Rock Wool and Refractory Ceramic Fibers
Authors: Rice Carol 
Published: August 2005

Man‐made vitreous fibers (MMVF) is a generic descriptor for a group of fibrous materials made from melting inorganic substances such as sand, clay, glass, or slag. Synthetic vitreous fibers (SVF) or man‐made synthetic vitreous fibers (MSVF) may also be used to describe these groups of materials. These terms have generally replaced earlier use of man‐made mineral fibers (MMMF). MMVF are further classified by the raw material used in production; major categories include glass fibers (glass wool or continuous filament), mineral wool (rock or slag), and refractory ceramic fibers. The latter two types are covered in this chapter; glass fibers are described in Chapter. Within each category, a variety of commercial products have been produced and may be identified by manufacturer and product name and number. Each has a slightly different formulation and characteristics; therefore it is important where possible to identify the particular product number.

Dimension, durability, and dose delivered to the target organ are critical factors in the toxicity of MMVF.

MMVF are characterized by length (L) and diameter (D). The arithmetic mean or median of the observed distribution of lengths and diameters may be given as the count mean or median diameter (CMD) or length (CML). If the observed values are transformed by taking the natural logarithm of the measured parameters, the geometric mean (GM) of each dimension may be given with a geometric standard deviation (GSD). The size determinations may be made by either scanning (SEM) or transmission (TEM) electron microscopy. TEM has the lower limits of detection by which investigators can characterize fibers with diameters in the nanometer range. Dose by some routes of administration may be further described by the mass of material, for example, in implantation or single bolus injection studies. For inhalation studies, GM and GSD length and diameter are usually listed for the exposure aerosol, and often the number of fibers within specific size ranges are listed.

Following inhalation, fibers may be deposited on surfaces within the respiratory tract or exhaled. For the fibers that are deposited, the site of deposition (dose) depends upon the characteristics of the fiber and results from one of five mechanisms: impaction, interception, sedimentation, electrostatic precipitation, or diffusion. The majority of the deposition of MMVF is probably governed by the first three mechanisms. Impaction and interception occur when the fiber is removed from the airstream by physically contacting the surface of the airway or a bifurcation. Sedimentation occurs in the lower airways, where the velocity of the fiber becomes low enough for it to settle on the airway surface. Electrostatic precipitation results when the fiber carries a charge opposite to that of the airway surface; for mineral wool fibers, no reports have been found on surface charge measurements. Deposition due to diffusion requires that the air molecules collide with the fiber, resulting in movement toward the surface. This mechanism could contribute to deposition of very thin fibers, e.g., those with diameters substantially less than one‐half micron, but few of them are expected in the work environment.

The clearance mechanism of the deposited fibers depends upon the characteristics of the fiber and the site of deposition. Fibers deposited in the tracheobronchial region are cleared with the mucous by the cilia and swallowed. This process is completed in a matter of days, during which little change in fiber dimensions would be anticipated. Fibers deposited lower in the respiratory tract are cleared more slowly. Here the fibers are cleared by translocation to another area of the lung or dissolve; translocation may be facilitated by partial dissolution of the fiber or breakage into particles of shorter length. When fibers recovered from the lung or other tissue are characterized by dimensions, comparison with the parent material provides information on deposition and distribution.

Solubility has been investigated as an indicator of durability.

The interpretation of short‐term bioassay results is still under study. Bernstein et al. suggested that the results of dissolution at neutral pH are correlated with
in vivo
biopersistence. Others report that the dissolution rates of MMVF that have high aluminum content are much greater in acidic environments. Evidence from animal studies shows that the macrophages may interact with long fibers and that multiple macrophages attach to a single fiber which can lead to dissolution.