risk assessment of extreme events

Columbia-Wharton/Penn Roundtable 2

Risk Assessment of

Extreme Events

Rae Zimmerman

(New York University)

Vicki M. Bier

(University of Wisconsin-Madison)


I. Introduction and Scope

• Risk assessment is a means to characterize and reduce uncertainty to support our ability to deal with catastrophe

• Scope of this paper: – Application of risk assessment to both the

built and natural environments under extreme events

– Understanding and management of human health, safety, and security


I. Introduction and Scope (cont.)

• Modern risk assessment for engineering began with Reactor Safety Study (1975):– Applications to engineered systems and

infrastructure are common

• Applications to chemical risks under dozens of federal environmental statutes:– E.g., drinking water, ambient water quality, and

air quality standards– Review and renewal of pesticide applications– Levels of site cleanup under Superfund


II. What is Risk Assessment?

• Definition of risk assessment: “A systematic approach to organizing

and analyzing scientific knowledge and information for potentially hazardous activities or for substances that might pose risks under specified circumstances”

National Research Council (NRC), 1994


II.A Definitions of Risk

• “Both uncertainty and some kind of loss or damage” (Kaplan and Garrick 1981)

• “The potential for realization of unwanted, negative consequences of an event” (Rowe 1976)

• “The probability per unit time of the occurrence of a unit cost burden” (Sage and White 1980)

• “The likelihood that a vulnerability will be exploited” (NRC 2002)


II.A Definitions of Risk (cont.)

• Terms to characterize acceptable risk in health and safety legislation: – Adequate– Imminent– Substantial– Reasonable (vs. unreasonable)– Posing grave danger– At a zero level– Significant (vs. de minimus)– An ample or adequate margin of safety


II.B Relationship of Risk to Other Concepts

• Merriam-Webster’s Collegiate Dictionary 2002:– Hazard (“a source of danger”) – Catastrophe (“a momentous tragic event”) – Chronic (“long duration or frequent recurrence”)

• NRC 2002: Threat (“an adversary”)– Vulnerability (“an error or a weakness”)

• Extreme events (low frequency and high severity)• Counter-expected events (believed to be unlikely)• Unexpected events (not even anticipated)• Uncertainty (lack of knowledge)• Variability (differences among a population)


II.C Paradigms for Risk Assessment

• A form of systems analysis• Answers three questions (Kaplan and

Garrick 1981): – “What can go wrong?” – “How likely is it that that will happen?” – “If it does happen, what are the

consequences?”

• Several integrated risk assessment/risk management frameworks have been proposed


II.C Paradigms for Risk Assessment (cont.)

• “Deliberation frames analysis and analysis informs deliberation” (Stern and Fineberg 1996): – The combination of these two steps is

termed the “analytic-deliberative” process

– An iterative process – Deliberation and analysis are viewed as

complementary


III.A Health Risk Assessment

• Hazard identification

• Risk estimation:– Exposure assessment– Dose/response relationships (toxicity

assessment)– Risk characterization or risk calculation


III.A Health Risk Assessment (cont.)

• Hazard identification: – Structure activity relationships

(structural toxicology)– Case clusters– Epidemiological studies– Experimental chemical tests on lower

order organisms (rapid screening)– Animal tests



• Exposure assessment:– Sources, pathways, and sinks (or

receptors)– Health effects assessment



• Sources, pathways, and sinks (receptors):– Source characterization (substances released,

rates of release, temporal variations, location)– Fate and transport– Routes or pathways of exposure from

environmental end points to human organisms– Size, type, and sensitivity of population at risk



– Health effects assessment:• Dose estimates or intake levels• Absorption by the body• General toxicity of the risk agent in the body

(e.g., target organs, types of effects)• State of health of the organism



• Dose/response relationships (toxicity assessment):– Dose/response models– Empirical relationships between levels

of exposure and effects



• Risk characterization or calculation:– Risk estimate– Characterization of uncertainties,

assumptions, and data quality


IIIB Engineering Risk Assessment

• Hazard identification • Assessment of accident occurrence

frequencies • Consequence analysis • Risk characterization• Uncertainty analysis


III.B Engineering Risk Assessment (cont.)

• Hazard identification:– System familiarization– Hazard and operability studies – Failure modes and effects analysis



• Assessment of accident occurrence frequencies:



• Consequence analysis has two stages: – Migration of hazardous materials from sources

to sinks– Consequences of those materials for public

health and safety

• Relevant consequence measures include: – Structural response of a building – Costs of property damage, loss of use, repair– Amount of hazardous material released – Numbers of fatalities or other health effects



• Risk characterization:– Results presented graphically

– Probability distribution, complementary cumulative


III.C Spatial Dimensions

• Proximity is a key factor in the exposure portion of the risk equation

• Proximity can also affect: – Perceived severity of particular

scenarios– Conditional failure probabilities


III.C Spatial Dimensions

• Despite this, risk analyses rarely use sophisticated spatial concepts or models:– Methodology for doing so tends to be

ad hoc– Takes little advantage of GIS systems


IV. Understanding Uncertainty

• Sources of uncertainty:– Statistical variation – Systematic error – Subjective judgment – Linguistic imprecision – Variability – Inherent randomness or unpredictability– Disagreement – Approximation


IV. Understanding Uncertainty (cont.)

• Uncertainty and variability have different implications for decision-making (NRC 1994):– “Uncertainty forces decision makers to judge

how probable it is that risks will be overestimated or underestimated”

– “Variability forces them to cope with the certainty that different individuals will be subjected to [different] risks”

• Large uncertainty suggests that further research may be desirable


V. Human Perceptions, Behavior, and Performance

• Evacuation responses in emergencies differ substantially from performance in tests and simulations

• Behavioral assumptions underlying many building codes and strategies are flawed

• Human behavior is extremely variable:– Healthy versus elderly, ill, or disabled– Familiarity with a particular environment

• Predicting the behavior of the public is a difficult challenge


V. Human Perceptions, Behavior, Performance (cont.)

• Intentional hazards:– Estimating the likelihood and nature of

intentional attacks “is needed for intelligent benefit-cost analysis” (Woo 2002)

• Protection from an adversary is different than protection against accidents:– Adversaries can choose to attack targets that

have not been hardened – Defensive measures may be less effective if

they are known – Optimal strategy depends on attacker behavior


VI. World Trade Center Disaster

• Unexpected or counter-expected• Past experiences could have helped to

identify risk of an attack (Barnett 2001):– “Lots of events…could be interpreted as

precursors of the calamity”– “All the elements of the Sept. 11 catastrophe…

had historical precedent”

• This points out the need for:– Methods of learning from past experience – Vigilance to signs of problems


VII. Conclusions

• Risk assessment is a vital tool for dealing with extreme events

• Capabilities of risk assessment are challenged when we attempt to apply it to extreme and unanticipated events

• Need for methodological improvements to more fully incorporate:– Spatial dimensions– Human values, attitudes, beliefs, and behavior– Past experience


Acknowledgments

• This material is based upon work supported in part by:– The U.S. Army Research Laboratory and the

U.S. Army Research Office under grant number DAAD19-01-1-0502

– The National Science Foundation under Cooperative Agreement No. CMS-9728805

• Any opinions, findings, conclusions, or recommendations expressed in this document are those of the authors

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