NYC-WTC_000199582

NYC 9/11 Public Portal Document

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setting, uncertainty factors were used to reduce the allowable airborne levels for the Tiered bench marks. The Tier I benchmark is set at one tenth of the TLV and the Tier III level is set at 1 hundredth of the TLV. It is believed these levels will prevent conditions which may cause irritant health effects for most individual.

Although fibrous glass in settled dust is known to be a contact irritant, little data exist to provide a threshold relating to what fiber loading may result in skin irritation, or airborne levels of concern. However, as with asbestos it was deemed preferable to have a Tier I benchmark available so that there was some upper bound to settled dust loadings that may trigger action. Here again Millet’s K factors were used to correlate dust loading levels to the air benchmarks. The Tier I level for fibrous glass in settled dust was based on Millette’s K factors, which are empirical relationships between concentrations of asbestos fibers in settled dust and indoor air. Millette developed the K factors by studying matched air and settled dust samples taken from various homes, at varying levels of activity in the home. The Tier I level was based on the K factor for a worst case scenario of a high level of activity. Although K. factors are based on activity data for asbestos, they are believed to be protective for fibrous glass, which is larger and less airborne. In addition, although K factors are not rigorous enough to rely upon for a firm health decision, they are used here only to define what might constitute elevated fiber counts, that would need to be addressed as a near-term concern. This approach may overestimate potential exposures, but as such, is protective.

The Tier I level for fibrous glass in indoor air was based on the American Conference of Governmental Industrial Hygienists’ (ACGIH) threshold limit value (TLV) of 1 f/cc. That TLV is based on limiting irritant effects of fibrous glass on workers. The TLV was divided by a factor of ten to account for the different exposure durations between ACGIH’s workers and the WTC area’s residents.

The Tier III level for fibrous glass in indoor air was based on the ACGIH TLV of 1 f/cc, divided by a factor of 100 to account for greater exposure and the different sensitivities between ACGIH’s healthy works and the WTC area’s more varied population.

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