow dynamics, designed to quantify particulate emissions from joint compound products, provided precise and reliable measurements of generated airborne dust mass, chrysotile fiber concentrations, and corresponding activity-specific emission rates. Subsequent chamber studies characterized fibers counted by phase contrast microscopy (PCM) per mass of respirable dusts and total suspended particulate dusts (total dusts), generated during JCC sanding or sweeping, as well as corresponding dust emission rates for JCC and JCN, and the ratio of total to respirable dust mass for JCN. From these data we estimated factors, F(CH-rd) and F(CH-td) (in units of f cm(-3) per mg m(-3)), by which respirable JCN dust mass concentrations collected during construction use can be converted to corresponding airborne PCM fiber concentrations generated by sanding or sweeping JCC. For sanding, median values (95% confidence limits) of F(CH-rd) and F(CH-td) were estimated to be 0.044 (0.039-0.050) and 0.212 (0.115-0.390) f cm(-3) per mg m(-3), respectively. The F(CH-td) to F(CH-rd) ratio indicates that approximately five times as many airborne PCM fibers are anticipated per unit air volume sampled when JCC dust is collected on cassettes (as done historically), than when respirable JCC dust is collected on cyclones. As the sizes of individual fibers collected appear to be primarily respirable, this difference may be a sampling artifact and suggests caution in interpreting historical fiber concentration measures made using cassettes during work with JCC-like materials. F(CH-rd) can be used with published and newly generated field measurements of respirable dust mass concentrations associated with the use of JCN or equivalent JCN materials to better characterize historical worker exposures to PCM fibers from use of JCC or equivalent JCCs. The experimental process described also can be used to develop conversion factors for other combinations of modern-day asbestos-free and historical chrysotile-containing products.     

空气净化系统回风口过滤装置贴附尼龙网的效果评价

目的探讨利用医用橡皮膏使用后废弃的尼龙网,对空气净化系统回风口过滤装置进行贴附的效果评价。方法对回风口滤过装置贴附上尼龙网前后手术室的风速、细菌浓度和尘埃粒子量测定。结果贴附尼龙网前后所测数值不影响空调洁净系统的功能、净化的效果。结论对空气净化系统回风口过滤装置贴附尼龙网比每周擦拭效果更明显,省时、省力、省能源。     

Assessment of measured respirable dust sampler penetration and the sampling convention for work environment measurement]

The relationship between dust size and penetration for a static horizontal elutriator (Sibata C-30) was measured in calm air. The elutriator as a low-volume air sampler is widely used as a dust size classifier in work environment measurements. The actual penetrations were compared with the theoretical models of the sampler and with sampling convention for respirable dust in work environment measurement. The sampling convention was recently introduced into the Japanese standard for work environment measurement and is based on the ISO 7708 respirable dust convention. The bias of sampled masses from the respirable dust was calculated for two flow rates of the sampler, i.e., 50% cut sizes of 4 microm and 5 microm, from measured penetration curves. The bias of the sampler was overestimated in the 5 microm, 50% cut condition and underestimated in the 4 microm, 50% cut condition for most workplace sampling situations.     

Differential pressure response of 25-mm-diameter glass fiber filters challenged with coal and limestone dust mixtures

This article summarizes results of research conducted by the National Institute for Occupational Safety and Health (NIOSH) at its Pittsburgh Research Laboratory. The objective of this work was to determine the correlation between the mass (M) of respirable coal and limestone dusts collected on 25-mm-diameter glass fiber filters mounted in cassettes and the increase in differential pressure (deltaP) that develops across the filters when drawing at constant air flow. Test aerosols were generated inside a laboratory dust chamber using various coal dusts, limestone dust, and mixes of the two. Dusts with different particle size distributions were deposited on the filters by sampling from the chamber through cyclone preclassifiers at different flow rates. Results show that the relationship between differential pressure increase (cm water) and dust mass (mg) is linear and can be approximated by the equation deltaP = KM. The K values (slopes) range from 1.14 to 1.64, depending on the parent coal of the samples. The influence of particle size distribution was also found. The overall K value for all the data summarized in this article is 1.35, with R2 = 0.84 for the summary equation. When calibrated for individual work sites, or other circumstances where great variability in dust characteristics is avoided, the relationship between collected dust mass and increase in differential pressure may provide an exploitable principle for measurement of respirable dust concentrations.     

Cellulosic building insulation versus mineral wool, fiberglass or perlite: installer's exposure by inhalation of fibers, dust, endotoxin and fire-retardant additives

A task-specific exposure matrix was designed for workers installing building insulation materials. A priori, a matrix element was defined by type of task (installer or helper), type of work area (attic spaces or wall cavities) and type of insulation material (slabs from mineral wool, fiberglass or flax; loose-fill cellulosic material or perlite). In the laboratory a mock-up (full scale) of a one-family house was used for simulated installation of insulation materials (four replicates per matrix element). Personal exposure to dust and fibers was measured. The dust was analyzed for content of endotoxin and some trace elements (boron and aluminum) from fire-retardant or mold-resistant additives. Fibers were characterized as WHO fibers or non-WHO fibers. In support of the exposure matrix, the dustiness of all the materials was measured in a rotating drum tester. For installers in attic spaces, risk of exposure was low for inhalation of dust and WHO fibers from slab materials of mineral wool or fiberglass. Slab materials from flax may cause high risk of exposure to endotoxin. The risk of exposure by inhalation of dust from loose-fill materials was high for installers in attic spaces and for some of the materials risk of exposure was high for boron and aluminum. Exposure by inhalation of cellulosic WHO fibers was high but little is known about the health effects and a risk assessment is not possible. For the insulation of walls, the risk of installers' exposure by inhalation of dust and fibers was low for the slab materials, while a high risk was observed for loose-fill materials. The exposure to WHO fibers was positively correlated to the dust exposure. A dust level of 6.1 mg/m3 was shown to be useful as a proxy for screening exposure to WHO fibers in excess of 10(6) fibers/m3. In the rotating drum, slabs of insulation material from mineral wool or fiberglass were tested as not dusty. Cellulosic loose-fill materials were tested as very dusty, and perlite proved to be extremely dusty.     

吸尘罩的优选及其在玉石雕刻防尘中的应用

目的对适用吸尘罩进行优选,并在实际通风除尘系统中应用。方法用示踪气体法对吸尘罩进行优选,比较不同吸尘罩捕集效率和风量的关系;对吸尘罩前风速分布进行测定,得出罩前风速分布模型。并对吸尘罩进行空气动力性能分析,测定雕刻排气中粉尘的粒径分级组成和除尘器的粉尘粒径分级除尘效率,评估除尘器的总除尘效率。结果优选出的雕刻吸尘罩,置于切割轮正前方0.11 m处,其控制风速为0.5 m/s,能有效地控制雕刻切割轮的粉尘不向周围扩散,工作地点空气粉尘浓度由无罩时的30-297 mg/m3降低至1.1-1.7 mg/m3,吸尘罩风量为255 m3/h,为其他类型吸尘罩的50%-77%。优选出的冲击水浴除尘器,其除尘效率为97%-98%,阻力为1.5 kPa;滤袋除尘器的效率为98%-99%,阻力为2.5 kPa;除尘后尾气的粉尘浓度为20-30 mg/m3。结论提出了优选吸尘罩的设计方法及实用的除尘器,可供实际应用。     

Use of fluidizing bed aerosol generators to establish a dust mixture of two substances at a fixed ratio for inhalation toxicity studies.

A method was developed to use two fluidizing bed generators to deliver a mixture of 1 mg cobalt + 15 mg tungsten carbide/m3 to an inhalation exposure chamber with the output from the cobalt generator split to provide the same cobalt concentration to a cobalt-only chamber. To provide a more uniform delivery of material and to minimize the amount of starting dust needed, a subsystem that produced timed bursts of compressed air was used to prevent the accumulation of dust along the aerosol transport tubes. The addition of an electrostatic precipitator placed in the exhaust lines reduced the amount of dust delivered to the high-efficiency particulate air filters, thereby reducing the number of filter changes.     

Comparison of coal mine dust size distributions and calibration standards for crystalline silica analysis

Since 1982 standard calibration materials recommended for respirable crystalline silica analysis by the Mine Safety and Health Administration (MSHA) P7 Infrared Method and the National Institute for Occupational Safety and Health (NIOSH) X-ray Diffraction (XRD) Analytical Method 7500 have undergone minor changes in size distribution. However, a critical assumption has been made that the crystalline silica in ambient mine atmosphere respirable dust samples has also remained essentially unchanged in particle size distribution. Therefore, this work compared recent particle size distributions of underground coal mine dust and the silica component of these dusts with estimated aerodynamic particle size distributions of calibration standard materials MIN-U-SIL 5, Berkeley 5, and SRM 1878 used by two crystalline silica analysis techniques. Dust impactor sampling data for various locations in 13 underground coal mines were analyzed for the respirable mass median aerodynamic diameters. The data suggest that the MSHA P7 method will underestimate the silica content of the sample by at most 7.4% in the median size range 0.9 to 3.6 microm, and that it is unlikely one would obtain any significant error in the MSHA P7 method analysis when the method uses Berkeley 5, MIN-U-SIL 5, or SRM 1878 as a calibration standard material. The results suggest that the NIOSH Analytical Method 7500 would be more appropriate for a dust sample that is representative of the total (no cyclone classifier) rather than the respirable airborne dust, particularly because the mass fraction in the size range below 4 microm is usually a small percentage of the total airborne dust mass. However, NIOSH Analytical Method 7500 is likely to underestimate the silica content of an airborne respirable dust sample by only 5 to 10%. The results of this study also suggest that any changes that may have occurred in the median respirable size of airborne coal mine dust are not significant enough to cause any appreciable error in the current methods used for respirable crystalline silica analysis.