Source: Indoor Environment Connections
Mankind has used asbestos fibers, primarily the chrysotile form, in manufacturing processes for over two mellenia. The name asbestos is derived from a Greek word translated loosely as “indestructible.” Ancient societies used the fibrous mineral for funeral shrouds, candle wicks, and as matrix binder in ceramic products. During the Industrial Revolution, the “indestructible” properties of the fibrous minerals were utilized to insulate boilers, as well as steam and heated water pipes; to create fire-resistant clothing and curtains; to improve the durability of construction materials such as roofing shingles, paints and plasters; and to insulate wires that conducted electricity.
In the mid-1800s, a company established by H.W. Johns manufactured and marketed building materials that contained asbestos fibers, because asbestos fibers (1) enhance cohesion of matrix ingredients, (2) are resistant to chemical and biological degradation, (3) are not affected by temperature extreme changes, and (4) do not conduct electricity. History notes that Mr. Johns succumbed to a disease of the respiratory system, later identified as asbestosis, in 1898.
Ancient societies used materials that were adaptable to their needs and simply accepted that some rock formations contained natural fibers that could be mined and used as an additive to the matrices of bricks, plasters and ceramics, as well as woven into textile-like materials. Today we define asbestos, for legal purposes, as silicate minerals of specified elemental content, crystalline pattern, and dimensional aspects.
The six mineral species categorized as “asbestos” by the Environmental Protection Agency (EPA) are silicate minerals, specifically chrysotile (white asbestos), amosite (brown asbestos), crocidolite (blue asbestos – color related to iron and magnesium content), anthophyslite, tremolite and actinolite (these last three rarely exhibit coloration other than off-white). Among the species of asbestos, only chrysotile is associated with serpentine rock, and the fibers resemble sheets of paper rolled into thin, tubular fibers. The other five species of asbestos are classified as amphibole minerals and the crystalline fibers split into very thin, needle-like splinters.
These minerals occur naturally in mountainous regions of the world, including the Appalachian and Rocky Mountain ranges of the North American content. Out-crops of these minerals are common, such as veins of tremolite in northern Virginia and chrysotile in New Idrea, California. Asbestos fibers are released into the atmosphere, from natural sources, by wind saltation and volcanic eruptions (such as Mount St. Helens); thereby creating a natural component to the mineral dust burden of the atmosphere.
Since the time of the Roman Empire, there have been indications that inhalation of asbestos fibers, in excess of the natural dust burden, was associated with disease, particularly of the lower respiratory system. With the increase in asbestos use in the late eighteenth through early twentieth centuries, the indications that exposure to excessive concentrations of asbestos fibers correlated to respiratory disease increased until the data was overwhelming that asbestos was a hazard to public health.
In 1927, Dr. W.E. Cooke published a case study of the death of Nellie Kershaw, a 33 year old worker in an asbestos textile factory, who died of a respiratory ailment for which Cooke coined the term asbestosis. Ms. Kershaw had worked in the factory for twenty years. Drs. Merewether and Price published a report on working conditions of laborers handling asbestos and the correlation to respiratory disease and early death in 1930 for the British government.
Asbestosis is generally characterized by development of fibrotic tissue resulting from attempts by the human immune system to remove, digest or isolate foreign objects in the lungs. Small particles of dust, including asbestos fibers, can slip past the respiratory system’s mechanical defenses and lodge among the alveoli of the lungs.
A review of health risks related to mineral dusts, including asbestos can be found in Health Effects of Mineral Dusts (Volume 28, Reviews in Mineralogy, 1993). In chapter 8, Dr. Hochella reviewed studies that indicate that chrysotile fibers dissolved in about six months; however, fibers of amphibole asbestos do not dissolve and are attacked by macrophage cells, eventually creating fibrous plaques to isolate the asbestos fiber. The plaques interfere with normal gas exhange by adjacent alveoli, initiating respiratory distress. Some plaques may become cancer lesions (studies by Seidman and Selikoff in the 1960s through mid-1980s).
The lag time between initial exposure to airborne asbestos fibers and diagnostic detection of cancers (i.e., in the lungs), may be as long as 20 years. Particulates of tobacco smoke also irritate lung tissues and exacerbate the condition. Studies by Suzuki (2000 and 2004) and others have identified a correlation between extremely small chrysotile fibers and mesothelioma.
Dr. Vanessa Vu, (Chapter 19, Volume 28, Reviews in Mineralogy, 1993), indicated that the EPA estimates one-fifth of public and commercial buildings in the United States contained friable asbestos-containing building materials (commonly abbreviated ACM or ACBM) that released fibers into the indoor environment. Many buildings and residences conserve energy use and loss with central heating, cooling and air-conditioning systems and inoperable windows. While these particular methods are effective in controlling energy usage and costs, they also serve to trap indoor environmental pollutants and concentrate them through recycling of conditioned indoor air.
Determining whether building HVAC components are contaminated with asbestos can be exceptionally obvious or exceedingly difficult. In buildings where fireproofing was sprayed onto structural metal framing or into HVAC systems because of incomplete duct installation, the source of asbestos fibers is obvious. Unfortunately, it is often not simple. HVAC system component contamination that occurred during original construction is often hidden.
Worse, many HVAC contamination situations occur well after original building construction and are difficult to detect. These contamination situations often occur during maintenance or renovation activities where ACM disturbance activities are not controlled. In addition, interior building renovation that disturbs ACM when HVAC systems are operational can cause extensive asbestos contamination to supply and return ducts and plenums, as well as other HVAC components and occupied building spaces.
Where asbestos contamination of HVAC components is difficult to assess, the reader is referred to Settled Asbestos Dust Sampling and Analysis by James R. Millette and Steve M. Hays. The authors present multiple procedures for assessing the presence of asbestos fibers in settled dust on surfaces. Relatively obvious situations (such as overspray, debris, or accumulations of dust) can be addressed by straight forward techniques including the collection of bulk samples, then analysis by Polarized Light Microscopy (PLM) and/or Transmission Electron Microscopy (TEM).
Millette and Hays also describe accepted procedures for “scrape and scoop” collection of debris (also for analysis by PLM and/or TEM), and for low dust level adhesive tape lift (PLM analysis), wipe samples (TEM analysis), and micro-vacuum sampling techniques (TEM analysis). Selection of an appropriate technique takes experience, but the authors suggest bulk sampling and scrape and scoop techniques for obvious accumulations of debris, and microvacuum for sampling dust accumulations. The reader is referred to the American Society for Testing and Materials (ASTM) Standard Test Method D5755-09 for procedures for the collection and analysis of microvacuum dust samples.
What exactly constitutes asbestos-contaminated HVAC components? There are no federal standards that define asbestos contamination in dust and debris in buildings. Options for assessing asbestos levels in dust include comparison with background levels (dust samples collected from a building of the same type and geographic location that has no ACM) or based on the experience of the industry professionals. Chapter 6 of the Millette and Hayes book provides a brief treatment of this subject.
Based on their experience, concentrations less than 1,000 asbestos structures per square centimeter of surface area are considered to be low. Concentrations greater than 10,000 are considered “above background” and concentrations above 100,000 are considered high. These concentration numbers are presented only as guidance, not as a standard.
Procedures for cleaning of HVAC components are well established and significant effort has been expended to draft asbestos-specific procedures for HVAC cleaning. For asbestos-specific review of the topic, the reader is directed to efforts focused on the assessment and cleaning of building HVAC systems contaminated as a result of the collapse of World Trade Center Towers, in New York City on September 11, 2001. The EPA published a detailed protocol on HVAC cleaning procedures as a addendum to the May 2003 document entitled Interim Final WTC Residential Confirmation Cleaning Study, Volume 1.
The procedures described in this EPA document draw heavily from the National Air Duct Cleaners Association (NADCA) Assessment, Cleaning and Restoration of HVAC Systems (ACR 2006). Although not specific to asbestos, ACR 2006 serves as the industry standard for cleaning HVAC ducts and other components. Another good document to review if pursuing HVAC cleaning activities is the NADCA General Specifications for the Cleaning of Commercial Heating, Ventilating and Air Conditioning Systems.
An excerpt from the EPA study: “In general, all cleaning of air moving equipment and active plenums shall be performed under negative pressure using HEPA-filtered air filtration devices (AFD). All ducts that are cleaned shall be maintained under negative pressure using HEPA-filtered vacuum collection equipment specific to the duct cleaning industry. Air filtration devices and vacuum collection equipment shall be exhausted to the outdoors” provides the basic concept of the document.
In essence, the HEPA-filtered AFD is located in an appropriate central location to HVAC duct systems and that room or area is placed under negative pressure with a HEPA air filtered air filtration device. The HEPA-filtered vacuum collection equipment is attached to the duct to be cleaned. At any location where ducts are to accessed (supply or return diffusers, service openings etc.), plastic sheeting is placed on the floor, 3 feet in all directions from the point of access.
The HEPA-filtered vaccum collection equipment has to provide enough airflow at the location where cleaning is being performed to “draw dislodged debris from the mechanical device to the vacuum’s collection chamber.” EPA states that a minimum air flow of 3000 feet per minute (fpm) is typical for vacuum cleaning, although ACR 2006 varies the recommended air flow based on the nature of the contaminate (from 2500 to 4500 fpm).
Air ducts are accessed at appropriate locations and the ducts are mechanically cleaned using manual, mechanical, pneumatic, or hydro-agitation methods. Work is performed towards the centrally located HEPA-filtered vacuum collection equipment.
Outside the scope of the discussion, it is important to remember that asbestos is a known human health hazard and that the assessment and remediation of asbestos-contaminated HVAC components is a potential exposure pathway to airborne asbestos fibers. Appropriate work practices, engineering controls, and personal protective equipment should be utilized.
As indicated, the assessment and remediation of asbestos-contaminated HVAC components can be complex. The reader is encouraged to pursue the resources available for a more in depth treatment of the subject.