Airborne Fiber Concentrations Research Articles

Estimation of Historical and Current Employee Exposure to Refractory Ceramic Fibers during Manufacturing and Related Operations

Abstract Refractory ceramic fibers (RCFs) have been produced in the United States since the early 1950s for use as a high temperature insulation material, or in the production of specialty products for insulation. Environmental data collected through 1991 have been assembled and used to reconstruct exposures at jobs in RCF manufacture or related operations. The algorithm incorporated consideration of the statistical distribution of the data, dates of implementation of engineering or process modifications potentially associated with a change in airborne fiber concentration, and the area in the facility (dust zone) where the sample was collected. Analysis of variance was used to test if differences between time periods within a dust zone were statistically significant. Time period was considered a fixed effect; the days of sample collection within a time period were considered a random, nested effect. Exposure estimates were constructed for 81 job tides in seven facilities for specified time periods. Overal...

Airborne fibre concentrations and lung burden compared to the tumour response in rats and humans exposed to asbestos

The excess risk of tumours exposed to asbestos were previously compared with the results of rat inhalation experiments. It could be demonstrated that humans at the workplace suffer from a tumour risk at fibre concentrations which are 300 times lower than those needed in the rat inhalation model to produce the same risk. However, the estimation of human risk was based on the study of workers at a chrysotile textile factory, whereas animal experimental results were related to exposure to amphiboles. Since for this comparison the risk of cancer due to exposure to amosite or crocidolite fibres at the workplace is of interest, quantitative exposure-response relationships for lung cancer and mesothelioma for the white workforce of South African amosite and crocidolite mines were discussed. On comparing the risk of lung cancer in this study with the risk of lung cancer for chrysotile textile workers, it can be concluded, that the risk of lung cancer and mesothelioma from crocidolite and amosite was higher than in the chrysotile textile factory.It could be also demonstrated, on the basis of a study of the lung burden of mesothelioma cases and of controls, that a significantly increased odds ratio of about 5 was established at amphibole concentrations of between 0.1 and 0.2 f μg−1 dry lung (WHO fibres longer than 5 μm from TEM analysis). On the other hand, carcinogenic response was observed at a fibre concentration 6000 times higher in animal inhalation experiments with crocidolite asbestos (SEM analysis of WHO fibres). As a result of these findings, it has been concluded that inhalation studies in rats are not sufficiently sensitive for the detection of hazards and risks to humans exposed to man-made fibres.

Airborne Fibre Concentrations and Lung Burden Compared to the Tumour Response in Rats and Humans Exposed to Asbestos

The excess risk of tumours exposed to asbestos were previously compared with the results of rat inhalation experiments. It could be demonstrated that humans at the workplace suffer from a tumour risk at fibre concentrations which are 300 times lower than those needed in the rat inhalation model to produce the same risk. However, the estimation of human risk was based on the study of workers at a chrysotile textile factory, whereas animal experimental results were related to exposure to amphiboles. Since for this comparison the risk of cancer due to exposure to amosite or crocidolite fibres at the workplace is of interest, quantitative exposure-response relationships for lung cancer and mesothelioma for the white workforce of South African amosite and crocidolite mines were discussed. On comparing the risk of lung cancer in this study with the risk of lung cancer for chrysotile textile workers, it can be concluded, that the risk of lung cancer and mesothelioma from crocidolite and amosite was higher than in the chrysotile textile factory. It could be also demonstrated, on the basis of a study of the lung burden of mesothelioma cases and of controls, that a significantly increased odds ratio of about 5 was established at amphibole concentrations of between 0.1 and 0.2 f micrograms-1 dry lung (WHO fibres longer than 5 microns from TEM analysis). On the other hand, carcinogenic response was observed at a fibre concentration 6000 times higher in animal inhalation experiments with crocidolite asbestos (SEM analysis of WHO fibres). As a result of these findings, it has been concluded that inhalation studies in rats are not sufficiently sensitive for the detection of hazards and risks to humans exposed to man-made fibres.

Physical characterization of MMVF for risk assessment

Size and other physical properties of MMVF play a central role in risk assessment. They can be affected by, for example, conditions during manufacturing, and preparation prior to administration to experimental animals or cells. It is suggested that a necessary requirement for stating that an experiment has been repeated should be documented evidence that: (i) the administered fibres are alike when analysed with not less than a minimum number of well-established methods on single fibre and on bulk sample level; and (ii) doses must be equal, and the fibre concentrations and sizes at the primary target tissue must be documented. Assessment of fibre toxicity and assessment of potential for exposure are equally important in the risk assessment process. The physics of fibrous dust release is described and methods for a priori assessment of exposure are classified into three levels depending on the analytical effort involved: (1) direct analysis of the fibrous bulk material; (2) bench-scale test; and (3) full-scale tests of prefabricated insulation material in a test room during standardized insulation work. For the first method, two approaches are investigated. One relates the size-dependent measures (the mass of fibres with D < 3 μm, the total length of fibres with D < 3 μm and the total number of WHO fibres per unit mass of fibres) and the other considers fractions (the fraction of total fibre length with D < 3 μm, the fraction of WHO fibre number, and the fraction of fibre number with D < 3 μm). The values of the size-dependent measures that are quoted span 2–4 orders of magnitude for the ranges of nominal diameter and GSD(D) considered, and show strong and similar dependence on GSD(D). The second group of measures, based on fractions only range up to 2 orders of magnitude and are almost independent on GSD(D) for nominal diameters ranging 2–4 μm. The combined effect on exposure of bulk fibre size, and of addition of oil and binder, should be assessed by the rotating drum dustiness test. The gold standard test of prefabricated MMVF products should be a full-scale simulation. The slope of the curve relating overall average product nominal diameter and airborne fibre concentrations on a log-log scale [Esmen et al., Am. ind. Hyg. Ass. J.40, 108–117 (1979)]has been confirmed on a qualitative basis using model calculations, so that this relation can be used as a first estimate of changes in exposure due to changes in nominal diameter of bulk material.

Analytical Limits of Asbestos Fiber Detection

Abstract Asbestos fiber deposition upon a filter can be statistically described by a Poisson frequency distribution. Poisson distributions can be developed to model gross, net, and blank fiber depositions. A statistical approach is developed to calculate the analytical limit of detection (ALOD). The ALOD, expressed as fibers/square millimeter, represents the true total sample fiber surface deposition that can be differentiated from a true blank filter at a 95 percent statistical confidence level. Based upon the ALOD concept, an environmental limit of detection (ELOD) is presented that defines the airborne fiber concentrations that are statistically discernible above background at a 95 percent confidence level. The ELOD is expressed in units of fibers/cubic centimeter and depends upon both the volume sampled and the characteristics of the microscope used for analysis. This work demonstrates the importance of collecting and analyzing a sufficient number of field blanks, as they determine the method detectio...

A Comparison of Airborne Man-Made Vitreous Fiber Concentrations Before and After Installation of Insulation in New Construction Housing

Abstract Airborne fiber concentrations were measured before and after installation of man-made vitreous fiber (MMVF) insulation products in 14 homes. Gravimetric, phase contrast microscopic (PCM), and scanning electron microscopic (SEM) methods were used to measure total dust, fiber, and MMVF concentrations, respectively. Preinstallation and postinstallation total dust concentrations, ranging from 0.01 to 1.17 and 0.03 to 0.96 mg/m3, respectively, were not statistically different. PCM analysis of preinstallation fiber concentrations calculated from the use of the National Institute for Occupational Safety and Health (NIOSH) Method 7400 A rules ranged from 0.002 to 0.011 fibers/cm3 (f/cm3); postinstallation concentrations ranged from 0.003 to 0.015 f/cm3. Fiber concentrations calculated from the use of the NIOSH 7400 Method B rules were slightly lower. Comparison of preinstallation and postinstallation concentrations using a one-sided, paired t-test indicated that there was no statistically significant difference for fiber concentrations calculated by either A or B rules. Airborne MMVF concentrations determined using SEM were all <0.01 f/cm3. Eleven of 14 preinstallation samples and 7 of 14 postinstallation samples were below the analytical limit of quantification (LOQ). SEM data are difficult to interpret because of the large number of samples <LOQ. Initially airborne fiber concentrations resulting from fiber counts <LOQ were calculated in the traditional manner by substituting 5.5 fibers for the actual number of fibers counted. This is termed the “LOQ-substituted” concentration. Nonparametric statistical analyses indicate that the postinstallation LOQ-substituted SEM airborne MMVF concentrations were significantly elevated as a result of insulation activities. Comparison of the pooled preinstallation and postinstallation concentrations, however, indicates that in both cases the concentration is low and the difference is slight. All airborne fiber concentrations detected and determined by the two methods used in this study were two to three orders of magnitude below occupational exposure limits proposed by the Occupational Safety and Health Administration and those recommended in industry guidelines. Miller, M.E.; Lees, P.S.J.; Breysse, P.N.: A Comparison of Airborne Man-Made Vitreous Fiber Concentrations Before and After Installation of Insulation in New Construction Housing. Appl. Occup. Environ. Hyg. 10(3): 182–187; 1995.

Bromo-deoxyuridine (BRDU) uptake in the lungs of rats inhaling amosite asbestos or vitreous fibres at equal airborne fibre concentrations.

Rats were exposed, by inhalation, to target airborne fibre concentrations of 1000 f/ml (PCOM fibres by WHO criteria) of a long amosite asbestos sample and a vitreous fibre sample; the target was closely attained for both fibre samples. The size distributions of the two fibre samples was closely similar. Rats were placed in the chambers for 7 hours and then, following a further 16 hours in room air, were injected with bromo-deoxyuridine (BRDU). The presence of BRDU-positive cells in terminal bronchioles/alveolar ducts was assessed in blocks taken from various parts of the left lung, from apex to base. There were significant differences in the proliferative responses between animals but there were also significant differences between the treatments. Lungs from rats exposed to vitreous fibres showed no greater response than the controls, but there was a markedly greater proliferative response in the lungs of rats inhaling long amosite. There was a decreasing gradient of proliferative response from the apex of the lung to the base with all treatments. This could be explained by different degrees of deposition in different areas of the lung. Similar amounts of fibre accumulated in the lungs of rats exposed to the two fibre types and it is unlikely that dissolution could be important over the timescale used here. We conclude that, when amosite asbestos deposits in the lungs of rats it stimulates a proliferative response and that deposition of an equal number of similar-sized vitreous fibres has no effect.

Glass fiber manufacturing and fiber safety: the producer's perspective.

Historically, the potential health effects of airborne fibers have been associated with the dose, dimension, and durability. Increasing focus is being placed on the latter category. Concern about airborne fiber safety could be reduced by manufacturing fibers that are not respirable; however, due to performance and manufacturing constraints on glasswool insulations, this is not possible today. These products are an important part of today's economy and as a major manufacturer, Owens-Corning is committed to producing and marketing materials that are both safe and effective in their intended use. To this end, manufacturing technology seeks to produce materials that generate low concentrations of airborne fibers, thus minimizing exposure and irritation. The range of fiber diameters is controlled to assure effective product performance and, as far as possible, to minimize respirability. Glass compositions are designed to allow effective fiber forming and ultimate product function. Fiber dissolution is primarily a function of composition; this too, can be controlled within certain constraints. Coupled with these broad parameters is an extensive product stewardship program to assure the safety of these materials. This article will discuss the factors that influence glasswool insulation production, use, and safety.

Comparison of Four Methods for the Determination of Asbestos Fiber Concentrations in Workplace Atmospheres by Phase Contrast Microscopy

Abstract To study the comparability of results of airborne fiber concentrations with field samples using the phase contrast optical microscopy methods [National Institute for Occupational Safety and Health (NIOSH) 7400, International Standards Organization (ISO), modified P&CAM 239, and Institut de Recherche en Santé et en Sécurité du Travail (IRSST) 243], samples were collected in asbestos mill, asbestos cement, and asbestos textile workplaces. These samples were counted by 12 experienced counters and the results compared between counts on the same sample for NIOSH Method 7400, ISO, and IRSST Method 243, and between counts of paired samples for all four methods. All statistical significance tests were performed at the 5 percent probability level (size of test). The geometric averages of the results in field samples of individual observations with their geometric standard deviations were in the following order: the modified P&CAM Method 239: 61 percent (1.19) larger, NIOSH Method 7400: 28 percent (1.23) larger, and ISO: 26 percent (1.23) lower than the IRSST Method 243 method. These results indicate that to improve comparability of results, an international consensus should be reached and maintained in all the details of the phase contrast optical microscopy method for the determination of fiber concentration in workplace atmospheres.

End User Exposures to Man-Made Vitreous Fibers: I. Installation of Residential Insulation Products

Abstract An investigation was undertaken to develop a comprehensive characterization of end user exposures to residential man-made vitreous fiber insulation products in terms useful for future study of potential health outcomes. Nearly 1200 air samples were collected and analyzed gravimetrically or by phase contrast microscopy or scanning electron microscopy to describe worker exposure in eight homogeneous exposure groups defined by man-made vitreous fiber product and occupation. These samples represent the exposure of 99 different workers insulating 107 different houses in 11 states in the eastern and central United States. Gravimetric and fiber count exposure concentrations are reported in terms of task length and full shift time-weighted averages (TWAs). Results of this study indicate mean task length airborne fiber concentrations determined by phase contrast microscopy (PCM) using the National Institute for Occupational Safety and Health (NIOSH) Method 7400 “B” rules to be less than 1.0 fibers/cm3 for all homogeneous exposure groups except installers and feeders of loose fiberglass without binder (7.67 and 1.74 f/cm3, respectively) and installers of loose mineral wool (1.94 f/cm3). PCM fiber exposure estimates using “A” counting rules, although slightly higher, provided generally similar results. Because residential insulators actively install insulation for only 1 to 4 hours on a typical day, comparable mean 8-hour TWA airborne fiber concentrations determined by PCM using NIOSH Method 7400 “B” rules were less than 0.3 f/cm3 for all homogeneous exposure groups except installers and feeders of loose fiberglass without binder (1.96 and 0.85 f/cm3, respectively) and installers of loose mineral wool (0.97 f/cm3). Because of the presence of other fibers, man-made vitreous fiber exposure estimates resulting from scanning electron microscopic analysis of samples using NIOSH counting rules were slightly less than corresponding PCM estimates for every homogeneous exposure group except installers and feeders of loose fiberglass without binder. The higher exposure estimates for these homogeneous exposure groups resulted from the presence of a significant number of thin fibers that were below the limit of resolution of PCM. Fiber size distributions determined by scanning electron microscopy showed geometric mean fiber diameters to range from approximately 1 to 2 μm for all homogeneous exposure groups except installers and feeders of loose fiberglass without binder that both had geometric mean fiber diameters of 0.6 μm. Lees, P.S.J.; Breysse, P.N.; McArthur, B.R.; Miller, M.E.; Rooney, B.C.; Robbins, C.A.; Corn, M.: End User Exposure to Man-Made Vitreous Fibers: I. Installation of Residential Insulation Products. Appl. Occup. Environ. Hyg. 8(12):1022–1030; 1993.

AIRBORNE GLASS FIBER CONCENTRATIONS DURING MANUFACTURING OPERATIONS INVOLVING GLASS WOOL INSULATION

In order to better characterize airborne fiber concentrations arising in manufacturing operations that use glass wool insulation, and to determine the proportion of these fibers that are glass fibers, airborne fiber concentrations were measured in a number of different operations involving Owens-Corning Fiberglas insulation products. The operations sampled included those that fabricate or assemble metal building insulation, manufactured housing, pipe insulation, kitchen ranges, air-handling ducts, and water heaters. Some operations in which pipe insulation and ceiling boards were removed and discarded were also measured. Sample collection and fiber-counting procedures followed National Institute for Occupational Safety and Health Method 7400 procedures (phase contrast light microscopy), with some modifications to allow identification of the fiber type. The arithmetic mean concentration of total airborne fibers during 11 different manufacturing operations ranged from 0.02–0.2 fibers/cm3, of which typically half or less were identified as glass fibers. The mean concentrations of glass fibers ranged from 0.002–0.14 fibers/cm3, whereas respirable glass fibers were from 0.001–0.07 fibers/cm3. For the removal of pipe insulation and ceiling boards, the mean concentration of total airborne fibers was 0.29 fibers/cm3 (95% confidence limits 0.20–0.41 fibers/cm3), whereas the mean respirable airborne fiber concentration was only 0.042 fibers/cm3 (0.023–0.059).

Airborne Glass Fiber Concentrations During Manufacturing Operations Involving Glass Wool Insulation

In order to better characterize airborne fiber concentrations arising in manufacturing operations that use glass wool insulation, and to determine the proportion of these fibers that are glass fibers, airborne fiber concentrations were measured in a number of different operations involving Owens-Corning Fiberglas insulation products. The operations sampled included those that fabricate or assemble metal building insulation, manufactured housing, pipe insulation, kitchen ranges, air-handling ducts, and water heaters. Some operations in which pipe insulation and ceiling boards were removed and discarded were also measured. Sample collection and fiber-counting procedures followed National Institute for Occupational Safety and Health Method 7400 procedures (phase contrast light microscopy), with some modifications to allow identification of the fiber type. The arithmetic mean concentration of total airborne fibers during 11 different manufacturing operations ranged from 0.02–0.2 fibers/cm3, of which typically half or less were identified as glass fibers. The mean concentrations of glass fibers ranged from 0.002–0.14 fibers/cm3, whereas respirable glass fibers were from 0.001–0.07 fibers/cm3. For the removal of pipe insulation and ceiling boards, the mean concentration of total airborne fibers was 0.29 fibers/cm3 (95% confidence limits 0.20–0.41 fibers/cm3), whereas the mean respirable airborne fiber concentration was only 0.042 fibers/cm3 (0.023–0.059).

11 O 01 Dilution sampler for evaluation of airborne fibre concentrations in asbetos stripping enclosures

11 O 01 Dilution sampler for evaluation of airborne fibre concentrations in asbetos stripping enclosures

Airborne Glass Fiber Concentrations during Installation of Residential Insulation

In an effort to better characterize airborne fiber levels associated with the installation of residential insulation and to determine the proportion of airborne fibers that are glass fibers, airborne fiber concentrations were measured during the installation of several Owens-Corning Fiberglas insulation products. Sample collection and fiber counting procedures followed National Institute for Occupational Safety and Health Method 7400 with some modifications to allow identification of the fiber type. The arithmetic mean concentration of total airborne fibers during installation of batt-type insulation was 0.22 fibers per cubic centimeter (f/cc) (95% confidence limits of 0.18–0.27 f/cc). Significantly, approximately 60% of these total fibers were glass fibers and approximately 20% were respirable glass fibers. For applications of blowing wool, the total airborne fiber concentrations were higher, with means of 1.0 f/cc (0.9–1.1) or 2.1 f/cc (15–2.7), depending on the product type. Glass fibers were 0.7 f/cc (0.6–0.8) or 1.7 f/cc (1.2–2.5) and respirable glass fibers were 0.3 f/cc (02–0.4) or 0.8 f/cc (05–1.3). Similar to the batt installation results, only 20–40% of the total airborne fibers were respirable glass fibers. The night after installation of the insulation was complete, the airborne fiber concentrations dropped to the levels measured before the insulation work began.

AIRBORNE GLASS FIBER CONCENTRATIONS DURING INSTALLATION OF RESIDENTIAL INSULATION

In an effort to better characterize airborne fiber levels associated with the installation of residential insulation and to determine the proportion of airborne fibers that are glass fibers, airborne fiber concentrations were measured during the installation of several Owens-Corning Fiberglas insulation products. Sample collection and fiber counting procedures followed National Institute for Occupational Safety and Health Method 7400 with some modifications to allow identification of the fiber type. The arithmetic mean concentration of total airborne fibers during installation of batt-type insulation was 0.22 fibers per cubic centimeter (f/cc) (95% confidence limits of 0.18–0.27 f/cc). Significantly, approximately 60% of these total fibers were glass fibers and approximately 20% were respirable glass fibers. For applications of blowing wool, the total airborne fiber concentrations were higher, with means of 1.0 f/cc (0.9–1.1) or 2.1 f/cc (15–2.7), depending on the product type. Glass fibers were 0.7 f/cc ...

Sampling and Analysis of Mineral Fibers on Construction Sites

Abstract Different organizations have expressed the need for more recent data on concentration determinations and the detailed characterization of airborne man-made mineral fibers (MMMF) at the users' sites. In this context, the concentrations and characterization of airborne MMMF were conducted using phase contrast optical microscopy and transmission electron microscopy (TEM) at construction sites where workers were installing or removing insulant materials. The size and composition of fibers were determined by TEM both in bulk materials and in the airborne samples. At five construction sites, ambient concentrations of airborne fibers were shown to increase from [glass wool] < [rock wool] < [ceramic fiber (industrial oven)] as a result of both the nature of the materials and the confinement of the sites. The characterization of the fibers in the bulk insulation materials is not a good representation of the sizes and elemental composition of the airborne fibers.

Effective Collection Area: A Potential Error in Asbestos Air Monitoring

Abstract The current National Institute for Occupational Safety and Health method to measure airborne fiber concentration has one of the highest variability levels of the analytical techniques used by industrial hygienists. It is common practice to assume that the effective collection area (ECA) of a 25-mm filter is 385 mm2. However, measurements of the inner diameter of filter cowls cast doubt on this assumption. Thus, a laboratory study was conducted to characterize the ECA of nine cassette types. The effective filter areas were blackened by sampling a carbon black aerosol. Computer image files were created from photocopies of the filters by means of a scanner at a resolution of 600 dots/inch, and the numbers of black pixels in the image files were determined. Areas were calculated by comparing the numbers of black pixels in filter images with the numbers in the images of circular area standards. Three cassette brands were found to be significantly larger than 385 mm2. Assuming that the number of fibers...

ANALYSIS OF AMPHIBOLE ASBESTOS IN CHRYSOTILE AND OTHER MINERALS

Chrysotile asbestos and many other mineral raw materials contain amphibole minerals which may be asbestiform. There is currently no analytical method which will detect the presence of amphibole at sufficiently low limits to preclude the possibility of inadvertent exposure of persons handling these materials to hazardous airborne fibre concentrations. A method of chemical digestion of chrysotiles has been tested with regard to the determination of their tremolite contaminant content and this has been applied to a range of chrysotile and other minerals. The method improves the sensitivity of the amphibole analysis at least 10-fold giving detection limits of 0.01-0.05% in chrysotile by X-ray diffractometry (XRD). The difficulties arising from compositional and morphological variations are discussed in the context of the potential hazards from airborne fibres and the relative values of analyses by XRD, infrared spectrophotometry (IR) and electron microscopy. It is concluded that XRD and IR are useful as screening methods for the detection of amphibole in chrysotile but other materials should be analysed by optical or electron microscopy.

Natural history of pleural thickening after exposure to crocidolite.

Serial plain chest radiographs of 384 men who worked at the Wittenoom crocidolite mine and mill between 1943 and 1966 and who applied for pneumoconiosis compensation between 1948 and 1982 have been examined independently by two trained observers for pleural disease using the 1980 ILO-UICC classification of radiographs to record width and extent of pleural disease. Radiographs covering follow up periods of from two to 38 years were examined (median number of films per subject was nine). The degree of crocidolite exposure was estimated from employment records and a survey of airborne fibre concentrations performed in 1966. Agreement between the observers on the presence and degree of pleural disease in the final film for each subject was moderately close (Kendall's tau B = 0.62) and was least for subjects with thickening less than 5 mm in width. Diffuse pleural thickening extending for greater than 50% of the lateral chest wall was the most common type recorded by both observers. Minor pleural thickening frequently progressed in extent along the lateral chest wall but progression beyond 5 mm in thickness was less common. Pleural plaques were not seen to progress beyond their initial thickness or extent. The rate of onset of thickening in this population increased continually from the time of first exposure and also increased slightly with age. There was evidence that the level of total cumulative exposure to crocidolite increased the rate of onset of pleural thickening in the period between five and 15 years after first exposure. Rate of progression of established thickening was greatest in subjects who first developed thickening early after first exposure. The relative rate of progression decreased slowly with time from first signs of thickening and there was no evidence of any progression more than 15 years after onset.

Comparison of the Fibrous Aerosol Monitor and the Optical Fiber Count Technique for Asbestos Measurement

Abstract The Fibrous Aerosol Monitor (Model FAM-1) was developed by GCA Corporation with joint funding from the Bureau of Mines and NIOSH. Although designed to “count” all fibers, the FAM is especially useful as a direct reading instrument for asbestos exposures. The purpose of this study was to compare airborne fiber concentrations determined by use of the FAM-1 with concentrations determined by the use of the standard optical membrane filter count technique. A 25-mm membrane filter cassette was placed directly downstream of the sensing tube at the exit port of the FAM and subsequently was evaluated using the NIOSH 7400 “B” counting rules. Thus, the same air sample could be evaluated by both the instrument and the optical count technique. Under laboratory conditions, 33 paired measurements were made at levels of approximately 0.1, 0.2, 0.5, 2, 5, and 7 fibers per cubic centimeter (f/cc). Comparison of the data (both raw and transformed) by linear regression analysis revealed a good correlation between th...