NYC-WTC_000164840

NYC 9/11 Public Portal Document

calculating an airborne asbestos concentration that relates to an upper-bound estimate of exposure duration and acceptable risk level.

J) Quantitative Risk Based on PCM or PCM Equivalence

Quantitative risk assessment based on PCM assigns cancer potency to fibers greater than 5 um in length with an aspect ratio >3:1. The AHERA standard directs that all fibers greater than 0.5 um in length be counted. Therefore, the risk associated with fibers counts performed as per AHERA is a function of fiber length distribution with only those fibers > 5 um contributing to risk (i.e., PCM equivalence, or PCME).

A worst-case scenario risk analysis can be performed on the AHERA standard of 70 S/mm2 by assuming that the entire distribution of fibers is > 5 um. An additional element of conservatism is provided by including in the count extremely thin fibers (<0.25 um width). Long fibers with width <0.25 um are, generally, not visualized by PCM.

2) Measurement Reporting Units: Conversion from f/mm2 to f/cc

To accommodate a quantitative risk assessment paradigm, the AHERA standard must also be converted from f/mm2 to f/cc. As per the AHERA sampling criteria (i.e., sample volume of 1,200 L, filter diameter of 25 mm), the standard of 70 f/mm2 converts to 0.022 f/cc as demonstrated below:

70 f X 385 mm2 (effective filter area of 25 itmii filter) 1,200 L X 1,000 cc/L

3) WTC Screening Levels

The draft risk-based screening analysis for WTC assumes an upper bound (continuous) exposure estimate of one year duration. Based on the IRIS inhalation unit risk of 2.3 E-1 per (f/cc), excess lifetime cancer risk of 1 E-04 (one-in ten thousand) for a 70 year exposure duration equates to an asbestos concentration of .0004 f/cc. Adjusting the exposure duration to 1 year at a 1 E-04 cancer risk (target risk used for WTC screening values) yields an airborne asbestos concentration of 0.028 f/cc.

Conclusion

The above analysis indicates that continuous exposure to 70 f/mm2 (under the worst case assumption that all fibers >5 um) for a one year exposure duration would result in less than a one-in ten thousand excess lifetime cancer risk.

It is worth noting that the unit (f/mm2 to f/cc) conversion of the AHERA standard is a function of sample volume. The relationship is inverse. That is, the lower the sample volume the higher the asbestos concentration (f/cc). Thus, if a sample volume is sufficiently less than 1,200 L, the AHERA standard of 70 f/mm2 would convert to an airborne asbestos concentration that exceeds

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