• 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.

Potential health effects
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.

Risks to building occupants
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.

Lifetime, continuous outdoor exposure
€ 0.00001 f/mL from birth (rural)                             4
€ 0.0001 f/mL from birth (high urban)                        40

Exposure in a school containing ACM, from age
5 to 18 years (180 days/year, 5 hours/day)
€ 0.0005 f/mL (average) (b)                                 6
€ 0.005 f/mL (high) (b)                                   60

Exposure in a public building containing ACM
age 25 to 45 years (240 days/year, 8 hours/day)
€ 0.0002 f/mL (average) (b)                                4
€ 0.002 f/mL (high) (b)                                   40

Occupational exposure from age 25 to 45
€ 0.1 f/mL (current occupational levels) (c)                 2,000
€ 10 f/mL (historical industrial exposures)                200,000

(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.

Man-made mineral fibers
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.

Research needs
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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