Individual risk measures

Risk Estimation There are a number of risk measures which can be estimated. The specific risk measures chosen are generally related to the study objective and depth of study, and any preferences or requirements established by the decision makers. Generally, risk measures can be broken down into three categories risk indices, individual risk measures, and societal risk measures. 

Ways of combining information on likelihood with the magnitude of loss or injury (e.g., risk indices, individual risk measures, and societal risk measures)... 

Individual Risk expresses the risk to a person exposed to a hazard. It is normally calculated as the frequency of serious or fatal injuries per year (fatalities/year). Three of the more common individual risk measurements are ... 

In evaluating measures of individual risk, it is necessary to specify at the outset whether the calculated frequencies relate to those most at risk from a given activity (for example, as a result of their location, habits, or time periods for which they remain vulnerable) or whether they relate to the average value, which is representative of all potentially affected individuals. The former individual risk measure concerns maximum risk levels for an individual or a group such as populations located along the transportation route. 

Through the approach used, it was possible to present a rich analysis which considers all risk dimensions and so instead of analyzing only individual risk measures or frequencies related to a specific consequence, it provides a ranking based on the multidimensional risk of each section of this gas pipeline. 

Results of assessment are individual risk per one transport and societal risk per one transport. Individual risk measures the risk to a single person along the transport route. Societal risk considers the overall likelihood of dead fraction of population distributed along the transport route and can be used for comparison of several possible transport routes (Vodnarek 2007). 

Biologieal exposure indiees (BEI) published by the ACGIH are given in Table 5.13. BEIs represent the levels of determinant whieh are most likely to be observed in speeimens eolleeted from a healthy worker who has been exposed to ehemieals to the same extent as a worker with inhalation exposure to the TLV. Due to biologieal variability it is possible for an individual s measurements to exeeed the BEI without ineuning inereased health risk. If, however, levels in speeimens obtained from a worker on different oeeasions persistently exeeed the BEI, or if the majority of levels in speeimens obtained from a group of workers at the same workplaee exeeed the BEI, the eause of the exeessive values must be investigated and proper aetion taken to reduee the exposure. 

Individual Risk - This provides a measure of tlie risk to a person in tlie vicinity of a liazard/accideiit, including tlie nature of the injury or otlier undesired outcomes, and tlie likelihood of occurrence. Individual risk is generally expressed in terms of a likelihood or probability of a specified undesired outcome per unit of time. For example (as indicated above), tlie... 

Societal Risk - This represents a measure of the risk to a group of people, including tlie risk of incidents potentially affecting more tlian one person. Individual risk (see above) is generally not significantly affected by the number of people involved in an incident. The risk to a person at a particular location depends on tlie probability of occurrence of the luizardous event, and on the probability of an adverse imptict at that location should the event occur. 

Zero sampling error Eq. (1.6) reduces to Vreprod = V repeat-Independent individual samples/measurements in the sense of Fig. 1.5. A result that is nearer to the SL(/ and an accepted risk of 5% (alpha/2 = 0.05 for the single-sided test use the alpha = 0.1 column in the t-table). 

Risk can be measured and expressed in a number of ways. CCPS s Guidelines for Chemical Process Quantitative Risk Analysis (Ref. 4) identifies three main categories of risk measure Risk Indices, Individual Risk, and Societal Risk. 

Population and individual risk can be determined in the same manner as discussed in Chapter 4. These risk measurements can then be compared with risk tolerance criteria, or decision methodologies can be used, to assist in making risk-reduction decisions about process plant buildings. 

Average Individual Risk (total population), defined as the individual risk over a predetermined population, whether or not people are actually exposed to the risk. If the population used is large, this risk measure can be deceiving, as it might depict very low risks while limited portions of the population in fact are exposed to high risks. 

Risk indices are single numbers or a tabulation of numbers that are correlated to the magnitude of the risk to people. Some risk indices are relative values with no specific units. The limitations on the use of indices are that they may not be an absolute criteria for accepting or rejecting the risk. Risk indices also do not communicate the same information as individual or societal risk measures. An example of risk indices is a risk ranking matrix. Table 6-4 (modified from CCPS, 1992) shows how severity and likelihood are combined to obtain risk indices. An example risk matrix is shown in Figure 6-3 (RRS, 2002). 

Average individual risk (exposed population) is the individual risk averaged over the population that is exposed to risk from the facility (e.g., all of the operators in a building, or those people within the largest incident effect zone). This risk measure is only useful if the risk is relatively uniformly distributed over the population, and can be extremely misleading if risk is not evenly distributed. 

Societal risks are single number measures, tabular sets of numbers, or graphical summaries that estimate risk to a group of people located in the effected zone of an incident. Since major incidents have the potential to affect many people, societal risk is a measure of risk to a group of people. It is most often expressed in terms of the frequency distribution of multiple casualty events, such as the F-N curve shown in Figure 6-5. The calculation of societal risk requires the same frequency and consequence information as individual risk. Additionally,... 

Individual and societal risks are different presentations of the same underlying combinations of incident frequency and consequences. Both of these measures may be of importance in assessing the benefits of risk reduction measures or in judging the acceptability of a facility in absolute terms. In general, it is impossible to derive one from the other. The underlying frequency and consequence information are the same, but individual and societal risk estimates can only be calculated directly from that basic data. A high societal risk does not necessarily mean any one person has a high individual risk and vise-versa. These are two very different criteria and should be treated separately. 

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