Pacific Southwest, Region 9
Serving: Arizona, California, Hawaii, Nevada, Pacific Islands, Tribal Nations
Naturally Occurring Asbestos
Clear Creek Management Area
What Do The Results Mean?
There was no combination of scenario, toxicity value, or visits per year that were below the EPA acceptable risk range of 1 in 1,000,000. Using the IRIS model, as shown in Figure 6, EPA’s risk estimations found that, with the exception of Scenario 3 Day Use Hiker, making five or more visits to CCMA per year over a 30-year period would put recreational users above the 10-4 risk range (1 in 10,000). Only Scenario 3 (Day Use Hiking) had risk calculations within the acceptable range. The highest IRIS risk estimations, 2 x 10-3 (2 in 1,000), were calculated using the 95% UCL exposure concentration for 12 visits per year for recreational Scenario 1 and 120 visits per year for worker Scenario 7 (SUV Patrol).
Figure 6 – Adult Cancer Risk, Scenarios 1 – 7: Mean and 95% UCL Exposures Using IRIS Unit Risk
Using the OEHHA model, even one visit per year for recreational scenarios 1, 2, 4, and 5, creates a risk that exceeds EPA’s acceptable range (Figure 7). The higher risks reflect the fact that the OEHHA asbestos toxicity value is 8 times higher than the value in IRIS. At the high end of the risk range, excess lifetime cancer risk estimations using the OEHHA model and the 95% UCL concentration indicate that recreational users riding motorcycles 12 weekends per year (Scenario 1), and workers performing SUV patrol duties at CCMA (Scenario 7) for 120 days per year during a 30-year career, could have as much as a 1 in 100 (1 x 10-2) chance of developing asbestos-related cancer. It should be noted that neither the IRIS nor OEHHA models are designed for very high exposure levels, so the absolute number calculated for the high-end risk has a higher degree of uncertainty than the numbers calculated for the lower exposure scenarios. However, the risks are still extremely high.
Figure 7 – Adult Cancer Risk, Scenarios 1 – 7: Mean and 95% UCL Exposures Using OEHHA Unit Risk
The Child/Adult estimations using the IRIS model found that five or more visits per year for Scenarios 1 through 4 was above the 10-4 risk range (Figure 8) and all visits were above the acceptable range using the OEHHA model (Figure 9).
Figure 8 – Child/Adult Cancer Risk, Scenarios 1 – 4: Mean and 95% UCL Exposures Using IRIS Unit Risk
Figure 9 – Child/Adult Cancer Risk, Scenarios 1 – 4: Mean and 95% UCL Exposures Using OEHHA Unit Risk
For the Child risks, which were calculated for a 12-year exposure from ages 6 to 18, less than five visits per year for Scenarios 1 and 2, one, five, and twelve visits for Scenario 3, and one and five visits per year for Scenario 4 were within the acceptable risk range using IRIS (Figure 10). Using the OEHHA model, only less than five visits per year for Scenario 3 Day Use Hiker was within the acceptable range (Figure 11).
Figure 10 – Child Cancer Risk, Scenarios 1 – 4: Mean and 95% UCL Exposures Using IRIS Unit Risk
Figure 11 – Child Cancer Risk, Scenarios 1 – 4: Mean and 95% UCL Exposures Using OEHHA Unit Risk
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