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Health Effects Institute-Asbestos Research (HEI-AR)

Asbestos in Public and Commercial Buildings

A Literature Review and Synthesis of Current Knowledge (1991)

CONTENTS

Introduction and background 
Asbestos 
Methodology 
Asbestos in public and commercial buildings 
Measurement of asbestos levels 
Exposure to asbestos in buildings 
   - Outdoor levels 
   - Ambient levels in buildings 
   - Exposures to C2 and C3 occupants 
   - Exposures to C4 and C5 occupants 

Control of asbestos exposure 
Potential health effects 
Risks to building occupants 
Man-made mineral fibres 
Research needs

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This report was prepared by the Literature Review Panel, a multidisciplinary group of experts under the auspices of the Health Effects Institute-Asbestos Research (HEI-AR). HEI-AR is an independent, nonprofit organization that was formed in 1990 to gather and generate reliable and objective information. HEI-AR is supported jointly by the Environmental Protection Agency and a broad range of private parties that have an interest in asbestos. The congressional mandate under which HEI-AR operates specifies that HEI-AR's research "effort shall in no way be construed to limit or alter [the Environmental Protection Agency's] authority or obligation to proceed with rulemakings and to issue rules as necessary."

This report represents the first step in the response to congressional mandate (August 3, 1988) to the Health Effects Institute (HEI), and through HEI to HEI-AR, for research to:

• "determine actual airborne (asbestos fiber) levels prevalent in buildings...
• "characterize peak exposure episodes and their significance, and
• "evaluate the effectiveness of asbestos management and abatement strategies in a scientifically meaningful manner."

The purpose of the present report is to review and synthesize the state of knowledge as reflected in scientific articles, reports, and additional unpublished data on four issues considered pertinent to the congressional mandate:

• the concentrations of airborne asbestos fibers found in public and commercial buildings;
• the concentrations of such fibers to which building occupants, including custodial workers, maintenance workers, abatement workers, and other occupants, are exposed; the situations causing such exposures, and the potential adverse health effects resulting therefrom;
• the possible impact that different asbestos remediation strategies may have on the exposure of building occupants to airborne asbestos and, in turn, on the risks of health effects in those exposed; and,
• the significance of each form of asbestos in terms of its potential ill health effects and its implications for different remediation options in buildings.

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The term asbestos is used for a group of fibrous, naturally occurring silicate minerals that exhibit properties rendering them useful in commerce. During the past century, asbestos has been mined, processed, and used in thousands of products. Because of the exceptionally effective insulating, fire-resistant, and reinforcing properties of asbestos-containing materials (ACM), they have been utilized widely as surface-applied finishes (for acoustical, decorative, and fire-retardant purposes), and as thermal insulation in the construction of buildings, as well as in equipment used in buildings. Although chrysotile is estimated to constitute approximately 95 percent of the asbestos used in the United States, building surveys have shown amosite and, to a lesser extent, crocidolite, to have been used with greater frequency in buildings than the total consumption figures would suggest. At least one common form of asbestos, chrysotile, is present naturally in the atmosphere.

The Literature Review Panel has reviewed and synthesized the diverse body of scientific and technical information that is germane to asbestos in public and commercial buildings. The relevant literature is extensive and has been augmented recently by new scientific and technical findings, which have not all been published in the peer-reviewed literature. Where appropriate, rather than attempting to compile an exhaustive bibliography, the report cites previous reviews of the literature. The information provided in such reviews has been evaluated critically, and has been extended and amplified as necessary to bridge gaps and to take into account these more recent data.

In subject areas where the Panel found a paucity of published data or reviews, it has made a concerted effort to obtain and review both published and unpublished data. Published information was obtained through searches of computerized databases. Unpublished information was sought from all possible sources through announcements in scientific journals and in the HEI-AR newsletter. All of the submitted data were reviewed and are summarized in this report, as appropriate. Where data were acknowledged to be in support of litigation, the Panel has clearly indicated their nature. A supplement to include many details of the unpublished data that the report has summarized is planned for publication in the near future.

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Under certain conditions, asbestos-containing materials (ACM) can release asbestos fibers into the air of buildings, which can be inhaled by and reach the lungs of occupants. The concentrations of airborne asbestos fibers to which building occupants may, therefore, be exposed can be categorized as follows:

• low ambient concentrations, such as have recently been found in many well-maintained public buildings, and which are similar to ambient levels found outside these buildings;
• generally elevated ambient asbestos fiber concentrations, such as those produced in certain buildings by abrasion or damage to ACM, and by resuspension of released material through human activities; and,
• locally elevated airborne fiber concentrations, resulting from damage, abnormal wear, or resuspension of dust; these often result from activities of certain occupational groups, including custodians and workers involved in building maintenance, or remodeling, or in asbestos removal.

For the purposes of this report, building occupants have been classified into the following five exposure categories: 

• C1 - General occupants, who spend time in buildings but who are unlikely to disturb asbestos in place; for example, office workers.
• C2 - Custodians and/or janitors, who may cause increased levels of airborne asbestos fibers as a result of housekeeping activities.
• C3 - Skilled maintenance workers, whose activities may disturb and displace ACM.
• C4 - Workers who are responsible for removal or remediation of ACM.
• C5 - Emergency personnel who may be required to enter buildings during or after extensive damage, for example, fire fighters.

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For determination of airborne concentrations of asbestos fibers in buildings, air is customarily filtered through a membrane filter. After some manipulations of the filter, the fibers are counted using an optical phase contrast microscope (PCM) or an electron microscope (EM); both the scanning electron microscope (SEM) and the transmission electron microscope (TEM) can be used for this purpose. Because of limitations of the PCM and SEM related to visibility and identification of small or thin asbestos fibers and structures, the analytical TEM is used for asbestos analysis. Only the TEM is capable of providing accurate information on fiber numbers, dimensions, and morphology. When combined with selected area electron diffraction and energy dispersive x-ray analysis, the structural nature and the mineralogical identity of fibers can also be ascertained with the TEM; this is a great advantage for environmental asbestos analysis where other types of fibers and mineral fragments are often present. Fiber counts determined by PCM and TEM represent different indices of measurement because the resolving power of the PCM is much lower.

TEM-based air measurements have been reported in the literature in terms of mass, fiber number, or structure number; however, the results expressed in the different units cannot easily be compared. In this report, the conventional measure of exposure (numbers of fibers longer than 5 mm) for both optical and TEM measurements of fiber concentrations is given in units of fibers per milliliter (f/mL). Measurements of concentrations of asbestos structures (fibers, bundles, clusters, as well as matrices) of all sizes per liter (s/L) and calculation of asbestos mass in nanogram per cubic meter (ng/m3) are also included where appropriate. Unless otherwise stated, measurements described in this report as f/mL refer to the counts of fibers (longer than 5 mm) and fiber-containing structures, as determined by TEM, and as reported by authors of the individual studies.

Two different protocols are used to prepare filters for TEM analysis. In the direct method, the specimen preparation procedures attempt to retain all particles in their unchanged physical state and in the same relative locations on the TEM specimen as they occupied on the original sample collection filter; thus these procedures endeavor to leave unaltered the size distribution and the state of aggregation of asbestos fibers. In the indirect method, the particulate matter is transferred from the original sample collection filter into a liquid suspension, of which an aliquot is redeposited onto a secondary filter The secondary filter is then used to prepare a specimen for TEM examination as in the direct protocol. A higher fiber count (particularly for short fibers), and a different fiber size distribution, is observed using the indirect protocol as compared to the direct protocol. Because of similarity of protocols, fiber counts obtained with the direct preparation methods can be more easily compared with those obtained with the optical PCM.

Measurements of the concentration of airborne asbestos fibers in buildings cannot be assumed to be adequately representative of the long-term average exposures of general building occupants (C1), unless they are made during normal periods of occupation of the buildings, normal operation of air handling and mechanical equipment, and with normal levels of maintenance (C3 and custodial (C2) activities. Maintenance and custodial work may result in localized increases in airborne fiber concentrations that can influence the exposure of building occupants.

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A large number of buildings in the United states and other countries have been examined for airborne asbestos fibers within the past 20 years, and have yielded many thousands of air measurements (most unpublished). However, few building environments have been individually characterized in sufficient detail or sampled with sufficient analytical sensitivity to describe adequately the exposures of general building (C1) occupants. Extensive efforts have been made to gather and interpret the available exposure data, but further research is required to establish the long-term means and distributions of asbestos fiber exposures in individual buildings. Specific details are especially lacking for episodic and point-source releases of fibers into the air of buildings from maintenance and engineering activities, from repair and renovation operations, and from normal custodial functions.

Outdoor levels
Such data as are now available on the airborne concentrations of asbestos fibers of the dimensions most relevant to human health (that is, fibers longer than 5 mm) generally show average concentrations in the order of 0.00001 f/mL for outdoor rural air (except near asbestos-containing rock outcroppings) and average concentrations up to about 10-fold higher in the outdoor air of urban environments. However, outdoor urban airborne concentrations above 0.0001 f/mL have been reported in certain circumstances as a result of local sources; for example, downwind from, or close to, areas of frequent vehicle braking or activities involving the demolition or spray application of asbestos products. Outdoor concentrations measured by the indirect method for TEM specimen preparation are higher than those obtained by the direct method.

Ambient levels in buildings
In the course of this review, the data on ambient outdoor levels of asbestos from a number of sources have been examined and analyzed, and the data from direct TEM measurements have been averaged for each of a number of individual buildings. The following data are based on 1,377 air samples obtained in 198 different ACM-containing buildings not involved in litigation (the data from buildings sampled for litigation purposes have been summarized separately in this report). The building means of the studies on the 198 buildings range from 0.00004 to 0.00243 f/mL. Grouped by building category, the mean concentrations are 0.00051, 0.00019, and 0.00020 f/mL in schools (including a few colleges), residences, and public and commercial buildings, respectively with 90th percentiles of 0.0016, 0.0005, and 0.0004, respectively (Fig. 1-1). For all data pooled, the mean exposure value is 0.00027 f/mL, with 90th and 95th percentiles of 0.0007 and 0.0014, respectively. Some of the higher values in the sampled buildings are derived from situations representative of custodial and maintenance activities. The averages reported here are sensitive to such high values; thus, if the sample with the highest value (which was collected in an area where cable was being installed) was excluded from the calculations, the average value for the concentration of fibers longer than 5 mm in public and commercial buildings would be reduced from 0.00020 to 0.00008 f/mL. Similarly, with respect to schools, if the sample with the highest value (which was collected in a mechanical room/closet) is excluded, the average is reduced from 0.00051 to 0.00038 f/mL. From the data collected for litigation purposes, arithmetic average values for 171 schools (including colleges), 10 residences, and 50 public and commercial buildings were 0.00011 f/mL, below the limit of detection, and 0.00006 f/mL, respectively. Little information was found on ambient indoor fiber counts using the indirect method for TEM sample preparation. In one study, fiber counts by the two methods of sample preparation were compared; for samples prepared using the indirect method, the fiber counts were substantially higher than the fiber counts with the direct method.

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The fiber concentrations (greater than 5 mm) from direct analysis are lower than the measurements inferred from the earlier mass studies reviewed in a 1984 report from the National Research Council which concluded, after converting mass measurements to fiber concentrations, that the median exposures corresponded to 0.00007 f/mL outdoors, 000.54 f/mL inside rooms without ACM, and 0.0006 f/mL in rooms with ACM (no estimates of average exposures were reported). The limited mass data since 1984 have median values that are lower than previously reported. The differences between the NRC estimates and those summarized here are due, in part, to the fact that the earlier studies utilized a mass-to-fiber conversion factor rather than direct counts of fibers longer than 5 mm, and were carried out in buildings which more often contained highly deteriorated, friable ACM surface treatments. The extent to which occupants of some unsampled buildings are currently exposed to conditions and levels similar to those reported in the earlier mass-based studies is not known.

The extent to which the data from the sampled buildings reviewed in this report are representative of the conditions generally found in U.S. public and commercial buildings is not known. Sources of uncertainty include: types of buildings sampled, building selection strategy, sampling location within buildings, types of ACM present, extent of ACM damage, level of building activity, whether an operations and maintenance (O&M) program was established, and the extent and level of maintenance activity undertaken. In addition, other sources of uncertainty in the data relate to analytical preparation, sensitivity, and measurement errors.

Exposures to C2 and C3 occupants
Janitorial, custodial, maintenance, and renovation personnel may disturb or damage ACM in the course of their work and thereby generate "peak" (brief, relatively high) exposure episodes. Such episodes have not often been reported and are poorly characterized as yet. With proper controls, the exposures to maintenance personnel can be kept below 0.1 f/mL, the permissible exposure limit proposed by the U.S. Occupational Safety and Health Administration; but without adequate controls, exposures can exceed 10 f/mL during some removal and repair work. Such exposures can, in principle, be reduced by an O&M program that includes both training of personnel and implementation of standard control procedures for activities that may disturb ACMs, and can also be reduced by one or several of the abatement strategies. Unless the location of ACM in a building is known, there is little opportunity for appropriate planning and implementation of procedures to avoid such "peak" exposures for workers.

Exposures of C4 and C5 occupants
For workers involved with asbestos removal (C4 occupants), available data indicate a potential for exposure to airborne concentrations as high as 10 to 100 f/mL during dry removal with air exhaust and as high as 1 f/mL during wet removal with air exhaust. Emergency (5) workers may also encounter situations in damaged buildings in which the airborne concentrations of asbestos fibers are high, although no data on the exposure of such workers were found. Good work practice and adequate respiratory protection are, therefore, essential to avoid exposure of such workers to high levels of asbestos.

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Although limited, the existing data suffice to support the following generalizations:

• ACM within buildings in good repair and undisturbed is unlikely to give rise to airborne asbestos fiber concentrations above the levels found outside those buildings;
• accessible ACM has the potential to be damaged;
• during processes that damage ACM, fibers can be released into the air, and the resulting elevation of fiber levels may persist subsequently for varying lengths of time;
• maintenance activities can result in localized increases in airborne asbestos levels in the vicinity of ACM, exposing the workers who are directly involved and also possibly nearby building occupants; O&M work procedures can reduce such exposures;
• removal of ACM from buildings, if improperly done, can cause generalized increases in airborne fiber levels, which may persist for varying lengths of time.

Determination of the potential for asbestos exposure in a given building situation is primarily concerned with the discovery of the physical situations that can lead directly or indirectly to the disturbance of ACM; such disturbances may be caused by untrained and unprotected individuals. This type of determination customarily involves a survey to catalogue the location, accessibility, quantity, condition, and type of each ACM in the building. The existing level of exposure in a building can be determined by air monitoring.

Determining which particular preventive measures and forms of remediation are warranted in a given situation is a site-specific and complex task. The general questions to be considered in such a determination include:

• whether the selected remediation option will be the most effective among the available options in reducing current or potential future exposures to general (C1), custodial (C2), or maintenance (C3) occupants;
• whether the process of remediation will cause workers or other occupants to experience exposures that exceed the exposures being prevented;
• whether, if ACM is left in place, the control measures will be effective and whether reasonably anticipated disturbances (whether generated by repair, renovation, or natural causes) will later create even higher levels of exposures; and,
• whether, if removal of ACM takes place, any replacement materials are safer than the materials being removed, and whether the disposal of removed asbestos materials does not simply move the potential danger from one location to another.

The data on exposures to custodial (C2) or maintenance (C3) workers during specific activities can help to determine the need for, and type of, remediation appropriate to prevent exposures; such data can also provide information on the potential for increased exposure of general (C1) occupants as the result of custodial and maintenance activities.

Remediation strategies vary in their potential for disturbing asbestos; the control of such disturbance, with the aim of preventing exposures to building occupants, is less difficult with O&M programs than with enclosure or encapsulation and is most difficult with removal. The effects of abatement work on the long-term asbestos exposures of building occupants, custodians, or maintenance workers depend on product design and execution as well as building circumstances. In well-maintained buildings with long-term airborne levels of asbestos fibers similar to ambient background levels, removal or other abatement action, if done improperly, can cause increases of fiber levels which may persist for varying periods of time. On the other hand, in buildings where ACM has undergone continuing disturbance, appropriate abatement action can lead to a reduction in the asbestos exposure of workers and other occupants.

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At the relatively low concentrations of airborne asbestos fibres encountered by general (C1) building occupants, lung cancer and mesothelioma are diseases of concern. The capacity of asbestos fibers to cause these diseases depends on a number of the physical and chemical characteristics of such fibers.

• Fiber length: While the differential responses to fibers of different lengths cannot yet be specified precisely, the data suggest that the risks of lung cancer and mesothelioma increase with increasing fiber length. In particular, a substantial body of experimental evidence suggests that the rates of induction of tumors and fibrosis in animals, as well as transformation of cells in vitro, increase sharply as fiber length increases above 5 mm. Thus, the conventional definition of an asbestos fiber used for industrial hygiene purposes (fibers longer than 5 mm with an aspect ratio of 3 and greater) continues to be a practical index for risk assessment; the use of this index also facilitates comparison of present observations with those in the earlier literature. Whether there is any threshold length below which there is no carcinogenic effect in humans is not known. Animal data suggest, however, that very short fibers have much less carcinogenic activity than longer fibers and may even be relatively inactive.
• Fiber diameter: There is clear experimental evidence that mesotheliomas occur more frequently following exposure to thin fibers than to thick fibers. Observations in humans are consistent with this finding; however, accurate human exposure data expressed in terms of fiber number and dimensions are not available, and in animal studies the dose has been measured in terms of dust mass, so that preparations containing thin fibers have included larger number of fibers per unit mass.
• Fiber type: When handled in similar ways (for example, in mining or in gas mask manufacture), crocidolite has caused a greater risk of pleural mesothelioma than chrysotile or amosite; however, in the absence of adequate fiber measurements in many of these occupational cohorts, it is not clear whether there are any differences in dose-specific risk. There is also suggestive evidence that most peritoneal mesotheliomas are caused by amosite or crocidolite. For lung cancer, no consistent differences between fiber types in dose-specific risk have been established; however, there are large, unexplained differences in dose-specific risk among different occupational groups exposed to chrysotile. In particular, the risk in chrysotile miners and millers is much lower than that in chrysotile textile workers.
• Other physico-chemical factors: Other physico-chemical factors, such as differences in durability in lung tissue, in surface chemistry, or in surface charge, may also contribute to fiber toxicity, although their precise role remains to be established.

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The health effects resulting from inhalation of airborne asbestos fibers by occupants in today's buildings, and the benefits to be obtained from appropriate ACM remediation strategies, cannot be estimated with confidence from the existing data, owing to uncertainties about the relevant exposure-response relations and difficulties in estimating levels of past and current exposures. Although a threshold cannot be excluded, if a linear (no threshold) relationship between exposure and risk is assumed to exist, then the asbestos-related cancer risk to general (C1) building occupants can in principle be computed from the overall mean of average exposures in buildings. There are, however, a number of serious limitations underlying such exposure estimates.

• Historical occupational exposure data, and hence epidemiological risk estimates, are based on estimates of fiber exposure that were derived from total particle counts and, in a few cases, on the concentrations of fibers longer than 5 mm counted by optical microscopy. This dictated the Panel's decision to base its conclusions only on studies reporting measurements of conventional (longer than 5 mm) fibers, on the premise that environmental measurements expressed in these terms are the only ones which can be related to the historical industrial measurements on which the dose-response relationships, and hence the risk assessments, are based. At the present time, measurements of fibers 5 mm and longer, by transmission electron microscopy using the direct method, constitute by far the most extensive data available to the Panel for assessing exposure and risk.
• It is not known how representative the data are of the conditions generally found in U.S. public and commercial buildings because of several variables. These include types of buildings sampled, building section strategy, sampling location within buildings, types of ACM present, extent of ACM damage, level of building activity, the level of building maintenance, and whether an O&M program was in force.
• In addition, interpretation of the data is complicated by uncertainties concerning certain aspects of the measurement techniques employed, such as the method of sample preparation, sensitivity of the analysis, and measurement errors; these uncertainties also apply to data obtained in work environments.

Within the constraints of the above reservations, estimates of risk based on linear extrapolation from effects resulting from heavy occupational exposure to asbestos in the past can, in principle, be calculated for building occupants at the different levels of exposure measured today. For asbestos workers who were exposed for 20 years at a level of 10 f/mL in the past, the lifetime increase in cancer risk is estimated on the basis of epidemiological studies to be about 200,000 per million, that is about 2 in 10. By linear extrapolation, therefore, it may be estimated that if workers were exposed to a level 100 times lower, that is, 0.1 f/mL (which is the permissible exposure limit proposed by OSHA), the risk would be 2 in 1,000 or 2,000 per million (Table 1-1). Because the average level in most asbestos-containing public buildings which have been surveyed herein is lower by a further factor of about 500 (0.00020 f/ mL, as noted above), the corresponding predicted lifetime risk for 20 years of exposure during working hours would be about 4 per million. If the highest sample was excluded from calulation of the average concentration, the risk estimate would be approximately 2 per million. Average levels in schools that have been surveyed herein are higher than those in other public buildings, approximating 0.0005 f/mL, for which the corresponding predicted lifetime risk to a child exposed during school hours would be about 6 per million. These risk estimates although highly uncertain for the reasons indicated, can be used to compare the public health hazard posed by different levels of indoor asbestos with the risks of other environmental agents for which control strategies may also be under consideration, as discussed in Chapter 8 of this report for the examples of indoor radon and environmental tobacco smoke.

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(a)  This table represents the combined risk (average for males and females) estimated for lung cancer and mesothelioma for building occupants exposed to airborne asbestos fibers under the circumstances specified. These estimates should be interpreted with caution because of the reservations concerning the reliability of the estimates of average levels and of the risk assessment models summarized in Chapter 8.

(b)  The "average" levels for the sampled schools and buildings represent the means of building averages for the buildings reviewed herein (Figure 1-1). The "high" levels for schools and public buildings, shown as 10 times the average, are approximately equal to the average airborne levels of asbestos recorded in approximately 5 percent of schools and buildings with asbestos-containing materials (ACM) (see Chapters 4 and 8). If the single highest sample value were excluded from calculation of the average indoor asbestos concentration in public and commercial buildings, the average value is reduced from 0.00021 to 0.00008 f/mL, and the lifetime risk is approximately halved.

(c)  The concentration shown (0.1 f/mL) represents the permissible exposure limit (PEL) proposed by the U.s. Occupational Safety and Health Administration. Actual worker exposure, expected to be lower, will depend on a variety of factors including work practices, and use and efficiency of respiratory protective equipment.

The above estimates apply to general building occupants (C1), and not to custodial (C2) and maintenance (C3) workers whose activities may result in episodic releases of asbestos fibers and dust. Such releases may contribute to the total exposure of all building occupants, and hence increase their long-term average exposure levels; however, there is no evidence that the occurrence of peaks in the exposure pattern has any effect on the overall risks of disease for general building occupants except insofar as they contribute to the long-term average exposures. As custodial and maintenance workers are more likely to be transiently exposed to higher levels, their added lifetime risks of cancer may be appreciably higher than those of general (C1) building occupants. However, representative data on exposures of C2 and C3 workers are not available; therefore, the Panel has not estimated the risks to such workers. Instead, the level of risk for workers that would be projected for the proposed OSHA permissible exposure limit is presented as a point of reference (Table 1-1) from which extrapolations can be made.

Although public concern over asbestos in buildings has focused primarily on potential risks to general building (C1) occupants, there does not appear to be sufficient justification on grounds of risk to the health of general occupants for arbitrarily removing intact ACM from well-maintained buildings. The potential risk to custodial and maintenance workers through exposure to airborne asbestos when ACM is disturbed is greater and, therefore, would appear to be the primary consideration in determining whether, and what type of, remedial action would be appropriate. The condition of the ACM and the circumstances of building use may also be considered in determining the appropriate control action. Measures to control the release of asbestos fibers from the disturbance of ACM, dust, or debris should be employed routinely where needed during the operation and maintenance of buildings. Uncontrolled disturbance of ACM should be avoided whenever possible.

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Man-made mineral fibers (MMMF) and other nonasbestos fibers are now often used as asbestos substitutes in building materials. Levels of exposure to man-made glass and wool fibers have been generally found to be low in public buildings. Although some MMMF types occur in fiber sizes that can be inhaled readily into the lung, most are nonrespirable. Ceramic fibers that are thin, respirable and durable may be of concern.

Because of limitations in the available data on the exposure of building occupants to airborne asbestos fibers, the assessment of such exposures calls for further research. The research should include: (a) studies to improve, compare and consolidate the methodology for analyzing the numbers, sizes and types of airborne asbestos fibres; (b) studies to define more adequately the characteristic sources and patterns of exposure -- long-term as well as short-term -- of building occupants in each of the various categories listed above; and (c) studies to determine how such patterns of exposure are affected by remediation strategies. HEI-AR has initiated a program of research aimed at addressing many of these issues in public and commercial buildings.

To reduce the uncertainty in estimates of the health impacts of asbestos on building occupants, there is need for further research on the biomedical effects of asbestos, with particular reference to the comparative potency of fibers of different sizes and types inhaled at low-to-intermediate levels of exposure; information yielded by lung dust measurements may be useful in this regard. The estimates of dose-response relations in this document and other published estimates are dominated by historical exposures, which were high and inadequately measured by modern standards. Such research should investigate the relevant dose-response relationships and mechanisms of asbestos-related disease, exploiting for this purpose experimental as well as epidemiological approaches. Systematic reanalysis and pooling of updated exposures and survival data on all available cohorts whose exposures were well-characterized and involved comparatively lower fiber concentrations -- for instance, less than 5 f/mL -- would be particularly useful.

In view of the growing numbers of different types of man-made fibers that are entering commerce to substitute for asbestos, as a result of the phase-out of asbestos itself, detailed material characterization and biological testing of such fibers should precede their widespread dissemination into human environment.

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