Risk Assessment Standards
Risk assessment is a process that evaluates the risk of a hazard,
estimates the population exposed to the hazard, and then determines
the relative importance of the public health risk. To determine
the health problems caused by pollutants at a contaminated site,
scientists must calculate the exposure to the pollutants multiplied
by the toxicity of the pollutants. A risk assessment is usually
presented as a range of probabilities. For instance, the risk of
cancer from exposure to a particular chemical might be 5-10 additional
illnesses per 10,000 people. As a result of this exposure, ten more
people in every 10,000 people are likely to get cancer.
One model to help predict risk is the Risk-Based Corrective Action
(RBCA). RBCA is a framework to develop a corrective action plan
based on risk and exposure assessment. Members of American Society
for Testing and Materials (ASTM) have developed the ASTM W 1739-95
and ASTM PS 104-98 guides for RBCA. The RBCA approach to corrective
action is intended to be flexible and to meet the needs of individual
states. These guides may be purchased from ASTM by calling 610-832-9585.
The steps of the RBCA approach are as follows:
- Involve stakeholders in determining what the acceptable cancer
risk will be.
- Conduct an initial site assessment. This assessment includes
reviewing the records associated with the site, conducting a visual
site inspection, and developing a conceptual model of the site.
- Classify the site in terms of the urgency of an initial response
needed. The site can be reclassified as more information is obtained.
- A Tier I evaluation is conducted to determine the Risk-Based
Screening Levels (RBSL). These RBSLs provide an estimate of the
risks from the type of chemicals found and their concentrations,
but is not site-specific. Rather the RBSL uses a standard exposure
level for a given contaminant to make an assumption about the
site. If needed, a Tier II evaluation can be conducted. This evaluation
uses additional data for fate and transport models to help replace
assumptions with site-specific information. Site Specific Target
Levels (SSTL) replace the RBSL. SSTLs are based on site-specific
values of chemicals and exposure points. A Tier III evaluation
goes beyond the analytical models and develops numerical models.
This effort requires substantial effort and cost.
- Remedial Action is then initiated to reduce the contaminant
concentration to an acceptable level. The contaminant may be removed,
treated and contained, or engineered or controlled to eliminate
the exposure pathway.
- The site is then monitored to ensure the remedial action is
effective over time.
Risk assessments can be somewhat uncertain and guided by assumptions.
One of the reasons for the uncertainty is that there are varying
levels of exposure to a wide variety of chemicals over widely varying
environmental circumstances. With so many variables, identifying
a single "cause" of illness is extremely difficult. An example of
the differences between individuals in a risk assessment is the
increased risk of asbestos workers that smoke cigarettes. In other
words, although all who work with asbestos are at risk, those who
smoke have a significantly greater risk
For the other risk-related pages, please see:
- Steps of Assessment. This page describes
detailed descriptions of the steps in risk assessment, with emphasis
on human health risk assessment.
- Ecological Risk Assessment This
page focuses on the unique aspects of ecological risk assessment.
- Risk Management This page provides
information about plans for managing and/or preventing risk.
- Risk Communication This page provides
information about communicating with communities facing contaminated