Societal 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. 

A common form of societal risk measure is an F-N curve, which is normally presented as a cumulative distribution plot of frequency F... 

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

Societal Risk measures the potential for incidents to affect many people. It has historically been applied as a measure of risk to the general public exposed to the same event or events. Societal risk is often presented as a frequency distribution of multiple-casualty events, called an F-N curve, showing the frequency of events F) leading to N or more fatalities. 

As will be shown in Section 4.2, published data are available on the application of societal risk measures, including the development of risk tolerability limits for F-N curves. However, much of this guidance has been developed for characterizing risks to the general public and would not normally be considered as a basis for assessing risks to on-site personnel. It is appropriate, therefore, to suggest another risk measure, similar in concept to societal risk, for on-site applications to process plant buildings ... 

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). 

F-N curve—A plot of cumulative frequency verses consequences (often expressed as number of fatalities). A societal risk measure. 

Societal risk is a measure of Uie risk to a group of people. It is most often expressed in terms of a frequency distribution of multiple casualty events and it can also be expressed in teniis similar to individual risk. Tlie calculation of societal risk requires the same frequency and consequence information as individual risk. For example, die likelihood of 10 fatalities at a specific location is a t pe of societal risk measure. Additionally, societal risk estimation requires a definition of the population at risk around tlie facility. Tliis definition can include die population tjpe (e.g., residential, industrial, school), tlie likelihood of people being present, or mitigation factors. 

If quantitative probabilistic safety criteria have been used in the development of the plant design, a comparison of the main PSA results with these criteria should be provided to demonstrate compliance. These criteria may relate to both individual and societal risk measures to ensure that all aspects of assessing the risks to the public due to the plant have been adequately considered. 

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. 

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. 

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. 

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.)... 

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. 

Societal risk—A measure of risk to a group of people. It is most often expressed in terms of the frequency distribution of multiple casualty events. 

Although there is no specific requirement to estimate group risk, the assessment principles for offshore safety cases, HSE (2006a), indicates a need for a safety case to demonstrate temporary refuge (TR) integrity -this could be considered as a measure of societal risk. 

In the Netherlands, Quantitative Risk Assessment (QRA) is used for decisions on land use planning around sites with dangerous substances, like Seveso stablishments, LPG filling stations, warehouses of dangerous substances and marshalling yards. Two measures are used the individual risk and the societal risk. 

Two measures are used in defining these policies the individual risk as a measure of the level of protection to each individual member of the public, and societal risk as a measure of the disaster potential for the society as a whole. The individual risk is expressed as the risk of fatality per year this is defined as the probability that an unprotected person residing permanently at a fixed location wiU be killed as aresult of an accident occmring at a hazardous source. The societal risk is defined as the probabilily that a certain number of victims wftl be exceeded during a single accident at a hazardous source it is expressed as the relationship between the mnnber of people killed (iV) and the frequency per year F) that this number wiU be exceeded. 

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