risk assessment l9

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Qualitative Risk Assessment

Chapter-9

Date: 31 October, 2012


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Examples of some commonplace risks in the United States

Risk Lifetime risk of mortality

Cancer from cigarette smoking (one pack per day) 1:4

Death in a motor vehicle accident 2:100

Homicide 1:100

Home accident deaths 1:100

Cancer from exposure to radon in homes 3:1000

Death from hepatitis A 3:1000

Exposure to the pesticide aflatoxin in peanut butter 6:10,000

Diarrhea from rotavirus 1:10,000

Exposure to typical EPA maximum chemical contaminant

levels

1:10,0001:10,000,000

What is risk?

A controversial but inherent property of everyday life


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Why do we need risk assessment?

Standards for levels of toxic chemicals or pathogenicmicroorganisms in water or food

Analyses of contaminated sites to determine the need for actionand the extent of cleanup

Constructing what-if scenarios to compare treatmentalternatives and to set priorities for corrective action.

Evaluating existing vs. new technologies

Articulating community public health concerns

Developing consistent public health expectations amongdifferent localities


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Microbial vs. chemical risk assessment

There are some inherent differences between microbial and chemical riskassessments. Usually disease due to chemical exposure is cumulative over a

long period of exposure. In contrast, for microbes, disease may occurfollowing exposure to a single pathogen and will depend on the virulence ofthe pathogen and the susceptibility of the host. Therefore, one mustestimate a risk of infection based on different factors. (For example, therisk of infection by Pseudomonas aeruginosais very small in general but islarge in a burn unit where burn patients are very susceptible to this

opportunistic pathogen. Thus, much more stringent (and expensive)disinfection precautions are taken in the burn unit)

Voluntary vs. involuntary risk

Voluntary risk (e.g., driving a car) is always more acceptable than involuntaryrisk (e.g., consuming hamburger contaminated with E. coli).

It is generally agreed that a lifetime involuntary risk on the order of

1:1,000,000 is small enough to be acceptable or is a tolerable risk.


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Risk Assessment

Definition: The process of estimating both the probability that an event

will occur and the probable magnitude of its adverse effects over aspecified time period.

Both chemical and microbial risk assessments can be performed. Eachconsists of four basic steps:

1) Hazard identification - identify the chemical (e.g, lead) ormicrobial (e.g, Polio virus) contaminant

2) Exposure assessment

3) Dose-response assessment

4) Risk characterization


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Step 2 - Exposure assessment

The process of measuring or estimating the intensity, frequency andduration of human exposures to a chemical or microbe

Exposure pathway the path from a source to the receptor air water

Exposure route intake pathway inhalation ingestion absorption through skin

Exposure response is mediated by concentration of chemical/microbe exposure rate (magnitude, frequency, duration) receptor characteristics (body weight, genetics, immunity)


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Event trees simplify modeling the infectivity of a pathogen. The following isan example of an event tree used to estimate the human exposure toSalmonellaas a result of biosolids applied to a lettuce crop.

Raw sewage 2.9 x 107

CFU/ton

Raw sewage sludge 2.4 x 107 CFU/ton

Anaerobic digestion 2.4 x 105 CFU/ton

Dilution after incorporation into soil 2.4 x 103 CFU/ton

Decay in the soil after 5 months 2.4 x 10-2 CFU/ton

Amount transferred to lettuce 4.8 x 10-4 CFU/ton

Assume 4500 g lettuce consumed/year:Salmonellaingested/person/year 2.1 x 10-6 Salmonella ingested/year


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Step 3 - Dose-response assessment

Quantitating adverse effects from exposure based on the degreeof exposure

The goal of a dose-response assessment is to obtain a mathematicalrelationship between the amount of a toxicant/microbe involved inan exposure to the risk of an adverse outcome.

To determine the capacity of an agent to cause harm, we need toquantify toxicity or infectivity.

Dose mg chemical/body weight# microbes/exposure

Possible responses no response temporary response permanent response chronic functional impairment

death


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If one looks at the four steps of risk assessment, there isuncertainty associated with each step of the assessment. The varioussources of uncertainty include:

extrapolation from high to low doses extrapolation from animal to human responses

extrapolation from one route of exposure to another

limitations of analytical methods

estimates of exposure

In addition, one must consider vulnerable populations that may beimpacted differently than the general population by the outcome of arisk analysis.

Step 4 - Risk characterization

Estimating the potential impact of a contaminants based on theseverity of its effects and the amount of exposure.


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Uncertainty can be assessed using

sensitivity analysis the uncertain quantities of each parameterare varied to find out how changes affect the final risk estimate.

Monte Carlo simulation assumes that all parameters are randomor uncertain. The computer chooses random variations of theparameters and generates risk estimates.

The final phase of risk assessment is to integrate exposure anddose-response assessments to yield probabilities of effects.Risk analysis can be quite accurate but most risk analysis is

associated with a great deal of uncertainty.


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Example: Infectious hepatitis and viral gastroenteritis are caused by consumption ofraw or, in some cases, cooked clams and oysters. The concentration of echovirus 12 wasfound to be 8 plaque-forming units (PFU) per 100 g in oysters collected from coastalNew England waters. What are the risks of becoming infected and ill from echovirus 12if the oysters are consumed? Assume that a person usually consumes 60 g of oyster

meat in a single serving:

It has been found that a modified exponential modelworks well for microbial risk assessment: P = 1 (1 + N/)-

where: P is the probability of infection, N is the number of organisms ingested, and and are parameters characterizing the host-virus interaction from the dose-responsecurve. For this example, = 0.374, = 186.69, these parameters were estimatedfrom ingestion studies for echovirus 12.

Recall there are 8 PFU/100 g oyster and 60 g are consumed: N = 4.8 PFU consumed

Using this model for this example: P = 1 (1 + 4.8/186.69)-0.374 = 9.4 x 10-3

If the percentage of infections that result in risk of clinical illness is 50%, then therisk of clinical illness is:

Risk of clinical illness = (9.4 x 10-3)(0.50) = 4.7 x 10-3

If a person consumes oyster 10 times a year with 4.8 PFU per serving, then one cancalculate the risk of infection in 1 year:

Annual risk = PA = 1 (1 9.4 x 10-3)365 = 9.7 x 10-1


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Comparison of outbreak data to model predictions forassessment of risks associated with exposure to Salmonella

Food Dose CFU Amount

consumed

Attack rate

(%)

Predicted P

(%)Water 17 1 liter 12 12

Pancretin 200 7 doses 100 77

Ice cream 102 1 portion 52 54

Cheese 100500 28 g 2836 5398

Cheese 105 100 g 100 >99.99

Ham 106 50100 g 100 >99.99

Example 2


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Risk assessment provides an effective framework for determining therelative urgency of problems and the allocation of resources toreduce risks.

Risk assessment is used routinely to make decisions by: FDA (Food and Drug Administration) OSHA (Occupational Safety and Health Administration) EPA (Environmental Protection Agency)

These agencies use risk assessment in a variety of situations: Setting standards for chemical or pathogens in water/food Assessing risk from GEMS (genetically engineered microbes) Conducting baseline analysis of contaminated sites to determine need for

cleanup

Cost/benefit analysis Development of cleanup goals Constructing what if scenarios Evaluation of existing and new technologies for pollution prevention and

control Articulation of public health concerns

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