Air Emissions of Contaminant Asbestos by qingyunliuliu


									Current Asbestos
 Related Issues
Aparna Koppikar, M.D., Ph. D.
        May 21, 2003
                        National Center for Environmental Assessment
                What is asbestos?
• There is a fibrous form and a nonfibrous form type of
• Asbestos is a term generally used to describe the fibrous
  forms of a family of hydrated metal silicate minerals
• Widely accepted definition of asbestos includes the six
  fibrous habits of these minerals which are currently
  federally regulated:
   – Serpentine: chrysotile - polymeric sheets which tend to wrap into
     tubular fibers that are curved and flexible
   – Amphibole: actinolite, amosite, anthophylite, rocidolite, tremolite
  Consists of two ribbons separated by a band of cat ion
• Accepted definition of fiber: length of >5 µm and diameter
  of <3 µm with an aspect ratio of 3:1
        Health Effects of Asbestos
    Asbestos is a known human carcinogen
•   Asbestos warts
•   Benign pleural plaques
•   Asbestosis
•   Lung cancer
•   Mesothelioma
    – pleural
    – peritoneal
    – other sites
• Gastrointestinal cancers
                  Current Issues
• For both lung cancer and mesothelioma
  – Influence of fiber type.
  – Influence of fiber length.
  – Does carcinogenic potency vary with fiber type and fiber
    length (suggestive evidence from animal studies)?
  – Do fibers <5 µm cause any effects?
  – Is potency a function of fiber diameter, aspect ratio, and/or
    surface properties (other than the fiber type and length)?
  – Are cleavage fragments toxicologically significant?
  – Are other amphiboles equally toxic as the five federally
    regulated ones?
       Current Exposure Issues

• Are exposure estimates from epidemiologic studies
• Which Lab method is reliable and useful for
  exposure measurement?
• Should fibers <5 µm in length be counted?
• The old fiber definition of fiber length of >5 µm
  and diameter of <3 µm with an aspect ratio of 3:1 is
  still valid?
        New Knowledge - Fiber
• Epidemiologic data suggest that fibers with diameter of 0.5
  µm to 0.7 µm can reach the respiratory zone of the lungs. In
  animals fibers with a diameter of 0.4 µm are critical based
  on rat data
• There is some indication from epidemiologic data that fibers
  with a diameter as high as 1.5 µm can reach the respiratory
  zone of the lungs in mouth breathers
• Thus, fibers with a diameter of <0.5 µm to 1.5 µm are
  considered to be relevant as they can reach the respiratory
  zone of the lungs in humans
         New Knowledge - Fiber
• Inhalation of fibers longer than 10 µm presents a considerably
  greater risk for lung cancer but the exact size cut-off for the
  length and magnitude of relative potency is uncertain
• There are two schools of thought about cancer toxicity of fibers
  <5 µm in length:
   – present a very low risk, possibly zero for cancer based on human
   – cause inflammation and may potentiate the pulmonary reaction to
     long fibers based on animal data and in vitro studies
• For mesothelioma, greater weight should be assigned to thinner
  fibers and fibers in the 5 µm to 10 µm in length range
       New Knowledge - Fiber
           Lung Cancer
• There are different opinions about the relative potency of
  chrysotile vs amphiboles for lung cancer based on
  epidemiologic data
• Some assert that amphiboles are 5 times more toxic than
  chrysotile for lung cancer
• Others assert that no real difference is observed in
  statistical analysis of epidemiolgic data
• The additional review of epidemiologic data to identify
  other factors such as industry in which exposure occurred
  might shed some light
         New Knowledge - Fiber
           Type Mesothelioma
• It is becoming apparent that there are different relative
  carcinogenic potencies for different fiber types
• The available epidemiologic data provides compelling
  evidence that potency of amphiboles is at least two orders of
  magnitude greater than that of chrysotile (ATSDR)
• Time since first exposure is an important factor for occurrence
  of mesothelioma
• Duration and intensity of exposure is also found to be
  important in epidemiologic studies
     New Knowledge - Cleavage
• Data indicate that durability and dimension are critical to
  pulmonary pathogenesis
• There are little data directly addressing similarities and
  dissimilarities between the original fibers and cleavage
  fragments for pulmonary pathogenesis
• Evidence suggests that it is prudent at this time to assume
  equivalent potency for lung cancer in the absence of other
• Similarly, evidence implies that for mesothelioma, thin diameter
  fibers and fibers >5 µm in length are found to be more
  important, thus, cleavage fragments that do not meet these
  criteria are not expected to contribute to the risk of
           New Knowledge -
         Other Amphibole Fibers
• Currently there are no data available either in humans or in
  animals about the toxicity of other amphiboles (e. g.,
  winchite and richterite)
• Other amphibole fibers with similar durability and
  dimension would be expected to result in similar
• Thus, it may be prudent to consider potency of currently
  regulated and unregulated amphiboles to be similar
  Current Knowledge - Exposure
• Historic exposure measurements in epidemiologic studies have
  too many uncertainties - measurements by PCM or MI
• Exposures to specific types of fibers or the % of each fiber type
  in mixture is unknown
• Information is usually lacking about other risk factors such as
  work processes, cigarette smoking, minerological, geological,
  and industrial hygiene data
• The fibers <5 µm in length should be counted
• The old fiber definition of fiber length of >5 µm and diameter of
  <3 µm is not valid
• Although minerolgy is considered to be important for toxicity,
  an aspect ratio of 3:1 is considered not to be relevant for toxicity
    Exposure Measurement Methods

•   MI: Midget impinger
•   PCM: Phase contrast microscopy
•   SEM: Scanning electron microscopy
•   TEM: Transmission electron microscopy
•   EDS: Energy dispersive x-raydetection
•   PLM: Polorized light microscopy
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