Membrane Filter Method for Estimating Asbestos Fiber Exposure
The following excerpt is from Wylie, A. G, from the book "Membrane Filter Method for Estimating Asbestos Fiber Exposure," Definitions for Asbestos and Other Health-Related Silicates, published by Benjamin Levadie, Ed., American Society for Testing and Materials. It is on the subject of Estimating Asbestos Fiber Exposure.
Asbestos exposure is usually evaluated by the membrane filter method recommended by the National Institute for Occupational Safety and Health (NIOSH). The method for estimating fiber exposure was originally applied only to environments in which commercially mined asbestos was being used, but it has now been extended to the mining environment and to other environments in which natural materials containing small quantities of asbestos are found. It has been incorporated into state regulations and procedures and adopted by the industrial hygiene community as an appropriate way to evaluate asbestos exposure. Nonetheless, the membrane filter method suffers from the following serious deficiencies:
(1) The aspect ratio criterion used to define and count mineral fibers was arbitrarily chosen and has never been shown to have any relevance to the etiology of asbestos-related diseases or to the actual dimensions of airborne asbestos fibers.
(2) Common cleavage fragments of many amphiboles are regulated as asbestos.
(3) Many fabricated products containing asbestos and many ores that may contain small quantities of asbestos also contain other substances that cannot be distinguished from asbestos by the membrane filter method.
(4) Most airborne asbestos fibers are invisible if the membrane filter method is used.
(5) There is no predictable relationship between exposure estimates based on the membrane filter method and the total airborne fiber concentration, as established by electron microscopy. The ratio varies between these two estimates of exposure according to (a) the particular asbestos mineral being evaluated, (b) the degree of development of the asbestiform habit, and (c) the degree of processing to which the mineral has been subjected. Therefore, different total exposures are allowed under existing regulations for different minerals, different mineral habits, and different work environments. A revised standard based on electron microscopy and applying appropriate dimensional criteria is recommended.
The National Institute for Occupational Safety and Health (NIOSH) has set forth the definitions and methods used in almost all regulation of asbestos. NIOSH defines a fiber as a particulate that has a physical dimension longer than 5 mm and a length-to-diameter ratio of 3 to 1 or greater, and asbestos as a fiber of chrysotile, amosite, crocidolite, anthophyllite, tremolite, or actinolite. Concentrations of airborne fiber are measured in units of fibers (F) per cubic centimetre or millilitre (F/cm2 = F/mL), where the fibers have been counted by phase contrast microscopy examination of membrane filters that have been rendered transparent by an immersion medium. The choice of a lower bound of 5 um for length was probably based on reproducibility considerations. Studies by Addingley [1]2 and Lynch et al [2] had shown that counting fibers less than 5 um in length could lead to imprecise results. The choice of an aspect ratio of 3:1, however, was apparently arbitrary and was not based on any systematic studies [3]. Phase contrast microscopy and the membrane filter method were probably selected by NIOSH because they had been used in the measurement and regulation of airborne quartz.
For many years, the NIOSH definition of a fiber and the membrane filter method have been used to measure and regulate concentrations of airborne fiber. All federal regulatory agencies use them, and most states have incorporated both into their own regulations, as have many foreign countries. The concentration of fiber as measured by the membrane method is used by industrial hygienists to assess occupational hazard, and their dose-response models are based on it as a unit of measure. Nonetheless, there have been complaints about both the definitions and the membrane filter method for the past ten years. These objections are raised by the mining industries, which have pointed out that the asbestos regulations are not appropriate for the mining and mineral-processing environment or for most mineral products; by certain industrial hygienists, who say that the regulations do not adequately protect workers, and by other academic, governmental, and industrial scientists, who maintain that the definitions do not conform to accepted scientific terminology and that the air monitoring techniques provide unreliable data.
Definitions
In this paper, the following definitions are used:
fiber—an acicular single crystal or a similarily elongated polycrystalline aggregate which displays some resemblance to organic fibers by having such properties as flexibility, high aspect ratio, silky luster, axial lineation, and others, and which has attained its shape primarily through growth rather than cleavage.
standard fiber--any particle with parallel sides and an aspect ratio of 3:1 or greater. The abbreviation SF will stand for a standard fiber, and the concentration of airborne standard fibers is expressed as SF/cm2.
NIOSH fiber—any particulate with a length greater than 5 um and an aspect
adxasbestos removal.com 125 S Clark St Chicago, IL
health-related silicates, asbestos, membrane filter method, asbestos fiber exposure
The following excerpt is from Wylie, A. G, from the book "Membrane Filter Method for Estimating Asbestos Fiber Exposure," Definitions for Asbestos and Other Health-Related Silicates, published by Benjamin Levadie, Ed., American Society for Testing and Materials. It is on the subject of Estimating Asbestos Fiber Exposure.
Asbestos exposure is usually evaluated by the membrane filter method recommended by the National Institute for Occupational Safety and Health (NIOSH). The method for estimating fiber exposure was originally applied only to environments in which commercially mined asbestos was being used, but it has now been extended to the mining environment and to other environments in which natural materials containing small quantities of asbestos are found. It has been incorporated into state regulations and procedures and adopted by the industrial hygiene community as an appropriate way to evaluate asbestos exposure. Nonetheless, the membrane filter method suffers from the following serious deficiencies:
(1) The aspect ratio criterion used to define and count mineral fibers was arbitrarily chosen and has never been shown to have any relevance to the etiology of asbestos-related diseases or to the actual dimensions of airborne asbestos fibers.
(2) Common cleavage fragments of many amphiboles are regulated as asbestos.
(3) Many fabricated products containing asbestos and many ores that may contain small quantities of asbestos also contain other substances that cannot be distinguished from asbestos by the membrane filter method.
(4) Most airborne asbestos fibers are invisible if the membrane filter method is used.
(5) There is no predictable relationship between exposure estimates based on the membrane filter method and the total airborne fiber concentration, as established by electron microscopy. The ratio varies between these two estimates of exposure according to (a) the particular asbestos mineral being evaluated, (b) the degree of development of the asbestiform habit, and (c) the degree of processing to which the mineral has been subjected. Therefore, different total exposures are allowed under existing regulations for different minerals, different mineral habits, and different work environments. A revised standard based on electron microscopy and applying appropriate dimensional criteria is recommended.
The National Institute for Occupational Safety and Health (NIOSH) has set forth the definitions and methods used in almost all regulation of asbestos. NIOSH defines a fiber as a particulate that has a physical dimension longer than 5 mm and a length-to-diameter ratio of 3 to 1 or greater, and asbestos as a fiber of chrysotile, amosite, crocidolite, anthophyllite, tremolite, or actinolite. Concentrations of airborne fiber are measured in units of fibers (F) per cubic centimetre or millilitre (F/cm2 = F/mL), where the fibers have been counted by phase contrast microscopy examination of membrane filters that have been rendered transparent by an immersion medium. The choice of a lower bound of 5 um for length was probably based on reproducibility considerations. Studies by Addingley [1]2 and Lynch et al [2] had shown that counting fibers less than 5 um in length could lead to imprecise results. The choice of an aspect ratio of 3:1, however, was apparently arbitrary and was not based on any systematic studies [3]. Phase contrast microscopy and the membrane filter method were probably selected by NIOSH because they had been used in the measurement and regulation of airborne quartz.
For many years, the NIOSH definition of a fiber and the membrane filter method have been used to measure and regulate concentrations of airborne fiber. All federal regulatory agencies use them, and most states have incorporated both into their own regulations, as have many foreign countries. The concentration of fiber as measured by the membrane method is used by industrial hygienists to assess occupational hazard, and their dose-response models are based on it as a unit of measure. Nonetheless, there have been complaints about both the definitions and the membrane filter method for the past ten years. These objections are raised by the mining industries, which have pointed out that the asbestos regulations are not appropriate for the mining and mineral-processing environment or for most mineral products; by certain industrial hygienists, who say that the regulations do not adequately protect workers, and by other academic, governmental, and industrial scientists, who maintain that the definitions do not conform to accepted scientific terminology and that the air monitoring techniques provide unreliable data.
Definitions
In this paper, the following definitions are used:
fiber—an acicular single crystal or a similarily elongated polycrystalline aggregate which displays some resemblance to organic fibers by having such properties as flexibility, high aspect ratio, silky luster, axial lineation, and others, and which has attained its shape primarily through growth rather than cleavage.
standard fiber--any particle with parallel sides and an aspect ratio of 3:1 or greater. The abbreviation SF will stand for a standard fiber, and the concentration of airborne standard fibers is expressed as SF/cm2.
NIOSH fiber—any particulate with a length greater than 5 um and an aspect
adxasbestos removal.com 125 S Clark St Chicago, IL
health-related silicates, asbestos, membrane filter method, asbestos fiber exposure