Health Effects Institute-Asbestos Research (HEI-AR)

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

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report is to review and synthesize the state of knowledge as reflected in scientific articles, reports, and additional unpublished data on four issues considered pertinent to the congressional mandate: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

Asbestos
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.

Methodology
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.

Asbestos in public and commercial buildings
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.

Measurement of asbestos levels
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.

Exposure to asbestos in buildings
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.

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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