For this sampling event september 2005 at least one

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anticipated air concentrations from previous site-specific experience. For this sampling event (September 2005), at least one filter, per activity sampling, not over- loaded was obtained, thereby eliminating this source of uncertainty. Laboratory Uncertainty . Laboratory uncertainty may result in either over- or underestimates of exposure concentrations. There are numerous potential sources of uncertainties in analyzing asbestos samples using transmission electron microscopy. These uncertainties are addressed to a large degree by the protocol for preparing and analyzing asbestos samples, developed by the International Organization for Standardization, known as ISO 10312. This method includes very specific definitions of structure types, which minimizes subjective decisions by analysts. In addition, it contains very specific counting rules and Quality Assurance/Quality Control (QA/QC) procedures. These include field duplicates, field blanks, and internal checks for consistency among analysts. Laboratory uncertainty may result in either over- or underestimates of exposure concentrations. Quality Control Sample Result Utilization . Selected QA/QC sample results were used to calculate the asbestos exposure concentrations for use in the risk assessment. For example, the QC samples classified as RD (recount different), RP (repreparation), and IL (interlab) were averaged with their original sample results. These samples results were independent evaluations of the same filter. The VA (verified analysis) results were used instead of the original sample result since the VA is a more accurate count. Use of QC sample counts may result in either over- or underestimates of exposure concentrations. UNCERTAINTIES IN CALCULATING RISK Exposure Parameters. The exposure parameters used in these risk calculations (hours/day, days/year use of the CCMA) were based on estimates reported by recreational motorcyclists at a CCMA public meeting (PTI, 1992). EPA also provided exposure parameters for some of the receptor activities including the post- decontamination driving and fence builder/repairer scenarios. The estimates by the recreational riders at the CCMA public meeting included the high use estimates of long-term OHV use by attendees. Variations in exposure parameters will exist. For example, the range of use (1 to 12 days per year), used herein, probably does not include extreme uses of the CCMA. The BLM ranger who patrols the CCMA, for example, may ride many more than 12 days per year. Uncertainties in exposure parameters can be easily addressed by producing tables of risk that encompass the broadest expected range of use. Future reports may expand the range of exposure parameters used to calculate risk, if warranted. U.S. EPA IRIS and OEHHA Cancer Inhalation Unit Risk for Asbestos Fibers. The U.S. EPA IRIS and OEHHA unit risk are derived from occupational studies where elevated rates of cancer were observed in workers whose occupations exposed them to high concentrations of asbestos for extended periods of time. Neither the actual dose each individual received in these studies nor the actual extent of their
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