the risk assessment process - part ijtaken.csoft.net/school/uic/engr494/introra-10.pdf ·...

The risk assessment process - Part I

byMichael D. Cailas, Ph.D., P.E.

Associate Professor, UIC-School of Public HealthJohn Bing-Canar, Ph.D.

Data Analyst, USA-EPA Superfund Division

This presentation is based on the seminar material presented at the annual

“Semaine Européenne” at CEREVE-ENPC (Champs-sur-Marne, France)

The risk assessment process, Part 1

Presentation outline:Risk assessment concepts and definitionsRisk assessment - Systemic toxicity Risk assessment - Carcinogenic toxicityRisk assessment and water quality regulations

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The risk assessment process

Why riskassessment?

The risk assessment process

The main issue :• More than 70,000 chemicals

in common use.• Production of 200 ~ 1,000

new synthetic chemicals every year.

• Trace chemicals found in food, air, and water.

Source: Population Reports: Volume XXVI, Number 1,September, 1998.

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Risk assessment process: Key concepts

Hazard (a few definitions): “a property or situation that in particular circumstances could lead to harm" (Royal Society, 1992)A condition or situation which has the potential to create harm to people, property, or the environment

► You should be able to make the distinction between hazard and risk

Royal Society, 1992. “Risk Analysis, Perception and Management.” The Royal Society, London.

Hazard Riskpathway

Hazard vs Risk :The term “hazard” implies the existence of some threat, whereas risk implies both the existence of the threat (hazard) and its potential for realization.Risk only occurs if a potential pathway for exposure exists. That is, there must be some path between the source of the toxic substance and humans for the risk to be meaningful.


Hazard Riskpathway

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The concepts of hazard and risk are inextricably connected, due, partially,to a translational confusion (risque signifies hazard in French) and themultitude of definitions of risk/hazard in the various disciplines (e.g.,engineering, epidemiology: absolute/relative risk, etc.) . A few relative definitions of risk:

"The combination of the probability, or frequency, of occurrence of a defined hazard and the magnitude of the consequences of the occurrence" (Royal Society, 1992), thus:

Risk = hazard:consequences

Note : : Indicates combination

the probability of hazard occurrence. the potential realization of unwanted consequences of an event. Both the probability (frequency) of occurrence and the magnitude of the consequences are involved.


Within the exposure human health framework:Risk is a measure of the probability that harm will occur under defined conditions of “exposure” to a toxic substance. Within this framework, risk is a function, f, of the hazard and the exposure :

Risk = f (hazard, exposure)

Notes: The definitions which are used here are based on the NRC-EPA framework; however, the philosophical concept of risk is under debate (see for example P. Thomson (1990) “Risk Objectivism and Risk Subjectivism: When Are Risks Real?”, RISK: Health, Safety & Environment 3, 22. Unfortunately, we will not cover this issue.Exposure = Concentration, amount, or intensity of a particular agent that reaches a target system. It is usually expressed in numerical terms of substance concentration, duration, frequency, and intensity.

In the majority of definitions, risk is a probability which provides a quantitative description of the likely occurrence of a particular event (harm).Probability is conventionally expressed on a scale from zero (no possibility)to one (certainty that the harm will occur). For example:

1E-4 = 1x10-4 = 1/ 10,000 = 0.0001

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Components of Risk (human health

framework):

A major component of risk is the uncertainty about the future outcome or event.

Voluntary / Involuntary

Objective / subjective (associated with risk perception and communication).

Statistically verifiable / nonverifiable

Source: RAIS http://risk.lsd.ornl.gov/

The risk assessment process: example of“individual” risks

From: MIT Technology Review, February, 1979 Analyzing the Daily Risks of Life by R. WilsonNote: The first study attempting to make individual risk understandable. Some risks have changed (e.g., Miami drinking water)

many others remain within the same range.

Cancer caused by radiationRisk of accident by living within 5 miles of a

nuclear reactor for 50 years

Cancer from benzopyreneEating 100 charcoal broiled steaks

Cancer caused by radiationLiving 150 years within 20 miles of a nuclear power plant

Cancer caused by saccharinDrinking 30 12 oz. cans of diet sodaCancer caused by chloroformDrinking Miami drinking-water for 1 yearLiver cancer caused by aflatoxin BEating 40 tablespoons of peanut butterCancer, heart diseaseLiving 2 months with a cigarette smokerCancer caused by cosmic radiationFlying 6000 miles by jetAccidentTraveling 300 miles by carAccidentTraveling 10 miles by bicycleCirrhosis of the liverDrinking ½ L of wineCancer, heart diseaseSmoking 1.4 cigarettes

Risks which increase chance of death by 0.000001 or 1/1,000,000

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What is the death “risk” from driving an automobile?

We know that the population is, approximately, 262.2 × 106 people and that each year 41.6 × 103 die in car accidents.

Individual death rate = (41.6 × 103 ) / (262.2 × 106 )= 1.586 × 10-4

The units are (deaths/person-year) and conceptually this can be construed as the annual population risk of dying in an automobile accident (approximately, 1/6,300). The lifetime risk, if we assume 50 years of driving is:

Lifetime risk = (1.586 × 10-4) × 50 = 0.0079 = 7.9 × 10-3

Risk assessment process: example

The underlying premise for considering these rates as risks is that both refer to the frequency of an event: the occurrence of death per year, per lifetime (hazard × exposure to driving), etc. in the population due to an automobile accident.

0.19 deaths per million aircraft departures 1 in 1,568,000 169

Commercial Air Carriers

Not applicable1 in 333,000 795Bicycles

Not applicable1 in 233,0001,135Firearms

Not applicable1 in 69,000 3,837Fires

4.9 deaths per 100,000 workers1 in 42,0006,275Work Related

22 deaths per 100 million veh. miles 1 in 119,0002,222Motorcycles

1.7 deaths per 100 million veh. miles 1 in 6,300 41,616 Motor Vehicle

Risk Based on Exposure and other measures

General Population Risk Per Year 5 Yr. Average Type

A COMPARISON OF RISK: Accidental Deaths, US 1994-1998

Source : National Safety Council; Bureau of Transportation

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Exposure:

• Concentration, amount, or intensity of a particular agent that

reaches a target system (e.g., outer boundary of an

organism). It is usually expressed in numerical terms of

substance concentration, duration, frequency, and intensity

(e.g., mg/kg-day).

Dose:

• The amount of the agent which is ingested (food and water),

inhaled (air) or absorbed (dermal absorption).

Risk assessment process: Exposure/Dose definition

Schematic of dose and exposure (Oral route)

chem

ical

Mouth

Exposure Dose

AppliedDose

Intake

G.I. tract

Uptake

PotentialDose

InternalDose

Met

abol

ism

Biological Effective Dose

ORGAN

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The risk assessment process: Key concepts

Risk assessment:

The process intended to calculate or estimate the risk for a given

target system (human, ecosystem) following exposure to a

particular toxic substance, taking into account the inherent

characteristics of a substance of concern as well as the

characteristics of the specific target system. In general, the

process includes four steps: hazard identification, dose-response

assessment, exposure assessment, and risk characterization.

Hazard identification

Exposureassessment

Dose-responseassessment

Riskcharacterization

The risk assessment process: Key concepts

The risk assessment process with its four components has been formalized by the National Research Council (NRC, 1983). Since then a number of variants appeared in specialized research projects; nevertheless, federal and state agencies continue to use this process with minor alterations.In this class we will cover the risk assessment process in terms of its application for assessing:• systemic toxicity outcomes (toxic end-points other than cancer and gene

mutations), and• cancer and gene mutations end-points.

For each case the risk assessment process is different in terms of the underlying assumptions, exposure/risk measures, and the sequencing of the components.

National Research Council (NRC), 1983. “Risk Assessment in the federal government: Managing the process.” National Academies Press: Washington, DC.

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Systemic toxicity:“Chemicals that give rise to toxic end-points other than cancer and gene mutations are often referred to as "systemic toxicants" because of their effects on the function of various organ systems. In addition, chemicals that cause cancer and gene mutations also commonly evoke other toxic effects (i.e., systemic toxicity). Based on our understanding ofhomeostatic and adaptive mechanisms, systemic toxicity is treated as if there is an identifiable exposure threshold (both for the individual and for populations) below which there are no observable adverse effects. This characteristic distinguishes systemic endpoints from carcinogenic and mutagenic endpoints, which are often treated as nonthreshold

processes.” (source: EPA, IRIS http://www.epa.gov/iris/rfd.htm)

The risk assessment process: systemic toxicity

► It is very important to understand the differences between systemic andcarcinogenic toxicity.

The risk assessment process: Carcinogenic toxicityCarcinogenic toxicity (carcinogenicity):

Chemicals that give rise to carcinogenic and/or mutagenic end-points.One of the basic underlying assumptions related to carcinogenic toxicity, is that there is NO level of exposure that does not pose a small, but finite, probability of generating a carcinogenic effect. This assumption is referred to as “nonthreshold” carcinogenesis (nonthreshold model).

Dose

Res

pons

e

Observable range

threshold

Carcinogeniceffect

Nonthresholdmodel

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Schematic of a dose response curve: systemic toxicity with a threshold level

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TA is NOT toleratedTA is tolerated

experimental observations

No statistically or biologically significant indication of effect

© 2003-2005 by Michael Cailas and John Bing-Canar

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Time

Molecular exposure events Carcinogenesis

Simplified schematic of a nonthreshold cellular mechanism

Time

Molecular exposure events

Simplified schematic of a threshold cellular mechanism


Homeostatic and adaptive mechanisms

Threshold range => No effect

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Risk assessment process: simplified schematic of threshold and nonthreshold dose response curves

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Experimental observations

Risk assessment process: systemic toxicity

Components (NRC-EPA):

Hazard identification

Dose-Response assessment

Exposure assessment

Risk characterization

National Research Council (NRC), 1983. “Risk Assessment in the federal government: Managing the process.” National Academies Press: Washington, DC.

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Risk assessment process: systemic toxicity

Hazardidentification

Dose-responseassessment

Exposureassessment

Riskcharacterization

RISK ASSESSMENT

RISK MANAGEMENT

Regulatorydecision

Risk reduction options

Cost-benefitanalysis

Schematic of NRC-EPA risk assessment process

Risk assessment process: systemic toxicityNRC-EPA Risk assessment components

Hazard identification involvesdetermining whether exposure to a chemical can cause an adverse health effect and whether the adverse health effect is likely to occur in humans.

Dose-Response assessment involvesdefining the relationship between the toxic agent and the incidence of adverse health effects in the exposed population, with particular emphasis on the quantitative relation between the dose and the response (i.e., dose response model).

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Exposure assessment involves• Specifying the exposed population to the agent of

concern, identifying the route of exposure to this agent, and estimating the magnitude, frequency, duration, and timing of the doses route of exposure to the toxic agent in the that people might receive as a result of their exposure.

Risk characterization involves• The development of an estimate of the likelihood that

any of the hazards associated with the agent of concern will be realized in exposed people by taking into account all the available information.


Hazard identificationDose-response

Exposure

Risk characterization

►

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Risk assessment process: systemic toxicityHazard identification

Main sources of information :• Principal studies: studies that contribute towards the qualitative

assessment of whether or not a particular chemical is a systemic toxicant in humans. These studies are of two types: studies of human populationssuch as epidemiological studies of the incidence (number of cases) of the health effects in exposed human populations and studies using laboratory animals (e.g., acute toxicity studies, chronic and subchronic toxicity studies, target organ systems). For most chemicals, the principal studies are drawn from experiments conducted on nonhuman mammals since human studies are rare.

• Supporting studies: studies providing supportive, rather than definitive, information from a wide variety of sources. For example, metabolic and other pharmacokinetic studies can provide insights into the mechanism of action of a particular compound. In vitro studies can provide insights into the chemical's potential for biological activity.

Source: EPA, IRIS


Main objectives:• Identify potential health hazards outcomes (i.e., cancer, chronic toxicity,

neurotoxicity, etc.).• Identify specific systems, organs, or tissues of the body that can be

damaged by the suspected chemical (e.g., target organ systems).• Identify specific abnormalities or diseases associated with the exposure

to the chemical (e.g., nervous disorders, behavioral problems, etc.). • Identification of reversible/irreversible effects.• Identify primary health hazard of concern (e.g., cancer at very low

doses).• Identify critical effect. The lowest dose or exposure at which a toxic effect

will occur (non-cancer end-point). For example, the lowest dose at which liver toxicity occurs. This dose is the Lowest-Observed-Adverse-Effect-Level (LOAEL).

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Main objectives:

• Biological and statistical significance of observed effects.

• Identify major data deficiencies.

• Evaluation of data quality and reliability of studies.

• Weight-of-evidence discussion. “As the culmination of the hazard identification step, a discussion of the weight-of-evidence summarizes the highlights of the information gleaned from the principal and supportive studies. Emphasis is given to examining the results from different studies to determine the extent to which a consistent, plausible picture of toxicity emerges.” (EPA, IRIS)


Exposure under

experimental conditions

Exposure under

environmental conditions

?


Major default assumption

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• Hazard identification

• Dose-response

• Exposure evaluation

• Risk characterization

►

Major tasks:

• Identification of the critical data.

• Analysis of the data which will be used for the dose response curve.

• Dose-response models.

– Non-cancer end-point outcome: point estimates of the "no-observed-adverse-effect-level" (NOAEL), LOAEL, etc., uncertainty factors, model selection, benchmark dose models, categorical regression models, assumptions, and model validation.

• Derivation of the reference dose (RfD) and uncertainty factors.

Risk assessment process: systemic toxicityDose-response

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Risk assessment process: systemic toxicity Dose-response assessment point estimates

NOAEL (mg/kg/day)*:• The “No-Observed-Adverse-Effect-Level” (NOAEL)

is an experimentally determined critical dose at which there was no statistically or biologically significant indication of the toxic (non-cancer end-point) effect of concern.

* The notation for mass per bodyweight per day is mg/Kg/day or mg/KgCday[in some texts mg/Kg-day, (mg/Kg)/day units are also used as well].

NOAEL and LOAEL (mg/ kg/ day):• The NOAEL level establishes the threshold range of

non-cancer end-point exposures which can be tolerated by organisms without any toxic effect.

• The lowest dose at which non-cancer end-point toxicity is manifested is the Lowest-Observed-Adverse-Effect-Level (LOAEL).


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e ( e

.g. r

atio

of l

iver

/ bo

dy w

eigh

t )


NOAEL

NOAEL : No Observed Adverse Effects Level

LOAEL

Dose response model



Risk assessment process: systemic toxicityDose-response assessment (experimental data)

Source: EPA Acrylamide study

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ADI (mg/ kg/ day) :

• Acceptable Daily Intake (ADI) is the amount of a chemical to which a person can be exposed to, on a daily basis, over an extended period of time (usually a lifetime) without suffering a deleterious effect.

• ADI = NOAEL / SF

• SF = safety factor [10, 100, 1000]

Risk assessment process:– systemic toxicityDose-response assessment point estimates: “safe

dose" approach

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SF

ADI = NOAEL / SF = NOAEL / (UF x MF)

Risk assessment process:– systemic toxicityDose-response assessment: “safe dose" estimates for non-

cancer toxicity outcomes

AD

I

© 2003-2005 by Michael D. Cailas and John Bing-Canar

0.5 1.5 2.5

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The ADI was commonly used by agencies for establishing allowablelevels of contaminants in food products and water. This measure has been replaced by the reference dose (RfD).

Before defining what a RfD is, it is important to discuss the underlying assumption for both measures. This assumption is based on the notion that there is a “safe dose” of a chemical to which a person can be exposed to on a daily basis over a lifetime without suffering a deleterious effect. Conceptually, the ADI is the limit at which the probability of harm tends, asymptotically, to an acceptable minimum. Thus, exposures less than the ADI are likely to yield outcomes that are not harmful.

Risk assessment process: systemic toxicityDose-response assessment point estimates: “safe

dose" approach


Thus the Risk min for exposures less than the ADI. The major problem with this measure is that, numerically, the response is zero because of the threshold (see dose response curve). Under these conditions it is difficult to estimate a frequency (i.e., Risk ≠ ADI), furthermore, it is difficult to define a minimum of absolute safety (i.e., absence of risk => min = 0).

The safe dose approach yields measures of “risk”, such as the ADI and the RfD, that are not probabilistic measures; in essence they are “limits of risk”.

Because of this characteristic the systemic toxicity risks are not assessed in terms of the probability of occurrence, they are assessed in terms of comparative ratios (e.g., margin of exposure, hazard ratio).


dose" approach


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RfD :

• Reference Dose (RfD), is an estimate of a daily lifetime dose which is likely to be without significant risk to the exposed population.

• RfD = NOAEL / (UF x MF)

• If (UF x MF) = SF, then RfD = ADI

• Units of milligrams per kilogram of body weight per day (mg/kg/day).


dose" approach

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R f

D

UF

R f D = NOAEL / SF = NOAEL / (UF x MF)



dose" approach

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Issue Value

To account for variation in the general population and protect sensitive subpopulations 10

When extrapolating from animals to humans 10 When NOAEL is derived from a subchronic study 10 When a "lowest-observed-adverse-effect level" LOAEL is used instead of a NOAEL

10

Uncertainty factors (UF)

Risk assessment process: systemic toxicity Dose-response assessment point estimates: “safe

dose" approach

Modifying factor (MF): the MF is an additional uncertainty factor that isgreater than zero and less than or equal to 10. The magnitude of the MFdepends upon the professional assessment of scientific uncertainties of the study and data base (e.g., the completeness of the overall data baseand the number of species tested). The default value for the MF is 1.

Tetrachloroethylene (CAS No. 127-18-4)ORAL RfD Chronic: 1E-2 mg/kg/day (U.S. EPA, 1990)UNCERTAINTY FACTOR: 1000NOAEL: 20 mg/kg/day (converted to 14 mg/kg/day)CONFIDENCE: Study: Low, Database: Medium, RfD: MediumVERIFICATION DATE: 9/17/87PRINCIPAL STUDY: Buben and O'Flaherty (1985)

COMMENTS: The study provided a NOAEL of 20 mg/kg/day that was converted to 14 mg/kg/day to account for noncontinuous exposure; an uncertainty factor of1000 results from factors of 10 to account for intraspecies variability, interspeciesvariability and extrapolation of a subchronic effect level to its chronic equivalent.The value is verified in IRIS.

Source: http://rais.ornl.gov/tox/profiles/tetrachloroethylene_f_V1.shtml

Risk assessment process: systemic toxicityDose-response assessment point estimates: example

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Chromium(VI) (Cr, CAS No 7440-47-3)ORAL RfD Chronic: 0.005 mg/kg/day UNCERTAINTY FACTOR: 500 NOAEL: 25 mg/L of chromium as K2CrO4 for one year,

converted to 2.4 mg of Cr(VI)/kg/day. CONFIDENCE: Study: Low, Data Base: Low, RfD: Low VERIFICATION DATE: 02/05/86 PRINCIPAL STUDY: MacKenzie et al., 1958

COMMENTS: The NOAEL was based on no effects reported at the highest dosetested in a one year drinking water study in rats. The RfD is limited to metallic Cr(VI) of soluble salts. The calculation assumed drinking water consumptionof 0.097 L/kg/day. The uncertainty factor of 500 reflects a factor of 10 to accountfor interspecies variability and a factor of 10 for interhuman variability in thetoxicity of the chemical in lieu of specific data, and an additional factor of 5

to compensate for the less-than-lifetime exposure duration of the study.

Risk assessment process: systemic toxicityDose-response assessment point estimates: example

Source: http://rais.ornl.gov/tox/profiles/chromium.shtml

Risk assessment process: systemic toxicityAssessment measures: HQ

Hazard Quotient (HQ, or Hazard ratio):the ratio of an actual exposure dose (ED) to the reference dose (RfD). This ratio provides information on the relative magnitude of risk.

HQ = ED / RfD

HQ = ED × (UF × MF) / NOAEL

If HQ > 1, then ED > RfD, this implies the likelihood of a risk for the exposed population.

As we have seen, the RfD, and ADI, provide a measure of a limit beyond which risk is not “acceptable”. The main issue with the HQ is how many times (in terms of order of magnitude: 1, 10,100, etc.) is the ED larger than the RfD.


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Risk assessment process: systemic toxicityAssessment measures: MOE

Margin of exposure (MOE):A similar measure is the margin of exposure (MOE), which is the magnitude by which the NOAEL of the critical toxic effect exceeds the exposure dose (ED), where both are expressed in the same units:

MOE = NOAEL / ED

When the MOE is equal to or greater than UF x MF, the need for concern is likely to be small since the MOE will be less than the RfD (why?). The following schematic present all the possible ranges of an exposure dose (ED) and the corresponding MOE ranges.

Risk assessment process: MOE


NO

AE

L

0

RfD

Dosescale

NOAEL> ED2 > RfDMOE # UF x MFED1 < RfD

MOE > UF x MF

RfD

NOAEL

?

?ED3 > NOAEL

MOE < UF x MF

;

ED1 ED2 ED3

Dosescale

Schematic of the relative MOE ranges

© 2005 Michael D. Cailas and John Bing-Canar

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Risk assessment process: measures of exposure

Quantifying the exposure dose (mg/ kg/ day):

the “Chronic Daily Intake” (CDI), quantifies an average intake amount of a toxic agent to which a person is exposed on a daily basis over an extended period of time (usually a lifetime). The Average Daily Dose (ADD) or the Estimated Exposure Dose (EED) have a similar application.

The basic components of the CDI, the ADD, or the EED are :

• Concentration of the chemical,

• intake (e.g., ingestion for liquids and food), and

• exposure time, duration, and frequency.

These measures are used for estimating the HQ and the MOE.

Risk assessment process: measures of exposure

CDI for ingestion of surface or groundwater water:

CDI (mg/kg/day) = [C×IR×EF×ET×ED] / [AT×BW]where:

C = Concentration (mg/L)

IR = Ingestion rate (L/hr; usually 0.05 L/hr)

EF = Exposure frequency (45 days/year)

ED = Exposure duration (30 years)

ET = Exposure time (1 hr/day)

AT = Averaging time (365 days)

BW = Body weight (adult; 70 kg)

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• Hazard identification

• Dose-response

• Exposure evaluation• Risk characterization

►

Risk assessment process: systemic toxicityexposure evaluation

“ Estimation of the number of people exposed and the magnitude, duration, and timing of their exposure to a compound in the environment.”

Example: sunbathing. To evaluate human exposure to the sun, one would need to determine the number of people sunbathing.One would also need to determine the intensity of the sun rays, the length of time exposed to the sun, and when the exposure occurred (e.g., morning and late afternoon sunlight is not as damaging as mid-day sunlight).

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Risk assessment process: systemic toxicityexposure evaluation

Sou

rce:

RA

IS s

ite

Information required to evaluate human exposure:factors controlling the production of the hazardous agent and itsrelease into the environment.quantities of the agent that are released, and location andtiming/frequency of release.

Information required to evaluate human exposure (cont’d):factors controlling human contact with the agent such as size ofvulnerable human population, distribution, and human activities which facilitate or prevent contact.human intake (exposure dose).

Factors controlling the fate of the agent in the environment after release:Movement, persistence, and degradation (the breakdown product may be more or less toxic than the original agent).

Risk assessment process:systemic toxicity exposure evaluation

Factors controlling the fate of the agent in the environment after release (example): Pesticides used in agriculture are a good example of the fate of an agent in the environment. Once applied to the soil, the pesticide may be taken up by the plant or it may infiltrate deeper into the soil following a rainfall. The pesticide might also degrade from exposure to sunlight or bacteria in the air and soil.

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Based on the previous three steps, risk characterization aims to

address the following major topics (EPA, 1995):

Risk assessment process:systemic toxicity example

Overall picture of risk and the specific risk estimates.Discussions on the dose response curves, uncertainty factors, risk measures (MOE). Major conclusions, strengths, limitations, and uncertainties of the data.Uncertainty of the risk estimates and implications.Qualitative characteristics of the hazard. Risk perception studies related to this type of hazard.Comparison of the studied risk with other similar risks.

Based on the previous three steps, risk characterization aims to

address the following major topics (EPA, 1987):

Risk assessment process: systemic toxicityrisk characterization

Overall picture of risk and the specific risk estimates.Discussions on the dose response curves, uncertainty factors, risk measures (MOE). Major conclusions, strengths, limitations, and uncertainties of the data.Uncertainty of the risk estimates and implications.Qualitative characteristics of the hazard. Risk perception studies related to this type of hazard.Comparison of the studied risk with other similar risks.

• U.S. EPA. 1987. The Risk Assessment Guidelines of 1986. Office of Health and Environmental Assessment, Washington, DC. EPA/600/8-87/045.

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