Mineral wool made of glass, rock or slag fibers comprise of the most effective, proven, existing technologies to conserve energy in the built environment. Over the last 50 years these fibrous insulation materials have found widespread use in thermal and acoustic applications. Concurrent with their widespread acceptance and use have been questions regarding possible health effects. Literally hundred of studies on the possible health effects of the fibers and the insulation wools have been reported in the scientific literature over the last several decades. These studies have evaluated exposures to airborne fibers, studied workers engaged in the manufacturing of the products, and reported on which animals studies in which animals were exposed to very large quantities of airborne fibers throughout their lives. In general, these studies have reported low airborne fiber levels, and have not established a relation between fibers and any disease in humans. Most of the animal inhalation studies have also not reported a relationship between airborne insulation fibers and disease in the test animals. The results of this extensive research have led to a general understanding that there are three major criteria that determine the potential of any airborne fiber to cause adverse effects:

1. The dose (how many fibers)
2. The dimension (size of the fibers)
3. The durability (if inhaled, the amount of time they last) of the airborne fibers.

The Three D's - Dose, Dimension and Durability

The Dose
Dose, as it relates to airborne fibers, is most appropriaTéléphoney defined as the number of respirable fibers in a given volume of air. Typically, results are expressed as fibers/cubic centimeter (f/cc) over an 8-hour workday. In insulation wool manufacturing facilities, these levels are generally well below 1f/cc. For most applications and installations these levels are well below lf/cc. (See Table 1.) The highest levels are found during the installation of blowing wool or loose fill insulation where levels may exceed 1 f/cc. Manufacturers have typically required the use of respiratory protection when exposures to respirable glass fibers exceed 1 f/cc. This leads to the question, "What is an acceptable exposure level?".

The American Conference of Governmental Industrial Hygienists (ACGIH) and the joint Occupational Safety and Health Administration (OSHA) and Industry Health Safety Partnership Program (HSPP) have both determined that 1f/cc is an appropriate exposure limit. It is of interest to compare the exposure levels in typical installation and fabrication operations with those exposure limits. (For details see Insulation Outlook, October 1999.)

Dimension
Fiber dimension, that is the length and diameter of airborne fibers, is also a very important factor in determining whether a fiber has any potential to produce adverse effects. Most importantly, for a fiber to be inhaled into the lower respiratory tract where it may have the potential to interact with the lung, it must be small enough to pass through the complex architecture of the respiratory tree. It is the diameter of the fiber rather than the length that is the most important determinant of such respirability. For fibers with the design of the insulation wools, the upper diameter limit is 3 microns (12 HT). Fibers with diameters larger than this cannot be inhaled into the lower lung. They are deposited in the upper airways and are removed efficiently by normal pulmonary clearance processes. Since the diameter rather than the length of fibers is the primary determinant of respirability, long fibers can still enter the lower lung if the diameter is small enough. Indeed, the fibrous shape is the only geometry that allows airborne material into the lower lung with a dimension (length) which is so large that it prevents normal clearance mechanisms from removing it. (See Figure 1.) It is these long fibers, generally considered to be 20 microns or more in length, that are now recognized as being the most potent in the induction of potential disease. Additionally, since normal biological processes cannot remove these long fibers, their durability in the body becomes the key to understanding fiber safety.

It should be noted that only about 15 to 20 percent of inhaled fibers are longer than that which can be engulfed by the macrophages. The short fibers are removed efficiently by the macrophages like any normal dust.

Durability-The New Understanding
The durability of fibers has been considered an important factor in determining the safety of inhaled fibers for some time. It is only during the last five to ten years that new knowledge of durability has led to the understanding of why some fibers can produce health effects while others do not. This new research has been directed primarily at the role played by the actual durability of the fibers in the lung.

To understand the role of durability, consider how the lung protects itself from the variety of airborne material that is inhaled daily.

All particles and short fibers that enter the lower lung are removed efficiently by cells called alveolar macrophages (AM). These scavenger cells reside in the lower lung and rapidly (minutes to hours) ingest inhaled particles. The AM have powerful biochemical mechanisms to kill bacteria and other biological material and a remarkable capacity to ingest particles. Following ingestion of particles, the AM migrate up the pulmonary tree to the back of the throat where they are swallowed and removed from the body.

This process is efficient for all inhaled materials that enter the lung and are small enough to be ingested by the AM (which are about 15 microns in diameter). As noted above, however, fibers can enter the lower lung with lengths of 20 microns or more. These long fibers cannot be removed by the AM, and with continuing exposure the long fibers will accumulate in the lung. If these long fibers remain in the lung for extended periods of time, they will lead to chronic inflammation and at high concentrations may result in permanent lung scarring (fibrosis) and pulmonary tumors.

Over the last several years, a major series of long term animal studies using very high concentrations of a variety of fibers with different durabilities demonstrated clearly that the effects of the fibers were directly related to the durability of the fibers. (See Table 2.) (McConnell et. al.) Very durable fibers produced lung scarring early in the study and lung tumors late in the study, while less durable fibers produced only lung scarring late in he study at the highest doses. Fibers that dissolved rapidly failed to produce any adverse effects in the animals even at the highest dose tested. It is now widely accept that the durability of the long fibers is directly related to the potential for disease.

The durability or the rate at which a fiber dissolves can be measured in the laboratory (Mattson 5,6). This rate, known as the dissolution rate or K dis can be obtained by determining at what rate the fibers dissolve in a buffered salt solution, which mimics the fluid lining the lung. These tests have shown that the dissolution rate is directly related to composition and that by controlling the composition the durability of a fiber can also be controlled. It is also possible to use the dissolution rate to calculate how long a fiber could remain in the lung before it dissolves.

It is now known that dissolution rates for a wide variety of fibers may range over a factor of 10,000 or more and the impact of this on the time to dissolve a fiber is impressive. Consider that for a very durable fiber such as asbestos with a K dis of less than 1, it can be estimated that a 1 micron diameter fiber would take over 5000 days (or about 14 years) to compleTéléphoney dissolve. On the other hand a typical 1 micron diameter insulation wool fiber with a dissolution rate of 100 would compleTéléphoney dissolve in 50 days. (See Table 2.) For reference, the time it takes for the macrophages to remove half of the particles or short fibers inhaled into the lung is about 70 days in rodents and even longer in man. Fibers with dissolution rates of 100 or more actually dissolve faster than the AM can remove normal dust from the lung.

Durability and Dose
One of the most important aspects of the new understanding of the significance of fiber durability is that it provides a unique means by which the actual dose of long fibers in the lung can be controlled by controlling the durability of the fibers.

Consider the example of two workers exposed to the same concentrations of airborne fibers but with different durabilities. In the first case the fiber has a durability that allows it to remain in the lung for 1000 days and in the second case the less durable fiber remains only for 50 days. Since the long fibers in each case are removed only by dissolution, in case one, the worker will accumulate fibers for the 1000 days it takes the fibers inhaled on day one of exposure to dissolve. In the second case the fibers will accumulate for only 50 days. All other parameters being equal, the worker exposed to the more durable fiber will accumulate 20 times more long fibers in his or her lungs than will the worker inhaling the more soluble fiber. This 20-fold increase in the concentration of long fibers in the lung occurs, even though both workers are exposed to the same concentration of fibers in the air.

This example shows that by controlling the composition and therefore durability of fibers, manufacturers can assure that workers do not develop high concentrations of long fibers in their lungs. This would be important for example if workers were exposed to temporary high concentrations of fibers, or failed to wear appropriate respiratory equipment when elevated fiber concentrations were possible.

Durability and Regulation
Though the understanding of the role of durability is very new, its importance has been recognized by the European Community (EC) and actually incorporated into regulations. In 1997, the EC published a document known as Directive 97/69/EC, which assigns a cancer classification to the insulation wool fibers. The Directive assigns mineral wool fibers to the category of EC 3 or "possibly carcinogenic", however, it also states that if the fibers can be shown to have a low durability they are exempt from any cancer classification. The Directive indicates a variety of animal tests that can be used to determine the low durability of the fibers. Importantly, each of the animal tests measures either directly or indirectly the durability of the long fibers within the animal. It is remarkable that even though the understanding of the importance of fiber durability is very new, its profound significance has been recognized, and used to exempt fibers from suspicion of carcinogenicity if they can be shown to be of low durability.

Durability and Product Design
It has become clear that the durability of fibers is a critical component of their potential to induce disease. Most fiber manufacturers now control fiber composition to assure that very durable fibers are not produced. In some cases entirely new fibers with reduced durability have been introduced to replace more durable fibers. For example, new, less durable compositions of rock wool products are on the market. In the glass industry, the chemistries historically used to produce these products have resulted in products with low durabilities. Glass manufacturers are now controlling chemistries to assure that very low durability fibers are used whenever possible.

Conclusions
Mineral wool insulation plays a major role in conserving energy in the built environment. Extensive research over the last several decades has led to growing assurances that the manufacture and use of these products is associated with very low exposures, and have not resulted in disease in humans. Recent scientific research has provided new understanding of the critical role of durability in the safety of these products. Though the traditional mineral wool insulations of rock, slag, and glass have generally been composed of chemistries that are not very durable in the body, today's manufacturers are controlling fiber durability by compositional control of their products to assure the continued safe manufacture and use of insulation wools in its many forms and applications.

References

Airborne Glass Fiber Concentrations during Manufacturing Operations Involving Glass Wool Insulation Jacob, TR.,Hadley, J. G., Bender, JR.,Eastes, W. 1993. Amencan Industrial Hygiene Association Journal, S4: 320-326.

Airborne Glass Fiber Concentrations during Installation of Residential Insulation. Jacob, TR., Hadley, J.G., Bender, J.R., Eastes, W, 1992, American Industrial Hygiene Association Journal, 53:Sl 9-523.

Results Of Life-Time Inhalation Studies of Glass Mineral and Slag Wools and Refractory Ceramic Fibers in Rodents. McConnell E.E., Hesterberg, T Chevaff, J., Theuenaz, R, Kotin, R, Mast, R., Musselman R., Karnstnip, O., Hadley, J., 1996, Austrahan/NZ Journal of Occupational Health and Safety, 12 (3): 327-332.

Biopersistence of Synthetic Vitreous Fibers and Amosite Asbestos in the Rat Lung following InhalationHesterbeig, TW, Chase, G., Axter4 C Miller, W-C, Musselman R.R, KamstrupO., Hadley, J. G., Morscheidt. C, Bernstein, D.M and Thevenaz, R, 1998, Toxicology and Applied Pharmacology, 151, 262-275.

Glass Fiber Dissolution in Simulated Lung Fluid and Measures Needed to Improve Consistency and Correspondence to In-Vivo Dissolution. Mattson, S.M, 1994, Environmental Health Perspectives, Supplement 5, 102: 87-90.

Glass Fibers in Simulated Lung Fluid:Dissolution Behavior and Analytical Requirements. Mattson, S.M, 1994, Annals of Occupational Hygiene, 38: 857-877.

Dr. Hadley joined Owens Coming in 1979. He received his doctorate in physiology and pharmacology from Duke University in 1977. His research concerned how the lung defends itself from airborne materials. He is a member of the Society of Toxicology, the Royal Society of Medicine. Dr. Hadley serves as a Chairman of the Health & Safety Sub Committee of the North American Insulation Manufacturers Association. In 1992, Dr. Hadley was appointed to the Expert Advisory Committee on Fibers to the World Health Organization/Regional Office for Europe.

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