NYC-WTC_000199582

NYC 9/11 Public Portal Document

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Reference Concentration (RfC) or IRIS Slope Factor exists, there are methodologies for examining risks of non-occupational exposures, on which to draw upon for a risk based approach to set benchmark levels. The approach taken here is a dual approach, where in the face of uncertainties the quantification of the benchmarks was based on the current occupational standards. Broader risk-based discussion of other methodologies provides support for the chosen benchmark levels although not quantitatively employed here to derive the benchmarks.

The Tier I benchmark is set at one tenth of the OSHA PEL. A single year of exposure at this benchmark (lOug/nr) would provide a cumulative silica loading to the lungs less than the Img/m^ work-year loading which is believed to be the departure point for adverse health effects. Additionally this level is also the effective National Ambient Air Quality Standard for silica, where PM,q at 50 ug/m^ contains no more than 10% crystalline silica. A recent review by supports the finding that this level would be protective of long-term non-cancer health effects [EPA, 1996]. The Tier III benchmark is set at one hundredth of the OSHA PEL. A 30 year continuous exposure at this level would not result in a cumulative silica loading to the lungs greater than the Img/m’ year loading which is believed to be the departure point for adverse health effects.

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CITY_21MC100_002452177

NYC-WTC_000199606