CHAPTER

14

QUANTIATIVE RISK ASSESSMENT(nh gi ri ro nh lng)The purpose of this chapter is to provide an understanding of risk, how to assess it at a specific site, and what the numerical estimates of risk represent. The National Research Council defines risk assessment as (mc ch ca chng ny l cung cp s hiu bit v s ri ro, nh gi n nh th no ti mt khu vc hin trng c th, v cc tnh ton s hc ca s ri ro biu th cho ci g. Hi ng nghin cu khoa hc quc gia nh ngha nh gi ri ro nh sau): the characterization of the potential adverse health effects of human exposures to environmental hazardsRisk assessment also includes characterization of the uncertainties inherent in the process of inferring risk (l s m t c im ca cc nh hng bt li c th xy ra n con ngi v nguy him cho mi trngnh gi ri ro cng bao gm s m t c im ca tnh khng n nh vn c trong qu trnh phng on ri ro).Risk assessment is a tool for understanding the health and environmental hazards associated with hazardous waste and can greatly improve the basis on which to make hazardous waste management decisions. Risk assessment can also assist the public in understanding the human health implications of the issues and available alternatives (nh gi ri ro l mt cng c hiu v cc nguy c c hi cho mi trng v sc khe do cht thi nguy hi gy nn v c th tng cng nn tng cho vic a ra nhng quyt sch qun l cht thi nguy hi. nh gi ri ro cng c th gip cng ng hiu v cc vn lin quan mt thit n sc khe con ngi v cc phng n sn c.).14-1 RISK (ri ro)In the classical sense, risk is defined as the probability of suffering harm or loss. When the resulting harm is measurable (e.g., worker-days lost to accidents), risk may be calculated as the probability of an action occurring multiplied by the severity of the harm if the action does occur: (theo ngha c in, ri ro c nh ngha nh l kh nng xy ra s mt mt hay s gy hi. Khi kt qu gy hi ny c th o lng c (v d, ngy cng b mt do tai nn)), th s ri ro ny c th c tnh nh l xc sut ca mt hnh ng xy ra nhn vi mc nghim trng ca s gy hi nu hnh ng xy ra)Risk = (probability) x (severity of consequence)

Ri ro = (xc sut) x (mc nghim trng ca kt qu)

Using this calculation suggests that the following two activities have the same risk (Vic s dng php tnh ny a ra 2 hnh ng c cng ri ro nh sau):

Activity no.Severity of outcome, worker-days lostProbability of outcomeRisk, worker-days lost

1200.12

2100.22

Often the undesirable consequence is not a quantifiable matter. An example of such a consequence is death; no one is ever slightly dead. Another example is the onset of cancer. For such consequences, risk is simply defines as the probability of the harm occurring. Because approximately 25 percent of the U.S population will experience cancer in their lifetime, the lifetime risk is background, incremental, or total risk. Background risk is what people are exposed to in the absence of the particular source of risk being studied, incremental risk is that caused by this source, and total risk is simply the sum of the two. The previous reference to 0.25 probability of cancer represents background risk. The U.S Environmental Protection Agency (EPA) target for Superfund sites of 1x10-6 excess lifetime cancer risk represents an incremental risk the probability of a person developing cancer from exposure to contaminants from a Superfund site is 1x10-6 (or one in a million) in excess of that persons risk of cancer from all other sources combined. It also implies that for every million people potential exposed to a site, one additional cancer case over their lifetimes would be expected. If the population exposed to a site is 10,000, this number can be interpreted as 0.01 additional occurrence of cancer (Cc kt qu khng mong mun thng khng phi l vn c th xc nh s lng. Mt v d ca kt qu ny l ci cht; khng c ai bao gi l hi cht. Mt v d khc l s xm nhp ca bnh ung th. Vi nhng kt qu nh vy, ri ro c nh ngha n gin l xc sut xy ra s tn hi. Bi v khong 25% dn s Hoa K s tri qua bnh ung th trong cuc i h, s ri ro trong cuc i l s ri ro nn, ri ro gia tng hay ri ro ton b. Ri ro nn l nhng g m con ngi tip xc trong iu kin khng c ngun pht sinh ri ro c bit no c nghin cu, ri ro gia tng l ci m gy ra bi ngun ri ro ny, v ri ro ton b n gin l tng ca 2 loi ri ro trn. S tham chiu trc n 0.25 xc sut ca bnh ung th i din cho ri ro nn. Ch tiu ca C quan Bo v Mi trng Hoa K cho cc a im Superfund nguy c ung th l vt qu 1x10-6 i din cho ri ro gia tng xc sut ca mt ngi pht trin ung th do phi nhim cc cht c t mt a im Superfund l 1x10-6 (hay mt phn mt triu) vt qu nguy c ung th t cc ngun khc kt hp li ca ngi . N cng hm rng mi mt triu ngi tip xc vi a im ny, th s c th c mt ngi mc thm bnh ung th trong cuc i ca h. Nu c 10,000 ngi tip xc vi mt a im, th s ny c th c th hin nh l thm 0.01 kh nng mc bnh ung th ).Table 14-1 provides some estimates of some common place risks (death in the case of accidents and cancer for the other actions). These present background risks on an annual basis. (Bng 14-1 cung cp mt vi c tnh ri ro cho mt vi ni ph bin (cht trong trng hp tai nn v ung th do cc hot ng khc). Nhng ci ny trnh by v nhng ri ro nn trn mt nn tng c bn thng thng).The terms risk and hazard are often confused. Hazard is a descriptive term, referring to the intrinsic capability of the waste to cause harm; it is the source of the risk. The hazard posed by a waste is a function of such variables as its toxicity, mobility, and persistence as well as how it is contained. The release or threatened release of a hazardous waste thus represents a hazard it is a source of risk; however, the waste does not represent a risk unless exposure has occurred or the possibility exists for future exposure. (thut ng ri ro (risk) v nguy hi (hazard) thng hay b ln ln vi nhau. Hazard l mt thut ng m t, cp n kh nng ni ti ca cht thi gy ra s thit hi hay tn hi; n l ngun gc sinh ra ri ro. S nguy hi gy ra bi cht thi l cc tnh cht c hi, tnh d bin i v s lu li lu di ca n cng vi vic n c cha ng nh th no. V vy, vic pht thi hay s gii phng mang tnh e da ca cht thi c hi i din cho s nguy hi n l ngun gc ca ri ro; tuy nhin, cht thi khng i din cho ri ro tr khi xy ra s phi nhim hoc kh nng xut hin s phi nhim trong tng lai).TABLE 14-1 Common risks (cc ri ro ph bin)Action (hot ng)Annual Risk (ri ro hang nm)Uncertainty (tnh khng n nh)

Motor vehicle accident (tai nn xe my)

Total (tng)

Pedestrian (ngi i b)Home accidents (tai nn nh)Cigarette smoking (one pack per day) (ht thuc l 1 gi mi ngy)Peanut butter (4 teaspoon per day) (b lc 4 mung mi ngy)Drinking water with EPA limit of TCE (ung nc c cht c TCE (Trichloroethylene) nm trong gii hn ca EPA)2.4 x 10-44.2 x 10-51.1 x 10-4

3.6x 10-38 x 10-62 x 10-910%

10%

5%

Factor of 3

Factor of 3

Factor of 10

Emergency as a Systematic Methodology (trong trng hp khn cp nh l mt phng php c h thng)Undoubtedly, the concept of risk has a long history of influencing environmental policy making. However, the initial efforts relied more on intuition than on the scientific principles of toxicology, chemistry, and fate and transport modeling inherent in modern risk assessments. Only in the past few years has a science-based risk assessment played a major role in environmental decision making (R rng, quan im v s ri ro c mt lch s lu di nh hng n cc quyt sch v mi trng. Tuy nhin, cc n lc ban u ph thuc nhiu hn vo trc gic hn l vo cc nguyn l khoa hc ca c cht hc, ha hc, v m phng s lun chuyn v hy dit vn c trong cc nh gi ri ro hin i. Cch y vi nm c mt nh gi ri ro da trn nn tng khoa hc tr thnh vai tr ch o duy nht trong vic a ra cc quyt sch v mi trng).In this chapter we will use the term quantitative to describe the process of using scientific principles to calculate quantitative estimates of risk. The widely recognized approach is a four-stage process (Trong chng ny, chng ta s s dng thut ng nh lng (quantitative) m t qu trnh s dng cc nguyn l khoa hc c tnh nh lng ri ro):

1. Hazard identification (which chemicals are important) (nhn dng mi nguy hi (cht ha hc no l quan trng))2. Exposure assessment (where the chemicals go, who might be exposed and how) (nh gi s phi nhim (cc cht ha hc i n u, nhng ai s c th b phi nhim v nh th no))3. Toxicity assessment (determining numerical indices of toxicity for computing risk) (nh gi c tnh (xc nh cc ch s s hc ca c tnh tnh ton ri ro))4. Risk characterization (estimating the magnitude of risk, and the uncertainty of the estimate) (m t c im ri ro (c tnh quy m ri ro, v s khng n nh ca c tnh ny ))Risk assessment, as performed under most hazardous waste regulatory programs, employs this concept of conducting the risk assessment process in these four-stage. The U.S National Academy of Sciences created the four-stage process and eventually the EPA codified it. The EPA first applied the concept in 1984 for carcinogens following a suit brought by the Natural Resources Defense Council regarding ambient water quality criteria. In response to this suit, the EPA issued slope factors (SFs) that could be utilized in a predictive capacity to estimate carcinogenic risk to potentially exposed populations. A similar approach was then applied to non-carcinogens by setting acceptable daily intake (ADI) levels, which are analogous to reference concentrations (RfCs). It should be noted that although SFs have also been named by various acronyms including CSF(carcinogenic slope factor) and CPF (carcinogenic potency factor), all terms refer to expected lifetime cancer risk if someone is exposed daily for 70 years to 1mg of substance per kilogram body weight (nh gi ri ro, c thc hin trong hu ht cc chng trnh iu tit cht thi nguy hi, s dng khi nim ny tin hnh cc quy trnh nh gi ri ro trong 4 giai on. C quan Khoa hc Quc gia Hoa K to ra qui trnh 4 giai on ny v cui cng l EPA h thng ha n. EPA p dng khi nim ny ln u tin nm 1984 vi cht gy ung th sau mt v kin c tin hnh bi Hi ng bo v Ti nguyn Quc gia lin quan n vn cht lng nc xung quanh. i ph vi vn ny, EPA ban hnh cc h s dc (SFs) ci m c th c s dng trong kh nng d on c tnh ri ro ca cht gy ung th n lng dn c c kh nng tip xc vi n. Mt cch tip cn tng t c p dng sau cho cc cht khng gy ung th bng cch thit lp cc mc tiu th hng ngy (ADI) c th chp nhn c, n tng t nh l cc nng tham chiu (RfCs). Nn nh rng, mc d SFs cng c nhiu tn vit tt khc nhau bao gm CSF (carcinogen slope factor) v CPR (carcinogenic potency factor), tuy nhin tt c cc thut ng ny u cp n kh nng ri ro mc bnh ung th trong i nu ai tip xc hng ngy trong 70 nm ti 1mg vt cht cho mi kg trng lng c th).Purposes of Risk Assessment (Cc mc ch ca vic nh gi ri ro)Using the estimates calculated by a risk assessment as a basis for making decisions is termed risk management. Risk management may be defines as (Vic s dng cc c tnh c tnh ton bi s nh gi ri ro nh l nn tng cho cc quyt sch c gi l qun l ri ro). Qun l ri ro c th nh ngha nh sau:

.evaluating alternativeactions and selecting among thementails consideration of political, social, economic, and engineering information with risk-related information to develop, analyze, and compare regulatory options and to select the appropriate regulatory response to a potential chronic health hazard. The selection process necessarily requires the use of value judgments on such issues as the acceptability of risk and the reasonableness of the costs of control (nh gi la chncc hot ng v la chn trong s chngi hi s quan tm ca cc thng tin chnh quyn, x hi, kinh t, v k thut vi cc thng tin lin quan n ri ro pht trin, phn tch v so snh cc ty chn iu tit v chn la ra cc chnh sch iu tit ph hp phn ng li vi mi nguy hi c kh nng xy ra thng xuyn n sc khe).

In the hazardous waste field, risk assessment provides information to decision makers as to the consequences of possible actions. Important decisions that could use risk estimates include selecting waste treatment/ disposal options, remediating contaminated sites, minimizing waste generation, sitting new facilities, and developing new products. It should be emphasized that risk estimates are only one type of information used, and hazardous waste decisions are often driven by political, social, economic, or other factors (trong lnh vc cht thi nguy hi, nh gi ri ro cung cp thng tin cho cc nh hoch nh chnh sch nh l kt qu ca cc hot ng c th xy ra. Cc quyt nh quan trng m c th s dng cc c tnh ri ro bao gm vic la chn cc phng n x l/ thi b cht thi, ty u cc a im b nhim cht c, gim thiu s pht sinh cht thi, la chn a im xy dng cc cng trnh dch v cng cng, v pht trin cc sn phm mi. Cng phi nhn mnh rng cc c tnh ri ro ch l mt trong nhng loi thng tin c s dng, v cc quyt sch cht thi nguy hi thng b chi phi bi chnh tr, x hi, kinh t hay cc yu t khc).Risk assessment plays a major role in the decision making for the remediation of contaminated sites. It provides an important part of the basis for selecting from alternative remedies (see Section 17-6). Risks from exposure are calculated for each alternative under the assumption that its remedial actions have been implemented fully. This enables a balanced comparison of the effectiveness of each alternative at reducing risks (nh gi ri ro tr thnh vai tr quan trng trong vic xy dng cc quyt sch v vn ty u cc a im b nhim c. N cung cp mt phn thng tin nn tng cho vic chn la cc phng n ty u khc nhau (xem Phn 17-6). Cc ri ro do s phi nhim c tnh ton cho mi phng n trong s gi nh rng cc hot ng ty u ca n c thc hin y . iu ny gip cho vic so snh cng bng hiu qu gia cc phng n gim thiu cc s ri ro).

Another application in site remediation is the establishment of cleanup standards. In many cases, definitive numerical standards for specific contaminants in soil or groundwater simply do not exist. Risk assessments are used to determine How clean is clean? In this instance the calculation procedure is reversed. The process starts with a numerical definition of acceptable risk and works back to the level of contamination that will produce the acceptable risk level. An example would be a determination at a specific site that any soil containing greater than 588mg/kg of toluene would pose an unacceptable risk and thus has to be remediated; soils containing smaller concentrations can be left in place (mt ng dng khc trong s ty u a im nhim l thit lp cc tiu chun lm sch. Trong nhiu trng hp, cc tiu chun s cui cng cho mi cht c hi trong t hoc nc ngm n gin l khng tn ti. Cc nh gi ri ro thng c s dng xc nh Sch nh th no l sch? Trong nhng hon cnh ny qu trnh tnh ton ny c nghch o. Qui trnh ny bt u vi mt nh ngha s hc ca s ri ro c th chp nhn c v nhng vic quay tr li mc nhim ci m s sn sinh ra mc ri ro c th chp nhn c. Mt v d l mt s xc nh ti mt a im c th rng bt k lng t no c lng toluene ln hn 588mg/kg s l ri ro khng th chp nhn c v v vy cn phi c ty u; loi t c cc nng nh hn c th c t vo mt ch).

The EPA has proposed guidelines for the preparation of hazardous waste site risk assessments in a number of publications. The initial publications, Handbook for Conducting Endangerment Assessments and the Superfund Public Health Evaluation Manual, have been superseded by Risk Assessment Guidance for Superfund (RAGS), the Superfund Exposure Assessment Manual, and the Exposure Factors Handbook.RAGS, which comprises Volumes I and II, provides the site team risk assessor, remedial project manager (RPM), and community involvement coordinator with information to improve community involvement in the Superfund risk assessment process.

Volume I, Part A, provides suggestions for how Superfund staff and community members can work together during the early stages of Superfund cleanup. It identifies where, within the framework of the human health risk assessment methodology, community input can augment and improve the EPAs estimates of exposure and risk; recommends questions the site team should ask the community; all illustrates why community involvement is valuable during the human health risk assessment at Superfund sites.

Volume I, Part B, was prepared for risk assessors, remedial project managers and others to assist them in developing preliminary remediation goals for National Priorities List (NPL) sites. Specifically, RAGS, Part B, provides guidance on using U.S Environmental Protection Agency toxicity values and exposure information to derive risk-based preliminary remediation goals that are protective of human health.Volume I, Part C, offers guidance on the human health risk evaluations of remedial alternatives conducted during the feasibility study, during selection and documentation of a remedy, and during and after remedy implementation. Part C assists users in preparing site-specific risk evaluations and maintaining flexibility during the analysis and decision-making process.Volume I, Part D, provides guidance on standardized risk assessment planning, reporting, and review throughout the CERCLA remedial process, from scoping through remedy selection and completion and periodic review of the remedial action. Part D strives for effective and efficient implementation of Superfund risk assessment practice described in Part A, B, and C, and in supplemental Office of Solid Waste and Emergency Response (OSWER) directives.

These risk assessments are conducted to quantify potential human health and ecological risks stemming from toxic contaminants that may be transported to potential receptor populations in both present and future use scenarios. Human health risks for carcinogens are calculated as the probability of developing cancer in a lifetime and non-carcinogens as hazard indices. In addition, an ecological assessment is attempted to quantify risk to on-site and off-site biota.The following four sections describe each of the four stages of the risk assessment process as an overview of the state of the art in risk assessment. The methods in general follow those recommended by the EPA because much of the risk assessment currently conducted adheres to these procedures. However, it must be noted that these procedures are constantly being revised, and it is our objective to explain the science of risk assessment and not to provide guidelines for regulatory compliance. To provide material for illustrating the process, the examples of exposure from a hazardous waste site are used.

14-2 HAZARD IDENTIFICATION (Nhn dng nguy him)It is not uncommon to detect as many as 100 different chemicals at a contaminated site. The hazard identification stage examines the data for all contaminants detected at a site and consolidates the data to stress the chemicals of concern (i.e., those representing the vast majority of risk posed by the site). Risk assessment requires a clear understanding of what chemicals are present at a site, their concentration and spatial distribution, and how they could move in the environment from the site to potential receptor points. The types of data typically sought at a hazardous waste site to address such needs are shown in Table 14-2, and the collection of such data is detailed in Chapter 15. A site investigation easily can produce a huge amount of data, necessitating that steps be taken in the hazard identification stage to facilitate subsequent analysis.

TABLE 14-2 Data needs

Site history (lch s khu t)Land use (hin trng s dng t)Contaminant levels in media (mc nhim trong cc lp thnh phn):

Air, groundwater, surface water, soils and sediments

(Khng kh, nc ngm, t v lng cn)

Environmental characteristics affecting chemical fate and transport(cc c im ca mi trng nh hng n qu trnh vn chuyn v hy dit ca ha cht)

Geologic (a cht)

Hydro-geologic (thy a cht, a cht thy vn)

Atmospheric (kh quyn)

Topographic (a hnh)Potentially affected population (khu dn c c nguy c b nh hng)Potentially affected biota (khu sinh vt c nguy c b nh hng)

One approach is to examine data for all chemical contaminants detected in any media and select a subset of chemicals, consisting of the specific chemicals of concern and representative of all detected chemicals. These selected chemicals thus represent indicators or surrogates for all chemicals detected at the site. Their purpose is to limit the number of chemicals that must be modeled in fate and transport analyses and to focus efforts on the most significant hazards. The advent of modern computer methods that allow a large number of chemicals to be modeled simultaneously may obviate the need for a surrogate chemical selection step in many instances. Nevertheless, the EPA endorses the data consolidation approach for sites having a large number of chemicals detected in different media; however, professional judgement is emphasized. In general, most constituents at hazardous waste sites should be carried through a quantitative risk assessment.

The surrogate chemicals are selected on the basis of which compounds best represent the risk posed be the site:

The most toxic, persistent, and mobile The most prevalent in terms of spatial distribution and concentration

Those involved in the more significant exposures

The list of surrogate chemicals should encompass those chemicals that are estimated to account for 99 percent of the risk at the site. It should contain compounds that will support an adequate evaluation of both carcinogenic and non-carcinogenic risk.

Initial Screening (sng lc ban u)One approach to selecting this subset from all detected chemicals begins with an initial screening of data which may be summarized as follows:1. Sort the contaminant data by medium (e.g., groundwater, soil, etc.) for both carcinogens and non-carcinogens.

2. Tabulate for each detected chemical the mean and range of concentration values observed at the site.

3. Identify the reference concentrations for non-carcinogens and slope factors for carcinogens see Chapter 5) for each potential exposure route.

4. Determine the toxicity scores for each chemical in each medium. For non-carcinogens:

(14-1)Where

TS= toxicity score

Cmax= maximum concentration

RfC= chronic reference concentration (i.e., an estimate of acceptable daily intake) (see Section 5-4, and www.epa.gov/iris/ for the latest qualitative and quantitative data)

For carcinogens:

(14-2)

Where

SF= slope factor (also called carcinogen potency factor)

(see Section 5-5, and and www.epa.gov/iris/ for the latest qualitative and quantitative data)

5. For each exposure route, rank the compounds by toxicity scores

6. For each exposure route, select those chemicals comprising 99 percent of the total score.

The process is illustrated by the following two examples. Data for surface soils appear in Table 14-3. Reference concentrations and slope factors may be found in the EPAs IRIS database, or in other sources of toxicological information.

EXAMPLE 14-1. SELECTION ON NONCARCINOGENIC CHEMICALS. Rank the chemicals for soil in Table 14-3.

ChemicalCmax, mg/kgOral RfC*Toxicity scoreRank

Chlorobenzene

Chloroform

1,2-Dichloroethane

BEHP6.40

4.10

ND

2.30 x 1022.00 x 10-21.00 x 10-2NA

2.00 x 10-23.20 x 1024.10 x 102NA

1.15 x 1043

2

NA

1

NA not applicable.

* Source: IRIS

EXAMPLE 14-2. SELECTION OF CARCINOGENIC CHEMICALS. Rank the chemicals for soil in Table 14-3.

ChemicalCmax, mg/kgOral SF*Toxicity scoreRank

Chlorobenzene

Chloroform

1,2-Dichloroethane

BEHP6.40

4.10

ND

2.30 x 102NA6.10 x 10-39.1 x 1021.40 x 10-2NA2.50 x 10-2NA

3.22NA2

NA

1

NA not applicable.

* Source: IRIS

Note in Examples 14-1 and 14-2 that BEHP (bis-2-ethyl hexyl phthalate) and chloroform have both carcinogenic and non-carcinogenic toxicity so that they appear in both rankings.

TABLE 14-3 Chemical concentrations at ABC landfill

ChemicalAirGround waterSoil

Mean, mg/m3Maximum, mg/m3Mean, mg/LMaximum, mg/LMean, mg/kgMaximum, mg/kg

Chlorobenzene

Chloroform

1,2-Dichloroethane

BEHP4.09 x 10-81.10 x 10-121.40 x 10-83.29 x 10-78.09 x 10-83.12 x 10-122.40 x 10-88.29 x 10-72.50 x 10-44.30 x 10-42.10 x 10-4ND1.10 x 10-27.60 x 10-32.00 x 10-3ND1.391.12

ND

1.03 x 1026.404.10

ND

2.30 x 102

ND not detected.Further Screening of Chemicals

The ranking of toxicity scores as illustrated by the previous examples indicate which compounds pose the greatest hazard based solely on their maximum concentration and toxicity. The selection of chemicals of concern requires further evaluation to consider for each chemical its range of concentrations, its mobility in the environment, and other issues. These additional considerations include:

Mean concentration (nng trung bnh) Frequency of detection (tn sut pht hin) Mobility (tnh d bin i) Persistence in the environment (s tn lu lu di trong mi trng) Chemicals associated with site operation (cc cht ha hc ) Treatability

In general, chemicals with a low frequency of detection or extremely low mean concentration are not deemed sufficiently significant to receive direct evaluation. The evaluation of mobility in the environment uses information such as half-lives and physical-chemical properties (Henrys law constant, vapor pressure, octanol-water partition coefficient and solubility) (see Section 3-2). Chemicals whose persistence and mobility make them significant risks would remain in the list of chemicals of concern. Also, the risk analyst can selectively add compounds that are degradation products of others on the list (e.g., vinyl chloride from TCE).

Although, the list of chemicals of concern should account for about 99 percent of the risk, it may account for less than 10 percent of the detected chemicals. It is not infrequent to reduce a total of 100 detected chemicals to 10 to 15 chemicals of concern. 14-3 EXPOSURE ASSESSMENT (nh gi phi nhim)The second stage of a quantitative risk assessment consists of estimating the exposure to the chemicals by the populations potentially at risk. To provide a comprehensive understanding of the sources of contamination, this stage begins with a delineation of the sources (e.g., sludge lagoons, contaminated soil) and the spatial distribution of contaminants at the site. The exposure assessment continues by analyzing how the contaminants might be released. Given the release of a chemical, it is necessary to estimate how it may migrate (e.g., via groundwater) to a potential receptor (e.g., users of groundwater). Having identified current and potential receptor points, considerable attention is given to (1) identification of general and sensitive populations of current and potential receptors and (2) estimation of both short- and long-term exposures in terms of doses by exposure route.

Environmental Pathways

Much of this stage consists of a fate and transport analysis as presented in depth in Chapter 4, and only a brief summary will be presented here. A chain of events must occur to result in exposure. This chain in a collective sense is termed an environmental pathway (i.e., the environment routes by which chemicals from the site can reach receptors). A pathway, consisting of the following elements, thus defines the framework of a fate and transport analysis: Source (e.g., a lagoon)

Chemical release mechanism (e.g., leaching)

Transport mechanisms (e.g., groundwater flow)

Transfer mechanisms (e.g., sorption) Transformation mechanisms (e.g., biodegradation)

Exposure point (e.g., residential well)

Receptors (e.g., residential consumers of drinking water)

Exposure route (e.g., ingestion)

An example of two simple but common pathways, one involving groundwater and the other atmospheric transport, is shown in Figure 14-1. Other examples of potential pathways, some of them quite complex, are discussed in Chapter 4 with various useful approaches for estimating fate and transport. It is necessary to establish potential pathways between sources and receptors for each chemical of concern. Figure 14-2 is a flowchart indicating the types of considerations necessary in analyzing fate and transport at hazardous waste sites.

Contaminant Release, Transport, Transfer, and Transformation

The release of contaminants from hazardous waste sites results from natural processes, such as leaching of soluble chemicals to groundwater; human activity, such as the construction of site drainage channels; and accidents such as chemical spills. Table 14-4 summarizes some potential mechanisms for release of chemicals.FIGURE 14-1 Examples of exposure pathways.

FIGURE 14-2A Flowchart for fate and transport assessments of the atmosphere.In addition to determining what chemicals are released from a source, it is important to determine what happens to these chemicals: how they are transported, transferred, or transformed. Transport involves movement by advection and diffusion. A great number of mechanisms could act to transfer the contaminants to another medium or to storage sites (e.g., sorption onto soils). In some instances, organic contaminants transform under environmental mechanisms to CO2 and water, while in others cases the degradation products may be more toxic than the original chemical (e.g., reductive dechlorination of tetrachloro- and trichloroethylene to vinyl chloride). Example mechanisms for the transfer and transformation of chemicals are presented in Chapter 4, and several examples are shown in Table 14-5.

Potentially Exposed Populations The next step in the exposure assessment is to determine potentially exposed populations. These would include the following:

Present population in vicinity of the site

Future population in vicinity of the site

Subpopulations of special concern (e.g., young children in the case of lead contamination)

Potential on-site workers during any remediation

The present population may be initially identified as those living within specified distances from site boundaries (e.g., 1 and 3 miles). However, any distance could be specified, depending on the results of the fate and transport analysis. Determining potentially exposed populations is based on the surrounding land use and documented sources of demographic information such as summarized in Table 14-6.FIGURE 14-2B Flowchart for fate and transport assessment in surface water and sediment

FIGURE 14-2C Flowchart for fate and transport assessment in surface water and sediment

TABLE 14-4 Contaminant Realease mechanisms

MediaMechanismTime Frame

Air

Soil

Surface and groundwaterVolatilization

Fugitive dust generation

Combustion

Erosion

Leachate generation

Spills

Leachate generation

SpillsC

C, E

E

C, E

C

E

C

E

C Chronic E - Episodic

TABLE 14-5 Mechanisms affecting environmental transfer and transformation of chemicals*

Fate mechanism

MediaTransferTransformation

Water

Soil

AtmosphereVolatilization

AdsorptionUptake by plants

Dissolution in rainwater

Washout by rain

Gravitational depositionBiodegradationPhotochemical degradation

Biodegradation

Oxidation by ozone

Adapted from R.A. Conway. 14After a preliminary overview of documents has been made, a thorough site visit to ground truth the current situation is critical. Patterns of human activity associate with various land and water uses should be determined, including answering questions such as: What sort of sensitive public facilities are located near the site (e.g., schools, hospitals, day care centers, nursing homes)?

TABLE 14-6 Sources of demographic informationTopographic maps (bn a hnh)

Tax maps (bn thu v)Census reports (bo co iu tra dn s)

County/regional demographic studies

(cc nghin cu nhn khu hc theo n v hnh chnh/ theo vng)Municipal zoning maps (bn vng th)

Land use data (s liu t s dng)Projected future land use

Site visit (thm hin trng)(din tch t d kin s dng trong tng lai)

Human activity patterns (cc m hnh sinh hot ca con ngi)Aerial photos (nh chp t trn khng)

What outdoor activities occur (e.g., parks, playgrounds, recreational facilities nearby)?

What fraction of time do various subpopulations spend in potentially contaminated area (e.g., children in a grade school)?

What is likely to change (e.g., seasonally, immediate future, long-term future)?

What secondary exposures are possible (e.g., contamination of crops at nearby farms)?

What are the dietary habits (e.g., consumption of recreational fish containing bio-accumulated contaminants)?

Certain subpopulations may require special consideration because of their higher sensitivity to toxic substances - children, pregnant women, the elderly, and people with chronic illnesses. An initial step in identifying such subpopulations is to locate sensitive facilities such as schools and nursing homes. It is critical to evaluate future populations to address how changes could affect the estimates of risks. For example, if groundwater is not currently utilized as a drinking water source but could be used in the future, this may dramatically affect future risk calculations.Development of Exposure Scenarios

After potential populations and exposure pathways have been defined, it is then necessary to characterize the conditions under which the populations may be potentially exposed. This involes an evaluation of both current and reasonable future uses of the site to establish a credible set of conditions under which the exposure could occur. After the scenarios have been established, then the specific parameters governing exposure (e.g., exposure frequency, duration, and intake rate) can be selected.

Some examples of commonly used exposure scenarios include a worker scenario, a trespasser scenario, a residential use scenario, a recreational use scenario, and a construction scenario. Each of these is discussed in the following listing, together with some pertinent questions for the risk assessor to consider in determining whether site conditions warrant evaluation of the given scenario. Worker scenario: Is the site currently used for industrial activities? Are workers exposed to site-related constituents under normal conditions? Could workers be exposed in the future, either because of a change in use of the site or because the workers would be involved in remedial activities? It should be note that workers are strictly protected under Occupational Safety and Health Administration (OSHA) regulations. The estimation of potential risks for workers is usually done as a hypothetical evaluation to determine whether the potential exists for unacceptable risks, rather than as a mandated exercise designed to ensure that workers are not placed at risk during the normal course of their activities. Trespasser scenario: Is there evidence that trespassing may routinely occur at the site? Is there a fence that would limit access to the site? If so, is the fence in good condition? Have other measures been taken to limit access to the site? Is the site close to a school, shopping, or residential area where area children would have reason or inclination to play at the site? Is the site attractive to children? Residential use scenario: Is the site currently used for residential purposes? Will it or could it be used for residential purposes in the future? Are there any zoning or deed restrictions that would limit its use for residential purposes? Are the residences singe-family dwelling? Is there the potential for residential use of groundwater? A residential scenario is frequently evaluated as a hypothetical condition in an effort to estimate worst-case risks. Exposure under a residential scenario is generally the most restrictive condition for any age group, thus resulting in the greatest potential exposures and the highest potential risks. Recreational use scenario: This scenario is particularly applicable for evaluation of potential risks associated with sureface water bodies where people may fish, swim, canoe, wade, etc. Observations made during site investigation activities can be useful in the development of a recreational exposure scenario. It may also be important to confirm the regulated use and classification of the water body with the appropriate state of federal agency. Construction scenario: Are construction activities planned or likely at the site? Will the construction result in potential exposures for both on-site receptors (e.g., direct contact with soils by construction workers) and off-site populations (e.g., exposure by off-site residents or workers to fugitive dust and volatiles released as a result of earth-moving activities)?The development of scenarios involves making assumptions and must include subjective decisions. Ideally, this should produce credible scenarios, but in an effort to ensure conservative risk assessments, unrealistic scenarios have been used at some hazardous waste sites.

As an example, pica behavior is a real phenomenon and should be considered, but the assumption that children will eat 200mg/day of soil for a 70-year lifetime is not realistic. Nondegradation policies, in many states, prohibit the discharge of contaminants into streams, independent of the present water quality in the stream. That policy has been used to suggest that a stream heavily contaminated with acid mine drainage be considered a primary drinking water source for the purpose of developing a credible scenario for a risk management. The point is that while nondegradation may be an appropriate public policy, drinking acid mine drainage is not realistic.

Exposure points define the locations of the receptors for the various scenarios. They are identified for each exposure scenario simply by overlaying the demographic information with the exposure pathway. An exposure point may be as close as the sources of waste at the site itself (e.g., the trespasser scenario) or at considerable distance, particularly for pathways involving the food chain.Exposure Point Concentrations

The risk analyst must next estimate the concentration of contaminants at the exposure points, including all pathways air, ground and surface water, soils, sediments, and food (e.g., plants and fish). For present exposures, actual monitoring data at the exposure point should be used wherever possible. For example, contaminant concentrations should be obtained for drinking water wells in the vicinity of a site. For on-site exposure points, representative concentrations in soil or groundwater may be calculated as the arithmetic or geometric mean (depending on the statistical distribution of the site analytical concentration data).

Future conditions can differ starkly. A plume may not have yet migrated to a potential exposure point. Remediation will, of course, drastically reduce migration. In a comprehensive risk assessment, the exposure concentrations for each remedial alternative would be estimated including the present situation (baseline conditions or no remedial action). Determining the concentration of a contaminant at the exposure point for future conditions often requires the use of fate and transport modeling methods of Chapter 4 and standard references. The major effort with these models is calibration; once calibrated, successive runs can be made relatively easily to estimate concentrations for a range of conditions and assumptions.For groundwater contaminants, hydrogeologic models can be used to estimate the future concentration at a downstream well. For volatile organic compounds released to the atmosphere, a gaussian diffusion model can be employed to estimate future downwind concentrations. In general, the level of effort employed in data collection and modeling will depend on the estimated severity of the risk. Nominal risks do not warrant the same level of analysis as the clearly significant risks.

All mathematical models require the making of assumptions. It is essential that the appropriateness of assumptions be reviewed carefully. The impact of assumptions on the exposure point concentrations and, ultimately, the risk values should be examined through a sensitive analysis. A sensitive analysis identifies which of the myriad of input variables have the most significant impact on the resulting risk value.Receptors Doses

The final step in the exposure assessment stage is to estimate the doses of the different chemicals of concern to which receptors are potentially exposed at the exposure points. As with the previous stages, three exposure routes are considered-insgetion, inhalation, and dermal contact. Also, there are three types of doses the administered dose (the amount ingested, inhaled, or in contact with the skin), the intake dose (the amount absorbed by the body), and the target dose (the amount reaching the target organ).

For purposes of calculating risks, the dose should be in the same form as that of the dose-response relationship reported for the specific chemical and the exposure route under study. This will almost always be either administered dose or absorbed dose. Given the concentration of the contaminant at the exposure point, the calculation of administered dose is straightforward. In contrast, the calculation of absorbed dose based on administered dose requires consideration of some complex factors (see Section 5-1). The key factors influencing the uptake of contaminants by the body are simplified as follows:

Ingestion

Inhalation

Dermal contact

Contaminant concentration in the ingested media

Amount of ingested material

Bioavailability to the gastrointestinal system

Concentration in air and dust

Particle size distribution

Bioavallability to the pulmonary system

Rate of respiration

Concentration in soil and dust

Rate of deposition of dust from air

Direct contact with soil

Bioavailability

Amount of skin exposed

Other factors to be considered in determining the intake of contaminants include considerations of life style, frequency and duration of exposure (e.g., chronic, subchronic, or acute), and the body weight of the receptor. In the majority of hazardous waste sites, long-term (i.e., chronic) exposures are frequently of greatest concern.The calculation of an administered dose is summarized in the following generic equation:

(14-3)

WhereI= intake (mg/kg of body weight . day)

C= concentration at exposure point (e.g., mg/L in water or mg/m3 in air)

CR= contact rate (e.g., L/day or m3/day)

EF= exposure frequency (days/year)

ED= exposure duration (years)

BD= body weight (kg)

AT= averaging time (days)

Equation 14-3 is typically modified for specific exposure pathways. For example, the intake dose from inhalation of fugitive dust may be calculated as:

(14-4)WhereRR= retention rate (decimal fraction)

Abs= absorbtion into bloodstream (decimal fraction)

For fugitive dust, the concentration in the air is determined by:

(14-5)

Where Cs= concentration of chemical in fugitive dust (mg/mg)

Pc= concentration of fugitive dust in air (mg/m3)

Often, appropriate parameters may be found in the literature. Considerable research has been done in recent years to define many basic parameters, such as skin surface areas, soil ingestion rates, and inhalation rates. Other parameters, such as exposure frequency and duration, are often based on site-specific information (if available) or professional judgment.

Exposure duration is used to estimate the total time of exposure. National statistics are available on the upper-bound (90th percentile) and average (50th percentile) numbers of years spent by individuals at one residence, for example. The exposure duration selected must be appropriate for the exposure pathway being evaluated. Overestimating the exposure duration will overestimate the calculated risk level and underestimating the exposure duration will underestimate the calculated risk level. For example,in the evaluation of a trespasser scenario, observations of trespassers during site investigation activities may dictate the values used for exposure frequency (e.g., number of days per year or number of events per year) and exposure duration (e.g., number of years that the activity occurred). Common sense also plays an important role in the selection of exposure parameters. For example, if one were evaluating residential use of two sites, one in North Dakota and one in Florida, it would not be reasonable to assume the exposure frequency (e.g., days per year that an activity ocurred) would be the same in the two risk assessments for a child potentially exposed to surface soils as a result of their playing outdoors in the backyard.Averaging time (AT) is another important parameter that must be defined in the intake equation. The averaging time selected will depend on the type of constituent being evaluated. For example, to assess long-term or chronic effects associated with exposure to noncarcinogens, the intake is averaged over the exposure duration (expressed in days). (The averaging time is equal to the exposure duration for noncarinogens.) Exposure to carcinogens, however, is averaged cover a lifetime (assumed to be 70 years or 25,550 days), to be consistent with the approach used to develop slope factors.TABLE 14-7 Standard parameters for calculation of dosage and intake determined for the ABC landfillParameterAdultsChild age 6-12Child age 2-6

Average body weight (kg)Skin surface area (cm2)

Water ingested (L/day)

Air breathed (m3/h)

Retention rate (inhaled air)

Absorption rate (inhaled air)

Soil ingested (mg/day)

Bathing duration (minutes)

Exposure frequency (days/year)

Exposure duration (years)70

18,150

2

0.83

100%

100%

100

30

365

302910,470

2

0.46

100%

100%

100

30

365

6166,980

1

0.25

100%

100%

200

30

365

4

An example of the parameters used for this type of calculation are shown in Table 14-7. It should be noted that these values can vary greatly, depending on the assumed exposure conditions (i.e., the selected exposure scenario). As an example, the air breathing rate for adult males is 0.83m3/h in Table 14-7. However, this rate can vary by an order of magnitude from 0.6m3/h at rest to 7.1m3/h for vigorous physical exercise. It should be noted that carinogenic and noncarcinogenic human health risks are often computed by using both average and maximal chemical intakes. The purpose of using both values is to calculate most probable and worst-case risks, two end points that may have meaning to either regulators or potential receptor populations.

EXAMPLE 14-3A. CALCULATION OF CONTAMINANT INTAKE. Determine the chronic daily inhalation intake, by adults in a residential setting, of a noncarcinogenic chemical as a function of concentration in fugitive dust at the ABC Landfill (Table 14-3).Solution. For an adult exposed to a noncarcinogenic constituent, the intake IN may be calculated from Equation 14-3:

From Table 14-7, air breathing rate for adults = 0.83 m3/h.

CR= 0.83 m3/h x 24 h/day = 19.92 m3/day

EF= 365 days/year

For residential exposures, a default value of ED = 30 years is typically used. In the absence of better information, a conservative approach would assume the retention rate (RR) and the absorption into bloodstream would both equal 100 percent or 1.0.

BW = 70kg (Table 14-7)

AT = 365 days/year x 30 years = 10,950 days

= 0.285 m3/(kg . day) x CWhere C = exposure point concentration (mg/m3)EXAMPLE 14-3B. CALCULATION OF CONTAMINANT INTAKE. Determine the chronic daily inhalation intake in children age 6-12 of a carcinogenic chemical (i.e., calculate IC).Solution.

From Table 14-7:

CR= 0.46 m3/h x 24 h/day = 11.04 m3/day

EF= 365 days/year

ED= 6 years

RR= Abs = 1.0 as in Example 14-3a

BW= 29 kg (Table 14-7)

The averaging time (AT) for determining carcinogenic risk is 70 years or 25,550 days. As noted in the text, this is to be consistent with the approach used to develop carcinogenic slope factors.

= 3.26 x 10-2 m3/(kg . day) x CWhere C = exposure point concentration (mg/m3)EXAMPLE 14-4A. AVERAGE DAILY INTAKE FROM DERMAL CONTACT WITH SOIL. Determine the average daily intake of chlorobenzene over 1 year of exposure for on-site workers from dermal contact of the soil in Table 14-3.Assume the following additional parameters:

A= skin exposed = 20% = 0.2 x 18,150 cm2 = 3630 cm2

DA= dust adherence = 0.51 mg/cm2

Abs= skin absorption rate = 6%

SM= effect of soil matrix = 15% (i.e., because of soil matrix, only 15% of contamination is actually available for contact)

EF= two exposure events per day; 156 exposure days per year

ED= 1 year

BW= 70 kg

AT= 365 days or 1 year

Solution.

=

= (2.03 x 10-7 x C) mg/(kg . day)

From Table 14-3, the average concentration of chlorobenzene in soil is 1.39 mg/kg. The daily intake of chlorobenzene is

1.39 mg/kg x 2.03 x 10-7Thus, IN = 2.82 x 10-7 mg/(kg . day)EXAMPLE 14-4B. AVERAGE DAILY INTAKE FROM DERMAL CONTACT WITH SOIL. Determine the average daily intake of chloroform for on-site workersfrom dermal contact os the soils in Table 14-3. Assume that the intake estimate will be used to assess the carcinogenic effects of chloroform (o.e., calculate IC).Solution.

=

= 2.91 x 10-9 x C

From Table 14-3, the daily intake of chloroform is 1.12 mg/kg x 2.91 x 10-9. Thus,

IC = 3.26 x 10-9 mg/(kg . day)It should be noted that unlike the oral and inhalation routes, there are no generally accepted reference concentrations or slope factors for the dermal route. In lieu of these, oral RfCs and SF values may be utilized, but the advice of a qualified toxicologist should be sought.Equation similar to those developed in Examples 14-3 and 14-4 are used in spreadsheets to calculate administered/absorbed doses for all chemicals of concern for all pathways. These are repeated for each exposure scenario.14-4 TOXICITY ASSESSMENTThis stage of the risk assessment process defines the toxicity (i.e., the dose-response relationship) for each chemical of concern. The output takes the form of mathematical constants for insertion into risk calculation equations. In addition to providing a set of mathematical constants for calculating risk, the toxicological assessment should also analyze the uncertainty inherent in these numbers, and describe how this uncertainty may affect the estimates of risk.

Chapter 5 explains in detail the scientific basic underlying the development and application of dose-response relationships. This section highlights some of the important concepts and quantitative methods involved in such undertakings and how they apply to the calculation of the risk from exposure to hazardous waste constituents.

Carcinogens versus NoncarcinogensFor the purposes of quantifying human health risks, chemicals are characterized as carcinogens (i.e., those with demonstrated propensity for cancer induction) and noncarcinogens. Some chemicals behave as both carcinogens and noncarcinogens and, hence, will appear in both types of calculation of potential human health risk.

Carcinogens tend to dominate public concerns about health risk; however, this is not the reason for distinguishing between chemicals that induce cancer and those that do not. The distinction is important because the two elicit toxic responses in different ways, giving rise to two quantitatively different models for induction of toxic response as a function of dose. Thus, the mathematical expressions of risk differ for the two categories of chemicals. Noncarcinogens have thredholds below which they fail to induce any discernible adverse health effect. In contrast, a linear nonthreshold model has been adopted for all carcinogens by the EPA (see Figure 5-24). According to this model, some risk is assumed for carcinogens at any dose, regardless of how small. There is increasing evidence, however, that some carcinogens do have thresholds, especially those that function as promoters.Sources of Toxicity DataThe experimental research effort involved in developing a new dose-response relationship for a toxic substance takes considerable time, much longer than associated with starting and completing a study of most hazardous waste issues. Rather than conduct experimental research, the risk analyst defers to existing data found in standard sources of toxicological data, and selects from them the appropriate mathematical descriptors of toxicity.Perhaps the most used source is IRIS (Intergrated Risk Information System). The toxicologic indices in the IRIS database are updated continuously and available on-line (www.epa.gov/iris/). The IRIS database contains both qualitative and quantitative data regarding noncarcinogens as well as carcinogens.

It should be noted that regulatory agencies will frequently specify the particular mathematical constants to be used. Whether taken from regulatory guidance or other sources, the toxicity constants must apply to the range of doses predicted by the exposure assessment. Also, because toxic response for the same dose can vary, depending on the exposure route, a separate toxicity constant frequently must be selected for each exposure route predicted by the exposure assessment.Slope Factors for CarcinogensReference Concentrations for Noncarcinogens

Protective Nature of EPA Approach

14-5 RISK CHARACTERIZATION

Risk for Average and Maximum Exposures

Carcinogenic Risk

Noncarcinogenic Risk

Background Risk

Iterative Nature of Risk Assessment

Uncertainty Inherent in Calculated Risks14-6 RISK COMMUNICATION

Having characterized the risk of exposure to consituents of hazardous waste and the degree of uncertainty associated with the risks, the next step is to use the information to improve the basis for making decisions. This often involves the public, or at least embraces public concerns and attitudes. It requires an examination of the question, What is acceptable risk? It enters the area of perception, and while not leaving the world of science, it is much different world and one not relished by many scientists and engineers. (Sau khi xc nh c cc c im ri ro ca s phi nhim i vi cc thnh phn ca cht thi nguy hi v mc khng n nh ng vi cc ri ro ny, bc tip theo l s dng cc thng tin ny ci thin nn tng cho cc quyt sch. iu ny thng lin quan n cng ng, hoc t nht l bao qut c thi v cc s lin quan n cng ng. N yu cu mt bi kim tra hi p, Ri ro c th chp nhn c l g? N bc vo khu vc ca s nhn thc, v trong khi khng ri b th gi khoa hc, n l mt th gii rt khc bit v mt c nhn no khng thch th vi cc nh khoa hc v cc k s.)What is Acceptable Risk?At present the EPA has defined acceptable risks for carcinogens as within the range of 10-4 to 10-6 excess lifetime cancer risk and for noncarcinogens as a hazard index of less than 1.0. As discussed in Section 5-5, there have been precedents with other issues which have defined acceptable risk outside this range. The U.S. FDA deemed that less than 10-7 cancer risk for saccharin was acceptable, and local citizens were willing to accept a risk much greater than 10-4 for a copper smelter. Clearly, acceptability is a personal concept and demands that the public, which ultimately must have jurisdiction over what level of risk is acceptable, be informed. At many sites it is ultimately the public that determines by its influence which levels of potential health risks are acceptable.The EPA uses 10-6 excess lifetime cancer risk as a point of departure, meaning that higher risk may be deemed aceptable only if there was special extenuating circumstances. For purposes of comparison, examples of actions that would increase the risk of death by a probability of 1 x 10-6 are illustrated in Table 14-10. Almost all of the actions shown in Table 14-10 seem highly acceptable if not commonplace; yet, the magnitude of their risk compares with that of the EPAs target risk for hazardous waste sites. It should be noted that risk of cancer is not the same as risk of death because not all cancer cases result in death. Still, cancer is the second greatest cause of death in the United States.

TABLE 14-10 Actions increasing risk of death by one in a million

ActionNature of risk

Smoking 1.4 cigarettes

Drinking 0.5 liter of wine

Spending 1 hour in a coal mine

Spending 3 hour in a coal mine

Living 2 days in New York or Boston

Traveling 6 minutes by canoe

Traveling 10 miles by bicycle

Traveling 30 miles by car

Flying 1000 miles by jet

One chest X-ray taken in a good hospital

Living 2 months with a cigarette smoker

Eating 40 tablespoons of improperly stored peanut butter

Drinking heavily chlorinated water (e.g., Miami) for 1 yearDrinking thirty 12-oz cans of diet soda

Living 5 years at site boundary of a typical nuclear power plant in the open

Living 150 years within 20 miles of a nuclear power plant

Eating 100 charcoal-broiled steaksCancer, heart diseaseCirrhosis of the liver

Black lung disease

Accident

Air pollution/ heart disease

Accident

Accident

Accident

Accident

Cancer caused by radiation

Cancer, heart disease

Liver cancer caused by Aflattoxin B1

Cancer caused by chloroform

Cancer caused by saccharin

Cancer caused by radiation

Cancer caused by chloroform

Cancer from benzo(a)pyrene

Another comparison is that of incremental risk and background risk. The 10-6 target represents an incremental risk of 0.0001 perent probability, an especially small level in comparison to the 25 percent background risk of the very same disease for which this regulatory target is directed. The total risk to an individual exposed at the EPAs target would increase from 25 percent to 25.0001 percent. This increase is hardly meaningful from a scientific perspective, especially considering that the exposed population is not the whole nation but isolated pockets.Risk PerceptionWhile an increase in cancer risk by an increment of 10-6 may not be significant from a scientific viewpoint, it easily can alarm the community near a hazardous waste site.The reason is perception. Some of the factors that affect the perception of risk are indicated in Table 14-11 and are discussed in Section 1-4. The factors work in such a manner that, for example, an action voluntarily undertaken by an individual is perceived as posing a smaller risk than one imposed on that individual, all else being equal. Almost all of the more risky items are associated with hazardous waste. Clearly, the four-stage risk assessment process described on the preceding pages can not incorporate such factors in a precise quantiative calculation.TABLE 14-11 Risk perceptionLess riskyMore risky

Voluntary

Familiar

Controlled by self

Chronic

Natural

Fair

Detectable

Not memorableInvoluntary

Unfamiliar

Controlled by others

Acute

Artificial

Unfair

Undetectable

Memorable

A concept accompanying public perception is the notion of acceptable risk. Fischoff, et al. identified five generic complexities that affect acceptable risk: Defining the decision problem

Assessing the facts

Assessing relative values

Addressing the human element in decision-making processes

Assessing the quality of the decisions

With each of these, there is a perception gap between the decision makers and the local citizens. Starting with the first item, a decision makers resposibility in site remediation climaxes with the evaluation of several remedial alternatives and the selection of one. The public likely will view this evaluation as an unnecessary exercise. They will ask, Why deliberate when it has been intuitively clear from the onset that the safest decision is to excavate the waste and haul it out of their community?The difficulties that surround assessing the facts are primarily those of dealing with the large magnitude of uncertainty inherent in any risk assessment. On the other hand, the public often expects that scientists and engineers should have absolute knowledge about risks before making decisions that affect them. The purposeful use of conservative safety factors at each stage to protect human health serves to inflate resulting risk numbers, and the public will often express more concern with the possible magnitude of the worst-case scenario, either not understanding or electing to ignore how unlikely if not incredible it may be. Merely learning that the assessment is filled with uncertainty may make the public uneasy about the estimates, believing that the true risk may be even greater than indicated.It is important to note that almost all of the resources applied in risk assessment are devoted to the first two items of Fischoffs list. The local citizensare just as influenced, perhaps more so, by the other three. Concentrating resources on the first two items is thought by decision makers to portray their decisions as purely objective. On the contrary, assessing risk and selecting from remedial alternatives is fraught with value judgments. Acknowledging the existence of values is far simpler than identifying and explaining the set of values underlying the decisions.It is critical that decision makers sufficiently address the human element. This probably represents the most important avenue for closing the perception gap between the decision maker and the public. To not do so is a failure to deal with risk in its broadest sense: Thinking about risks may be more productive than calculating them. Finally, rarely do decision makers revisit the issue long after the decisions have been made to determine if the decision was a good one and whether their decision-making process needs adjustment. Were the right questions asked? Did we solve the real problems?Risk CommunicationIt is perhaps now understandable that, given the five complexities listed by Fischoff, The public often sees proponents of risk assessment as trying to convince people to accept risks that the proponentsdo not face rather than acting to remove them. Not suprisingly, the remedy desired by the public is to remove the hazardous waste to somewhere else even though in scientific terms this may pose the largest risk.While it is possible to provide reasonable quantiative estimates of risks, it is clear from the previous discussion that it is very difficult to explain these risks to the affected public. This derives only in part from the highly technical nature of the risks assessment process. At many site, the selection of an alternative is driven by the demands of the citizens living in the vicinity of the site. It is therefore essential that consideration be given to communicating risks as well as calculating them.Santos and Edwards suggest that to achieve effective risk communication three questions must be answered: Is the communicator listening and acknowledging the concerns of the audience?

How capable is the spokesperson?

Can the objectives of the presentation be met while still meeting the information needs of the public?

14-7 ECOLOGICAL RISK ASSESSMENT*

Charaterization of Baseline Ecology Ecological Toxicity Assessment

Ecological Exposure Assessment

Ecological Risk Characterization

14-8 MONTE CARLO METHODS

Stochastic versus Deterministic Calculations

Method Description

An Example

14-9 CASE STUDY

Hazard Identification

Exposure Assessment

Toxicity AssessmentRisk Characterization

DISCUSSION TOPICS AND PROBLEMS

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