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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

Risk Assessment


Seven typical CCMA use scenarios were created from the individual activities for which EPA collected air samples.  Risk estimate calculations were then conducted for the scenarios.  The scenarios were designed to make the risk estimations better reflect typical CCMA use patterns and provide more useable information to Bureau of Land Management (BLM) and the public.  The scenarios were developed with input from BLM and the California Department of Toxic Substances Control.  Five of the seven scenarios represent recreational/volunteer use of CCMA, and two represent typical worker use.  The five recreational scenarios are:

  • Scenario 1 Weekend Rider:  Drive in, motorcycle on Saturday, camp on Saturday, sleep in tent, camp on Sunday, motorcycle on Sunday, drive out, vehicle wash, vehicle vacuum.
  • Scenario 2 Day Use Rider:  Drive in, stage (prepare for riding), ATV or motorcycle riding, stage, drive out, vehicle wash, vehicle vacuum.
  • Scenario 3 Day Use Hiker:  Drive in, stage, hike, stage, drive out.
  • Scenario 4 Weekend Hunter:  Drive in, hike/hunt on Saturday, camp on Saturday, sleep in tent, camp on Sunday, hike/hunt on Sunday, drive out, vehicle wash, vehicle vacuum.
  • Scenario 5 Combined Rider/Workday:  Drive in, stage, ATV or motorcycle riding, fence building/repair, stage, drive out, vehicle wash, vehicle vacuum. 

The typical worker scenarios are:

  • Scenario 6 Patrol:  Stage at Section 8, drive in and stage at CCMA (lead SUV only, ATV or motorcycle patrolling (lead rider only), stage and drive out(lead SUV only), vehicle wash, vehicle vacuum, unpacking at Section 8. 
  • Scenario 7 SUV/Truck Patrol:  SUV/truck patrol (lead SUV only), vehicle wash, vehicle vacuum.

Risk Assessment Methods - Excess Lifetime Cancer Risk estimates were calculated for the scenarios using both the U.S. EPA Integrated Risk Information System (IRIS) and the California EPA Office of Environmental Health Hazard Assessment (OEHHA) toxicity values for asbestos. These are standard methods for estimating risk.

Adult, Child, and Child/Adult Risk Estimates - Consistent with the EPA Risk Assessment Guidance for Superfund (RAGS), a 30-year exposure duration was used for estimating excess cancer risks from the CCMA adult recreational and worker exposures.  The risk assessment estimates risks for an adult who visits CCMA for 30 years, a child who visits for 12 years (ages 6 to 18) with his/her parents and then continues to visit for an additional 18 years as an adult (30 years total exposure), and a child who visits for 12 years from ages 6 to 18.

CCMA Use Frequency - The EPA RAGS guidance requires that risks be estimated for the reasonable maximum exposure (RME) that is expected to occur at a site under both current and future land-use conditions.  Based on surveys and interviews, an earlier risk assessment conducted by BLM estimated a CCMA recreational RME of 5 off-road vehicle rides a year.  Because some users indicated that they rode more frequently, the BLM assessment also used a “high” estimate of 12 days per year.  Risks were also calculated for one-day per year to provide a range of estimates and exposures.  The EPA risk assessment incorporates the 1, 5, and 12 visit per year frequency of the earlier BLM assessment for Scenarios 1 through 5 and, at BLM’s request, uses a 1, 60, and 120 day per year frequency for the worker scenarios, Scenarios 6 and 7.  

Risk Assessment Results - Excess Lifetime Cancer Risk estimates for Adult, Adult/Child, and Child exposures using the U.S. EPA IRIS risk model are shown in Figures 6 through 8.  The Excess Lifetime Cancer Risk estimates using the Cal/EPA OEHHA model are shown in Figures 9 through 11.  For reasons that are explained in more detail in the risk assessment report, the OEHHA toxicity value for asbestos is eight times higher than the IRIS value, and the OEHHA risk calculations reflect the greater toxicity.  The IRIS and OEHHA risk estimates can be thought of as bracketing the range of possible risks from CCMA asbestos exposure. 

The EPA Superfund program defines the acceptable risk range for exposure to a carcinogen, like asbestos, as 10-4 (1 in 10,000) to 10-6 (1 in 1,000,000) excess lifetime cancer risk.  Exposures which are calculated to cause more than 1 in 10,000 excess cancers are considered to be of concern and may require action to reduce the exposure and resulting risk.  It is important to note that the risk assessment present quantitative estimates of excess cancer risk over a lifetime in a population based on the defined exposure scenarios.  The scenarios have been designed to represent current and future exposures for recreational and working users of CCMA.  The numbers do not predict individual exposures or individual health outcomes.

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