Aggregate risk

Note that most societal F-N curves extend out to hundreds or even thousands of fatalities. Such events involving process plant buildings are extremely unlikely, since few buildings, if any, within process facilities have such large concentrations of people. Aggregate risk curves should reflect site-specific conditions and realistic events. 

The company has developed internal aggregate risk criteria. A comparison of these criteria with the calculated aggregate risk for personnel occupying the cafeteria shown in Figure 4.7 indicates the calculated aggregate risk curve is above the upper limit of tolerability for high values of N. 

On simple inspection, it appears that the risks presented by the cafeteria exceed the company criteria. However, F-N curves are not always the most effective tools for identifying the major contributors to the risk. More detailed analysis, or use of other aggregate risk methodologies, such as determining expected numbers of fatalities per year, may reveal the risk to be more (or less) tolerable than that shown by the F-N curves. 

For screening purposes, however, the analysis has shown that a building of low blast resistance and high episodic occupancy, and that is potentially impacted by three different process units may present an undue aggregate risk and should be evaluated further. In retrospect, this conclusion should have been obvious without the need to resort to the risk-screening analysis. 

Section 4.2.2 discussed the use of methods other than F-N curves for determining aggregate risk. An aggregate risk measure can be calculated for Example 10 by multiplying each incident frequency by the expected number of serious injuries or fatalities. This results in a parameter of "fatalities per year."... 

Summing the expected fatalities results in a total of 6.46 x 10 3 expected fatalities per year for the occupants of the cafeteria. This aggregate risk index can be inverted ... 

Buildings 2 and 3 require further evaluation after the consequence screening is completed. Risk screening can be performed first by calculating the maximum individual risk and then by determining the aggregate risk for occupants of the buildings. 

As discussed in Chapter 4, one approach to determining aggregate risk involves arranging the above data in a table, as follows ... 

Figure 6.3 Aggregate risks for occupants of Building 2—before and after risk reduction.

Occupant vulnerabilities are one of the key factors in determining the levels of both individual and aggregate risks for building occupants. Occupant vulnerabilities are used in risk screening as discussed in Chapter 4 and are used in detailed risk assessments. 

Risk screening relies upon generic frequency data to provide conservative approximations of individual and aggregate risks. Table 4.9 offers an example of generic frequencies for major explosions for certain types of refinery process units. A methodology is also offered to predict expected event frequencies where generic data are not available. 

Aggregate risk is used to measure the collective risk to people in a facility who could be exposed to an event or events. It indicates the frequency that a specified number of people will suffer a specific level of harm (e.g., death). Just as is the case with societal risk, aggregate risk can be expressed in terms of the frequency distribution of multiple-casualty events. 

The individual or aggregate risk to personnel in plant buildings... 

Since the aggregate risks from Process Unit 2 are largely the result of single event—brittle fracture—the qualitative hazard assessment identified potential safeguards that could be put in place to prevent a brittle fracture occurrence. It was decided that the best option would be to install an emergency shutdown system in Process Unit 2 to prevent pressuring the nitrogen vapor vessel if a cold temperature situation was present. 

Prior to installing a new shutdown system, however, a fault tree analysis was performed on the proposed modifications. From this study, it was concluded that the overall frequency of brittle fracture was lowered from 5x10"4 to 5 x 10-5 (occurrences/year). Using this new frequency in the calculation for aggregate risk would result in revised outcome frequencies and F-N data points, as shown below. 

Figures 6.3 and 6.4 show the adjusted F-N curves. With this risk reduction, the company s aggregate risk criteria would be met Therefore, the emergency shutdown system was installed and no further action was required.

For companies to make risk-based decisions from risk screening or quantitative risk assessments, company-specific risk tolerance criteria or methodologies should be developed for both individual risk and aggregate risk. Chapter 4 provides a discussion of risk criteria and methodologies, and offers guidance for companies to consider in developing their own approaches to risk tolerability. 

Aggregate risk A measure of the risk to personnel within a building or buildings or within a facility who are impacted by the same events, taking into account the total time spent in the building(s) or facility. (Aggregate risk is "societal risk" applied to a specific group of people within a facility.)... 

Although selection of the appropriate analysis techniques is often very problem specific, the basic elements of human health risk analysis are few, as presented in Figure 1. The figure shows that the aggregate risk to human health from exposure to an airborne pollutant results from two factors (1) the spread of the primary agent (and/or its... 

TEFs were also used by the NRC Committee on Pesticides in the Diet of Infants and Children to estimate the aggregate risk to children from dietary exposure to a mixture of pesticides (NRC 1993). 

The cumulative risk index (CRI), also referred to as the aggregate risk index (ARI) has been suggested by the US-EPA (Whalan and Pettigrew 1997) to combine MOEs for chemicals with different AFs. The risk index (RI) of a chemical is the MOE divided by the AF or simply the reference dose divided by exposure, and is the reciprocal of the HQ ... 

To conclude, the bureaucratic regulation of pollution has the possibility of creating a result similar to the pollution levels provided by an emissions market if the bureaucracy maximizes expected years of life saved for a given cost. However, if health-related issues evoke an emotional response not evoked by cellular phone rights, the concept of efficient aggregate risk reduction itself becomes problematic because it could be determined by the ownership of initial property rights. 

Global specialty chemicals companies already operate plants in many different countries and have the expertise to navigate in the respective business environments. Therefore, typically political risks are not incorporated into quantitative models but considered when selecting the potential investment candidate countries. If desired, the model can be extended to include political risk based on an aggregate risk parameter to analyze risk/return profiles for alternative network configurations. For a more detailed discussion of the major elements of political risk see Appendix 3. 

The methodology in the case study for chronic exposure, as well as several advances in probabilistic assessment methodology for acute exposure (e.g., a person s exposure on a single day), are being incorporated into the Cumulative and Aggregate Risk Evaluation System (CARES) begun in 2000 and being further developed with the International Life Sciences Institute (ILSI) in 2004. 

ILSI (2004). International Life Science Institute. Cumulative and Aggregate Risk Evaluation System (CARES ) Research Foundation Project. 

Aggregate risk assessment The evaluation of risk from exposure to a given chemical from multiple routes of exposure. 

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