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Pacific Southwest, Region 9

Serving: Arizona, California, Hawaii, Nevada, Pacific Islands, Tribal Nations

Naturally Occurring Asbestos

Clear Creek Management Area

Sampling Photos | Study Area

Limitations of the Assessment

With any assessment of risk, there are assumptions and variables that can cause the calculations to either overestimate or underestimate the actual risk.  The CCMA risk assessment report contains a more detailed discussion of the exposure and toxicity parameters which affect the calculations of estimated risk. 

The CCMA assessment may overestimate or underestimate risk if EPA’s measurements of exposure and the assumptions of exposure frequency are either greater or less than actual conditions.  Additional uncertainty is introduced because both the Inegrated Risk Information System (IRIS) and the OEHHA toxicity values for asbestos are based on epidemiological studies of work place exposures to intermittent high asbestos concentrations over extended periods.  While the concentrations measured for activities at CCMA are significantly elevated, the exposure is infrequent and episodic.  Because there is no clear mode of action for asbestos-induced disease and no threshold for cancer health effects, using a direct time-weighted extrapolation from the longer, chronic occupational exposures to shorter-term, episodic exposures may underestimate or overestimate the risk.  The risks could be much lower because the exposures may be too infrequent or the total retained fiber burden too few to initiate the asbestos disease process. 

On the other hand, EPA risk calculations may underestimate the risk because take-home exposures and non-cancer health effects were not considered.  Asbestos can adhere to equipment, clothes, and the interior and exterior of vehicles, and can be tracked out of CCMA, resulting in future exposures to CCMA users, families, and communities.  The off-site exposure could increase the risk, proportional to the time of exposure and the concentration of asbestos tracked off-site.  Perhaps most important, there is currently no reference value for calculating non-cancer risks from asbestos exposures and non-cancer risks were therefore not addressed in the EPA assessment.  However, epidemiological studies indicate that non-cancer respiratory health effects from exposure to asbestos can be significant and in some studies exceed the cancer cases.  Therefore, the general probability of developing disease from exposure related to activities at Clear Creek may be significantly underestimated in EPA risk estimations.
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