Risk Assessment:Risk Assessment:A Conceptual A Conceptual IntroductionIntroduction

Tee L. Guidotti

Occupational Health Program

University of Alberta

Risk Assessment

• Risk assessment is the systematic evaluation of the likelihood of an adverse effect arising from exposure in a defined population.

• Risk assessment is “a systematic process for describing and qualifying the risks associated with hazardous substances, action or events.” (Covello)

Other Important Terms

• Hazard– the intrinsic potential for harm– toxicity, injury, damage, cost– specific to an exposure level

• Risk– probability of (adverse) outcome

• Opportunity– likelihood that significant exposure will occur

A Definition of Risk

Risk = (H, E, O, P)

Hazard Exposure Opportunity Population (at Risk)

Risk Analysis, Risk Management

• Risk Analysis– Hazard identification – Risk assessment – Risk evaluation

• Risk Management– Option generation

– Option evaluation

– Option selection

– Implementation and enforcement

Types of Risk Assessment

Qualitative• knowledge of hazard• knowledge of exposure

opportunity• estimates of exposure

magnitude• knowledge of exposure-

response• informed judgement

Quantitative• quantification of hazard• estimates of population

at risk• exposure assessment• known or extrapolated

exposure-response• estimate of risk

Steps in Risk Assessment - 1

1 Release assessment– potential of emission, effluent, contact– pathways of exposure– subject to change

2 Exposure assessment– intensity– frequency– duration

Steps in Risk Assessment - 2

3 Consequence assessment– human health-related outcomes– ecological impact– psychological impact– cost impact

4 Risk estimation– probability of adverse effect for individual– probability of adverse effect in population

Step by Step through Risk Assessment

Step 1. Release Assessment - 1

Identification of the characteristics of “risk source” or “risk agent” that may predict failure or release:

• monitoring (recording past experience)• performance (simulation or experiential)• testing (systematic trials)• incident evaluation (detailed investigation)• modeling (e.g. fault trees , failure analysis)

Step 1. Release Assessment - 2

Outcomes:• probability of failure or exposure event• understanding of how this may happen• identification of critical faults or failures

Problems:• analysis is counterintuitive, complicated• may miss multifactoral outcomes• assumes a perfect understanding of the system

Step 2. Exposure Assessment - 1

Essential elements:• pathway and route of exposure• “fate and disposition”, dilution, degradation• distribution, duration and magnitude of exposures at

point of contact• multiple exposures and multiple sources of exposure• susceptible populations

Step 2. Exposure Assessment - 2

Methods:• Direct measurement

– environmental measurement

– personal measurement

– biological exposure monitoring

• Indirect measurement– biological effect monitoring

– biomarkers of exposure (narrowly defined)

– surveys, consumption records

Step 2. Exposure Assessment - 3

• Indirect Exposure Estimation– extrapolation, ecological association– categorization (exposure matrices)– simulation– surrogate measurements– modeling

• surface water

• groundwater

• air dispersion

Step 3. Consequence AssessmentGeneral framework:

Probability ofOutcome/Magnitude ofOutcome

High Low

High Typically astructuralproblem

Unusualcatastrophicevents

Low Commonminorproblem

Seldom aconcern

Step 3. Consequence Assessment

• Identify plausible outcomes associated with exposure levels implied by exposure assessment

• Consider susceptible populations and atypical responses

• Consider health effects as: acute, chronic, lethal

• Interest in subclinical effects growing

Step 3. Consequence Assessment

Key information in Consequence Assessment:• exposure-response relationships

– extrapolation from animal studies

– toxicological exposure-response relationship

– epidemiologic exposure-response relationship

– “low-dose” extrapolation problem

• stochastic effects• modeling

Step 4. Risk Estimation - 1

Expressions of risk estimates:• most likely, nominal, “best guess”• worst case, conservative assumptions• range of probabilities

Analyses of sensitivity:• critical assumptions• behaviour of model (e.g. Monte Carlo)• multiple models, assess convergence of prediction

Step 4. Risk Estimation - 2

Assumptions:• Human (epidemiological) data preferable to

extrapolated data from animal studies• If animal data must be used, correct for BSA• Conservative assumptions; 95% UL• Humans considered as sensitive as most sensitive

known animal species• Linear, non-threshold model is the UL of response

Step 4. Risk Estimation - 3

A generic mathematical model:

• Correcting for BSA, assume B = slope

• B = p/d, p = 95% UL frequency, d = dose

• Exposure assessment derives estimate for d, based on 70 y lifetime

• Risk = Bd, if set at 10-6, then derive

• D (allowable) = 106•B per lifetime

Some Problems of Risk Assessment

• Quantification (conversion to lives, $, P)

• Comparability

• Exposure-response relationships at low levels

• Chronic low-level v. rare catastrophes

• Uncertainty bands

What Determines When a Risk is Acceptable?

Acceptability of Risk - 1

• Catastrophic potential –

• Familiarity +

• Understanding of risk +

• Scientific uncertainty –

• Controllability + (may be an illusion!)

• Voluntariness +

• Obvious benefit to community +

Acceptability of Risk - 2

• Impact on children, future generations –

• Victim identity – (put a face to the number)

• Dread –

• Institutional trust + (few institutions trusted)

• Media attention –

• History +

• Equity + (inequity, maldistribution of risk –)

Acceptability of Risk - 3

• Reversibility +

• Personal or family stake in outcome –

• Attributability – (responsible party)

Also:

• Cultural differences in acceptability ±

• Outrage (Sandeman), distinct from equity –

• Profundity – (threat to social stability)

Utility of Risk Assessment

• Facilitates communication

• Framework for critical review of data, assumptions

• Allows problem to be disaggregated into component parts

• Gaps analysis

• Comparison of policy options

• Risk comparison

Standardization of Risk Assessment

• U.S. federal approach– EPA– NAS/NRC/IOM– Interagency Regulatory Liaison Group: EPA,

OSHA, CPSC, FDA, FSQS (now defunct)

• Health Canada, Health Protection Branch

• European Community

• World Health Organization