Risk-Based Tiering

The DHS has developed a risk-based tiering structure that will allow it to focus resources on the high-risk chemical facilities. To that end, the DHS will assign facilities to one of four risk-based tiers ranging from high (Tier 1) to low (Tier 4) risk 

Assignment of tiers is based on an assessment of the potential consequences of a snccessfnl attack on assets associated with chemicals of interest. The DHS uses information snbmitted by facilities through the Top-Screen and security vulnerability assessment processes to identify a facility s risk, which is a function of the potential impacts of an attack, the likelihood that an attack on the facility would be successful, and the likelihood that such an attack would occur at the facility. 

Facilities that complete the CSAT Top-Screen and do not meet the consequence thresholds do not need to comply with CFATS. The DHS recognizes that facilities have dedicated and invested time, resources, and capital to identify vulnerabilities and improve overall security. Facilities will be able to make use of information from these improvements. Facilities may also leverage their existing security measures to work toward compliance with CFATS, specifically the risk-based performance standards. 

The DHS considers a variety of factors in determining the appropriate tier for each high-risk facility, including information about the public health and safety risk, as well as the presence of chemicals with a critical impact on the governance mission and the economy. 

The security measures needed to satisfy the risk-based performance standards for each covered facility correspond to the security risks presented by the facility. Accordingly, facilities that present a higher risk will be required to meet more rigorous risk-based performance standards. 

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Identify and describe how security measures selected and utilized by the facility will meet or exceed each applicable performance standard for the appropriate risk-based tier for the facility. 

Covered facilities must satisfy the performance standards identified in this section. The assistant secretary will issue guidance on the application of these standards to the risk-based tiers of covered facilities, and the acceptable layering of measures used to meet these standards will vary by risk-based tier. Each covered facility must select, develop in their site security plan, and implement appropriately risk-based measures designed to satisfy the following performance standards ... 

Another example of exposure and corresponding risk reduction using Tier III methodology is provided in Table 27.4 for simazine (the triazine with the most registered uses in the United States). The Tier III analysis - using average field trial residues, realistic residue estimates in animal commodities, and percent of crop treated - showed at least a 1000-fold reduction in exposure for the United States population and for the most sensitive population subgroups as compared to the tolerance-based Tier I assessment. 

A tiered approach is also used for calculating estimated residues in animal commodities (meat, milk, and eggs), and higher-tier calculations can have a significant impact in decreasing estimates of dietary exposure and risk. The Tier III assessment for atrazine and simazine (Tables 27.3 and 27.4) is based on calculations of the estimated theoretical residue in animal commodities, whereas the Tier I assessments use tolerance values. These theoretical residues are often referred to as secondary residues. Calculations for estimating secondary residues in animal commodities are performed by constructing livestock (beef, dairy, and poultry) diets comprised of treated feed items to obtain a... 

Tier 1 uses conservative, health-protective levels to compare to site-specific data to eliminate those sites not requiring remediation beyond monitoring (the obvious monitoring-only sites). The Tier 1 risk-based screening levels (RBSLs) typically incorporate very conservative or worst-case exposure assumptions. However, many sites with concentrations exceeding the RBSLs do not necessarily pose a threat to human health. Therefore, exceeding values of RBSLs by themselves do not indicate that remedial action should be actively undertaken to protect human health and the environment. 

The data evaluation step in Tier 1 identifies the constituents of potential concern and the concentrations at which they occur in impacted media as determined by investigations conducted at the site. Most states do not require evaluation of historical data as part of the Tier 1 evaluation, although they may require submission of these data in the RBCA report as a basis for comparison (e.g., to demonstrate the decrease in constituent concentrations over time and/or demonstrate plume stability). However, the data that best represent the current environmental conditions at the site should be used in the risk-based decision-making approach. 

ASTM s risk-based corrective action (RBCA) uses a tiered approach to data collection and analysis in supporting decisions on site assessment and response to petroleum. The RBCA procedure begins with the assessment of the site (see Figure 5-2 for RBCA process flow-chart). 

Following a facility s submission of the security vulnerability assessment and its analysis by the DHS, the DHS will either confirm that a facility is high risk or inform a facility that the DHS no longer considers the facility to be high risk or subject to further regulation under CFATS. For facilities confirmed to be high risk, the DHS will communicate the final facility tier determination, and the facilities must develop and implement site security plans that satisfy the risk-based performance standards enumerated in 6 CFR 27.230. 

It is advisable, then, in a tiered approach to concentrate first on crops and activities (scenarios) that are considered to be relevant with respect to the expected level of exposure and to exclude those not relevant. Second, whether or not the toxicological properties of the product may lead to general restrictions on re-entry should be investigated. If both the likelihood of reentry and the hazard due to the toxicity of the compound cannot generally be neglected, a risk assessment over several steps should be carried out. The assessment may be based on surrogate data and "worst-case" assumptions at first and then refined, if necessary. One possible approach to a tiered evaluation procedure is presented in Figure 1. 

Data used to describe variation are ideally representative of some population of risk assessment interest. Representativeness was a focus of an earlier workshop on selection of distributions (USEPA 1998). The role of problem formulation is emphasized. In case of representativeness issues, some adjustment of the data may be possible, perhaps based on a mechanistic or statistical model. Statistical random-effects models may be useful in situations where the model includes distributions among as well as within populations. However, simple approaches may be adequate, depending on the assessment tier, such as an attempt to characterize quantitatively the consequences of assuming the data to be representative. 

Tier 2 PRA process involved developing environmental exposure data and chronic toxicity data distributions for individual POPs. The mean concentrations of POPs in local marine water measured at various locations were used as exposure data in the construction of the exposure distribution. The chronic toxicity data distribution was established based on published international acute toxicity data (LC50, EC50) on a variety of aquatic organisms tested in many jurisdictions, drawn primarily from the USEPA ECOTOX database (2002) (available at http //www.epa.gov/ ecotox). If the upper 5th centile of the measured chemical exposure data distribution did not exceed the lower 5th centile of its estimated chronic toxicity distribution, the potential ecological risk posed by the chemical was judged to be tolerable (Hall and Giddings, 2000). 

Current procedures of higher tier risk assessment are often based on the extrapolation of responses observed in relatively simple and short-term (weeks) cosm tests to structurally more complex ecosystems in the field. The predictive value of studies in small cosms (microcosms), however, depends on factors such as fate and exposure of the stressor and the sensitivity and recovery potential of the populations present. The role of cosm studies in extrapolation is discussed in more detail in Chapter 4. 

Tiering is often applied in risk assessment in order to reduce expenditures in time, money, and labor when the assessment requires only simple and possibly conservative output. Table 5.3 provides a suggested tiered approach in mixture extrapolation and is further described in the bulleted list below. The tiering is based on the way that mixture mechanisms are addressed in the approach. It is assumed that issues such as matrix and media extrapolation have been addressed according to the methods described in the pertinent chapters. 

The TU-based approaches (Tier-1) have to date most widely been used in practice. A critique of this approach is that it operates under the assumption that a linear relationship exists between risk and criterion exceedance or concentration, and that... 

Assessors should base their selection of methods on clearly defined decision criteria, and they need to communicate the results using clear and transparent language. This includes statements on the extrapolation issues that were considered but not addressed, and the magnitude and direction of the bias that may have been introduced by the extrapolation or lack thereof. In lower tiers and prospective risk assessment, this should lead to setting more appropriate UFs and ensure that lower tier approaches are more conservative than higher tier approaches. All this helps assessors to make informed decisions, on one hand, but it also allows the identification of future research needs, on the other hand, especially when methods are not available. 

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