COMPARATIVE EVALUATION

OF CHEMICAL RANKING

AND SCORING METHODOLOGIES

EPA Order No. 3N-3545-NAEX

Prepared by

University of Tennessee

Center for Clean Products and Clean Technologies

Gary A. Davis, Principal Investigator

Mary Swanson, Senior Research Associate

Sheila Jones, Research Assistant

April 7, 1994

Printed on Recycled Paper
ii

TABLE OF CONTENTS

LIST OF TABLES ............................................................ iii

LIST OF FIGURES ........................................................... iii

LIST OF ACRONYMS ........................................................ iv

SECTION 1: INTRODUCTION ................................................. 1

SECTION 2: METHODS ...................................................... 3

2.1 SUMMARY OF SYSTEMS ......................................... 3

2.2 DETAILED DESCRIPTION OF SYSTEMS ............................ 8

2.3 DESCRIPTION OF IMPORTANT ELEMENTS AND APPLICATIONS

OF RANKING SYSTEMS .......................................... 9

2.3.1 Purpose and Application of Chemical Ranking and Scoring Systems ..... 10

2.3.2 Human Health Criteria and Endpoints ............................ 12

2.3.3 Criteria and Endpoints for Environmental Effects .................... 22

2.3.4 Measures of Exposure ....................................... 24

2.3.5 Missing Data, Data Selection Approach .......................... 29

2.3.6 Aggregation and Weighing .................................... 31

2.3.7 Other Issues ............................................... 33

SECTION 3: DISCUSSION ................................................... 38

3.1 RISK ASSESSMENT PRINCIPLES APPLIED TO CHEMICAL

RANKING AND SCORING ....................................... 38

3.2 SIMILARITIES AND DIFFERENCES AMONG SYSTEMS REVIEWED .... 39

3.2.1 Most Common Effects and Exposure Endpoints Used ............... 39

3.2.2 Ranking versus Categorization ................................. 39

3.2.3 Quantitative versus Qualitative Endpoints ......................... 41

3.2.4 Assigning Scores to Endpoint Data .............................. 41

3.3 GENERAL STRENGTHS AND WEAKNESSES OF EVALUATED

SYSTEMS ..................................................... 44

SECTION 4: CONCLUSIONS ................................................. 46

REFERENCES .............................................................. 49

APPENDIX A SUMMARIES OF RANKING AND SCORING SYSTEMS ........ A-1

APPENDIX B RANKING AND SCORING SYSTEMS IDENTIFIED ................. B-1

APPENDIX C DETAILED TABLES OF CRITERIA, ENDPOINTS,AND DATA

APPROACH USED BY EACH EVALUATED SYSTEM .......... C-1
iii

LIST OF TABLES

Table No. page

1. Chemical Ranking and Scoring Systems Evaluated ................................ 4

2. Endpoints Used for Scoring Environmental and Human Health Effects ................. 16

3. Endpoints Used for Scoring Exposure ......................................... 26

4. WP Factors (WP) Scores for Several Systems .................................. 42

B-1 Overview of Purpose and Developers/Users of Ranking and Scoring

Systems Evaluated ...................................................... B-1

B-2 Overview of Purpose and Developers/Users of Additional Ranking

and Scoring Systems Identified ............................................. B-2

C-1 Toxicity Criteria and Endpoints Used in Evaluated Systems ....................... C-1

C-2 Overview of Exposure Criteria and Endpoints Used in Evaluated Systems ........... C-26

C-3 Overview of Data Selection Approach Used in Evaluated Systems ................ C-33

LIST OF FIGURES

Figure No. page

1. Comparison of Scoring for Several Systems .................................... 43

2. Development of a Chemical Ranking and Scoring System .......................... 47
iv

LIST OF ACRONYMS

AALAC: ambient aquatic life advisory concentration

ARET: Accelerated Reduction/Elimination of Toxics 

ADI: allowable daily intake

ATSDR: Agency for Toxic Substances and Disease Registry

AWQC: aquatic water quality criteria

BAF: bioaccumulation factor

BCF: bioconcentration factor

BP: boiling point

BUA: Beratergremium für Umweltrelevante Altstoffe

CAG: carcinogen assessment group

CCHTE: chemical category human toxicity estimate

CFC: chlorofluorocarbon

CMR: Critical Materials Register

DfE: Design for the Environment

EC: European Communities

EC50: the median effect concentration; the concentration at which 50% of the test population exhibit a

specified response during a specified time period

ED10: estimated dose associated with a lifetime increased cancer risk of 10%

EEC: European Community Commission

ELU: Environmental load units

EMPPL: Effluent Monitoring Priority Pollutants List

EPA: United States Environmental Protection Agency

EPS: environmental priority strategies

ERRP: ecological risk reduction potential

GWU: George Washington University

HA: health advisory

HEAST: Health Effects Assessment Summary Tables

HHSATR: human health structure activity team rank

HPV: high production volume

HRRP: human risk reduction potential

HRS: Hazard Ranking System

HWQC: human health water quality criteria

IARC: International Agency for Research on Cancer

IC50: inhibitory concentration for 50% test population

IRIS: Integrated Risk Information System

ITC: Interagency Testing Committee

Kow: octanol-water partition coefficient

LC50: median lethal concentration; the concentration at which 50% of the test population die during a

specified time period

LCA: life cycle assessment

LD50: lethal dose for 50% test population

LDlo: lethal dose to some value < 50%
v

LIST OF ACRONYMS, continued

LET: lethality

LOAEL: lowest observable adverse effect level

MATC: maximum acceptable toxicant concentration

MED: minimum effective dose

MOE: Ministry of the Environment (Ontario)

NA: not applicable

NAAQS: national ambient air quality standard

NEL: no effect level

NFPA: National Fire Protection Association

NOAEL: no observable adverse effect level

NOEC: no observable effect concentration

NOEL: no observable effect level

NOx: nitrous oxides

NPL: National Priority List

NTP: National Toxicology Program

OECD: Organization for Economic Cooperation and Development

PGR: population growth rate

q1*: cancer potency slope factor 

QSAR: quantitative structure-activity relationship

RfC: reference concentration

RfD: reference dose

RQ: reportable quantity

SAR: structure-activity relationship

SARA: Superfund Amendments and Reauthorization Act

SATR: structure activity team rank

SIDS: screening information data set

SRS: Site Ranking System

TDlo: total dose resulting in a sublethal effect

TPQ: threshold planning quantity

TRI: Toxics Release Inventory

TSCA: Toxic Substances Control Act

TSCA CSSC: TSCA chemical scoring system category

UCR: unit cancer risk

URF: unit risk factor

UT: University of Tennessee (Knoxville, Tennessee)

VOC: volatile organic compound

WMS: Wet Milieugevaarlijke Stoffen

WOE: weight of evidence
 

1

 "Data selection approach" is a term used here to describe the methods used to build a comparable

data set for a specific measurable characteristic across a set of chemicals of interest. As an example, to

characterize the bioconcentration potential for a set of 100 organic compounds, measured

bioconcentration factors are selected first for all chemicals where this information is available. For all

other chemicals, a bioconcentration value is estimated from the octanol-water partitioning coefficient or

from water solubility data.

1

SECTION 1: INTRODUCTION

Between 60,000 and 100,000 of the over 8,000,000 chemicals listed by the Chemical Abstracts

Services Registry are commercially produced and are potential environmental pollutants. Some kind of

risk-based evaluation for these chemicals is often required to evaluate the impacts of chemical use or

releases, for regulatory action, and to set priorities for pollution prevention. The time and resources,

however, are not reasonably available to test all of these chemicals for their potential health and

environmental effects. During the last decade there have been vast improvements in the methods used

to test chemicals for toxicity and environmental fate and to interpret these data within a risk assessment

framework. We have not, however, developed generally accepted and widely used tools to better

enable us to set priorities and focus limited resources on selecting those chemicals for study that would

yield the greatest environmental benefits. 

Risk-based ranking and scoring systems can be used to focus attention and resources on the largest

potential hazards. Risk-based chemical ranking and scoring combines an assessment of both the toxic

effects of chemicals (human and/or environmental) and the potential exposure to those chemicals to

provide a relative evaluation of risk. Along with toxicity and exposure, ranking and scoring systems

may include other environmental impacts (e.g. ozone depletion) and some measure of economic impact

and/or societal value.

Although numerous ranking and scoring systems have been or are being developed, there is currently

no scientific consensus on risk ranking methods. Chemical risk ranking has received the most attention,

and several systems have been used, for example, to determine which chemicals should be included in

various regulatory pollutant lists. To facilitate development of a framework for overall human health and

environmental risk ranking, this report presents an evaluation of existing chemical ranking and scoring

systems. Approximately fifty systems are compared and evaluated in terms of:

• the purpose and application of the ranking and scoring system;

• the human health criteria and endpoints included;

• the criteria and endpoints included for environmental effects;

• whether measures of exposure are included;

• the data selection approach1 and handling of missing data;
2

• the use of aggregation and weighing of different health and environmental impacts;

• methods of accounting for chemical potency and severity of effects; and

• inclusion of other impacts or issues.

An evaluation of the important issues inherent to the development of any consensus ranking and scoring

system is presented, and important similarities and differences among the systems evaluated are

discussed.

In this document, following the introduction, Section 2 presents the methods used by the evaluated

systems. This includes a list and summary of the systems, a more detailed description of the systems,

and a description of important elements and applications of ranking systems. Section 3 contains a

discussion, based on the analysis of systems in Section 2, of the application of risk assessment

principles to chemical ranking and scoring, similarities and differences among the systems, and general

strengths and weaknesses of the evaluated systems. Section 4 presents conclusions and suggestions for

future work.
3

SECTION 2: METHODS

2.1 SUMMARY OF SYSTEMS

Fifty-one ranking and scoring systems have been reviewed for this report (listed in Table 1). A

summary is provided for each of these in Appendix A. The systems selected for evaluation span a wide

range of methodologies and levels of complexity (from simple and straight-forward to fantastically

elaborate). Evaluating this variety of systems is intended to provide a representative sampling of the

approaches to chemical ranking and scoring and the issues involved in developing a standardized (or

consensus) framework.

It is necessary to distinguish chemical ranking and scoring systems from other risk ranking systems

such as those that primarily rank sites (e.g. potential sites for listing on the National Priorities List) or

issues (e.g. risk from hazardous waste sites versus air pollution, etc.). The majority of ranking and

scoring systems that we have evaluated are based on a chemical-specific approach, which involves

ranking, scoring, or categorizing a list of chemicals based on chemical-specific attributes. A system is

considered a chemical ranking or scoring system if it meets the following criteria:

• it ranks or scores a list of chemicals;

• it results in a relative ranking or scoring, not quantitative measure(s) of risk;

• it includes measures of either toxicity alone or toxicity and exposure; or

• if it is a subsystem of a site or issue ranking method, the approach ranks sites or issues

based on an initial chemical-specific evaluation.

Thirteen of the fifty-one systems reviewed here did not meet these conditions. The following systems

reviewed did not meet the criteria of a chemical scoring or ranking system as defined above:

• CERCLA Hazard Ranking System (HRS) (EPA 1990): a site ranking system;

• Modified Hazard Ranking System (Hawley & Napier, 1985): a site ranking system;

• A Groundwater Pollution Priority System (Hutchinson & Hoffman): a site ranking

system;

• EPA Unfinished Business Report (Morgenstern et. al., 1987): used to rank

environmental issues which pose societal risks;

• Coastal Hazardous Waste Site Review (Beckvar & Harris, 1985): a site ranking

system;

• Site Ranking System (SRS) for Chemical and Radioactive Waste (Rechard et. al.,

1988, 1991): a site ranking system;

• Evaluating Contamination Potential of Surface Impoundments (Silka &

Swearingen, 1978): a site ranking system;
4

TABLE 1. List of Ranking and Scoring Systems Evaluated

System

no. a System Name or Title Reference

1 TRI Environmental Indicators Methodology (draft) Abt Associates, Inc., 1992

2 ATSDR, "CERCLA Section 104 Third Priority List" ATSDR, 1992

3 Existing Chemicals of Environmental Relevance Behret, 1989a

4 Existing Chemicals of Environmental Relevance II, Selection Criteria

and Second Priority List

Behret, 1989b

5 Review of Region VII TRI Strategy Bouchard, 1991

6 Candidate Substance List for Bans or Phase-outs Socha et al, 1992

7 Criteria Identifying High Risk Pollutants BNA, 1991

8 A Classification System for Hazardous Chemical Wastes Crutcher & Parker, 1990

9 CERCLA Hazard Ranking System (HRS) EPA, 1990

10 Identifying Chemical Candidates for Sunsetting: George Washington

University 

Foran & Glenn, 1993

11 Existing Chemicals: Systematic Data Collection and Handling for

Priority Setting

Gjøs et al., 1989

12 Substances and Preparations Dangerous for the Environment: A

System for Classification, Labeling and Safety Data Sheets

Gustafsson & Ljung, 1990

13 Notes on Ranking Chemicals for Environmental Hazard Halfon & Reggiani, 1986

14 Application of the Hazard Ranking System to the Prioritization of

Organic Compounds Identified at Hazardous Waste Remedial Action

Sites 

Hallstedt et al., 1986

15 Modified Hazard Ranking System (mHRS); A Ranking System for

Hazardous Waste Sites with Mixed Radioactive and Hazardous

Wastes 

Hawley & Napier, 1985

16 A Groundwater Pollution Priority System (GWPPS) Hutchinson & Hoffman,

1983

17 The Great Lakes Water Quality Agreement (GLWQA) Annex 1, Lists

1,2,3 

IJC, 1989
TABLE 1. List of Ranking and Scoring Systems Evaluated

System

no. a System Name or Title Reference

5

18 Chemical Scoring by a Rapid Screen of Hazard (RASH) Method Jones et al., 1987

19 Systematic Approach for Environmental Hazard Ranking of New

Chemicals ("Schmallenberg")

Klein et al., 1988 

20 WMS Scoring System Könemann & Visser, 1988;

Timmer et al., 1988

21 Benchmark Comparisons Laskowski et al., 1982

22 Michigan Critical Materials Register (MCMR) Michigan DNR, 1987

23 USEPA Unfinished Business Report: A Comparative Assessment of

Environmental Problems 

Morgenstern et al., 1987

24 Chemical Scoring System for Hazard and Exposure Identification O'Bryan & Ross, 1988

25 Effluent Monitoring Priority Pollutant List (EMPPL) Environment Ontario, 1987,

1988

26 Coastal Hazardous Waste Site Review Beckvar & Harris, 1985

27 Site Ranking System (SRS) for Chemical and Radioactive Waste Rechard et al., 1988;

Rechard et al., 1991

28 A Practical Method for Priority Selections and Risk Assessment

Among Existing Chemicals 

Sampaolo & Binetti, 1986,

1989

29 UT Chemical Ranking System (draft) Davis et al., 1993

30 A Manual for Evaluating Contamination Potential of Surface

Impoundments 

Silka & Swearingen, 1978

31 The EPS Enviro-Accounting Method Steen & Ryding, 1992

32 Defense Priority Model (FY 1993 Version) U.S. DOD, 1991

33 Hazardous Air Pollutants: Proposed Regulations Governing

Constructed, Reconstructed and Modified Major Sources

EPA, 1993b (40 CFR 63)

34 Ranking System for Clean Water Act Section 307(a) List of Priority

Pollutants 

Poston & Prohammer, 1985;

Cornaby et al, 1986
TABLE 1. List of Ranking and Scoring Systems Evaluated

System

no. a System Name or Title Reference

6

35 CERCLA Section 102 Reportable Quantity Ranking Process EMS, 1985;

EPA, 1989c

36 The Source Category Ranking System Radian Corp., 1990

37 Sax Toxicity Ratings (or Hazard Index) Sax & Lewis, 1989

38 Examination of the Severity of Toxic Effects and Recommendations

of a Systematic Approach to Rank Adverse Effects 

Environ 1986

39 Screening Procedure for Chemicals of Importance to the Office of

Water 

EPA, 1986

40 Measuring Air Quality: The New Pollutants Standards Index EPA, 1978

41 Ranking the Relative Hazards of Industrial Discharges to POTWs

and Surface Waters

Abt Assoc., 1991

42 Targeting Pollution Prevention Opportunities Using the 1988 TRI ICF, 1990

43 EPA Design for the Environment Program Use Cluster Scoring

System (also called: Chemical Use Clusters Scoring Methodology)

EPA, 1993a

44 Screening Methodology for Pollution Prevention Targeting EPA (date unknown)

45 Toxic Chemical Release Inventory Risk Screening Guide EPA, 1989a

46 TSCA's TRI Chemical Risk Assessment Pre-Screening Methodology EPA (date unknown)

47 Priority Setting of Existing Chemicals ("Schmallenberg") Weiss et al., 1988

48 Multi-Media Environmental Pollutant Assessment System

(MEPAS), formerly the Remedial Action Priority System (RAPS)

Whelan et al., 1987; Droppo

et al., 1989; Strenge et al.,

1989;

Whelan et al., 1992

49 Canadian Accelerated Reduction/Elimination of Toxics (ARETS)

Scoring Protocol 

CLC, 1992; Canadian ARET,

1993

50 Risk Assessment Guidance under CERCLA, screening chemicals of

potential concern

EPA, 1989d
TABLE 1. List of Ranking and Scoring Systems Evaluated

System

no. a System Name or Title Reference

7

51 EC Proposal for Priority Setting of Existing Chemical Substances

(Draft)

van der Zandt & van

Leeuwen, 1992 

(a) System numbers are used as a shorthand method of tracking the systems. They were based on a preliminary

alphabetical list of references, but at this time are essentially an arbitrarily assigned number.
8

• The EPS Enviro-Accounting Method (Steen & Ryding, 1992): developed for life

cycle impact assessment;

• Defense Priority Model (U.S. DOD, 1991): a site ranking system;

• Examination of the Severity of Toxic Effects (Environ Corp., 1986): not chemical

specific;

• Ranking the Relative Hazards of Industrial Discharges to POTWs and Surface

Waters (Abt Assoc., 1991): a site ranking system;

• Targeting Pollution Prevention Opportunities Using the 1988 TRI (ICF, 1990): a

site ranking system; and

• Multi-media Environmental Pollutant Assessment System (MEPAS) (Whelan et.

al., 1987; Droppo et. al., 1989; Strenge et. al., 1989; Whelan et. al., 1992): a site

ranking system.

For the sake of completeness, and to illustrate the diversity among what are called risk ranking systems,

these thirteen systems are also summarized in Appendix A.

2.2 DETAILED DESCRIPTION OF SYSTEMS

Descriptions of the fifty-one systems have been distilled into two-page summaries (Appendix A) that

include the following information:

• system name or title;

• publication references;

• the intended user of the system;

• the purpose or application;

• chemicals addressed or used for demonstration;

• summary of method or algorithm;

• an outline of the criteria, subcriteria and endpoints used; and

• data selection approach.

Several other surveys of existing (primarily chemical) ranking and scoring systems have been identified:

• Waters et. al. (1993) describes seventeen ranking and scoring systems, including nine

methods for ranking existing sites and eight "various chemical and substance ranking

methodologies";

• Foran and Glenn (1993) analyze eight "existing chemical scoring systems" in detail, and

include summary tables of effects criteria, endpoints, and scoring methods for the

screening systems analyzed;
9

• ICF (1993) provides detailed summaries and comparisons of five chemical scoring

systems with brief descriptions of fifteen additional systems in a bibliography;

• Abt Associates, Inc. (1992) summarizes, in some detail, twenty-one U.S.

Environmental Protection Agency (EPA) ranking systems. An additional thirteen

"ranking and indexing efforts" from outside of the EPA are described briefly. All of the

systems surveyed make use of TRI data;

• The Organization for Economic Cooperation and Development (OECD, 1986)

describes fifteen ranking and scoring systems in some detail and lists an additional fifty

"known priority ranking systems" by system name, developer, and intended user of the

system. Full references for the fifty systems were not included;

• Environ Corp. (1986) describes thirty-four "chemical ranking schemes", including

eleven from the EPA, eight from other federal agencies, four state ranking schemes, two

"degree of hazard" ranking schemes, three from international agencies, two "other

ranking schemes", and four carcinogen ranking schemes; and

• Hushon and Kornreich (1984) summarize thirty-four "existing chemical scoring

systems" in a table that includes the system title, purpose, developer(s), user(s), criteria

used for scoring, algorithms used, substances scored, and references.

When all of the systems described by these authors are included, a total of 147 ranking and scoring

systems have been identified. A complete listing of these is provided in Appendix B, with the system

name, the developers and intended users of the system, the purpose or application of the system, and

the system approach (where the information was available).

2.3 DESCRIPTION OF IMPORTANT ELEMENTS AND APPLICATIONS OF

RANKING SYSTEMS

Important components of any chemical ranking and scoring system include:

• the purpose and application of the ranking and scoring system;

• the human health criteria and endpoints included;

• the criteria and endpoints included for environmental effects;

• whether measures of exposure are included;

• the data selection approach and handling of missing data; and

• the use of aggregation and weighing of different impacts.
10

Other issues addressed include major ecological impacts, chemical mixtures, application to life cycle

assessment (LCA), and economic and social considerations. These are discussed below with examples

of various approaches used by the evaluated systems.

2.3.1 Purpose and Application of Chemical Ranking and Scoring Systems

Chemical ranking and scoring systems are typically used as screening tools for a rapid assessment of

relative chemical hazards. They consider the toxic effects of chemicals and some measure of exposure,

but are not intended to serve the same purpose as a quantitative risk assessment.

The purpose or intended use of the ranking and scoring system affects all of the aspects of the system,

including the human health and environmental impacts considered, the use of aggregation and weighing,

and how missing data are handled. Chemical ranking and scoring systems have been developed for the

broad purposes of regulatory action, priority setting, and impact evaluations. Each of these is discussed

below.

Regulatory Action

The need for the regulation of chemicals of concern has been the driving force behind the development

of many of the chemical ranking and scoring systems evaluated. Examples of this application are

described below:

The CERCLA Section 102 Reportable Quantity Ranking Process (EMS, 1985; EPA, 1989)

established reportable quantities (RQs) through a scoring process that takes into account environmental

and human health hazards of chemicals on the CERCLA list. RQs are threshold quantities at which

certain chemical releases must be reported to the EPA.

The Michigan Critical Materials Register (CMR) (MDNR, 1987) ranking process results in a list of

chemicals that may threaten water quality in Michigan. Chemicals included in the register are

considered to pose a high degree of environmental concern, and companies must report their use and

discharge of these chemicals.

The Ontario Ministry of the Environment (MOE) Candidate Substance List for Bans or Phaseouts (Socha et. al., 1992) is a ranking system that identifies chemical substances for release reduction,

bans, or phase-outs.

The George Washington University (GWU) scoring system (Foran and Glenn, 1993) is intended to

serve as a tool for pollution prevention in the Great Lakes region through the identification of chemical

substances for Sunsetting (bans or phase-outs) and other activities.
11

Priority Setting

Several ranking and scoring systems have been developed specifically for assessing chemical hazards

for priority setting for non-regulatory purposes. For example:

The EPA Design for the Environment Program (DfE) Use Cluster Scoring System (EPA, 1993) is

aimed at measuring potential health and environmental risks of chemicals that are used in industry

clusters (i.e., certain common industrial processes) as a way of evaluating the benefits of substitutes for

those chemicals. The relative risk scores are provided to industry as a means of encouraging the

voluntary adoption of pollution prevention measures.

The Wet Milieugevaarlijke Stoffen (WMS) Scoring System (Könemann and Visser, 1988; Timmer

et. al., 1988) was developed by the Directorate General for Environmental Protection of the

Netherlands Ministry of Housing, Physical Planning, and the Environment. It was intended for use in

industry, government, and academia for the selection of a limited number of chemicals as priorities for

further investigation. 

The Agency for Toxic Substances and Disease Registry (ATSDR) System (ATSDR, 1992): The

Superfund Amendments and Reauthorization Act (SARA) of 1986 required the ATSDR to prepare a

list, in order of priority, of the hazardous substances commonly found at National Priority List sites that

pose the most significant potential human health threats. Substances on this priority list become

candidates for the preparation of toxicological profile reports prepared by ATSDR.

The University of Tennessee (UT) Center for Clean Products and Clean Technologies chemical

ranking system (Davis et. al., 1993) prioritizes chemicals for safe substitutes assessments. The system

has also been used for impact evaluation, as discussed below, and for scoring of TRI releases from

chemical production facilities.

Impact Evaluation

Other scoring systems have been developed to evaluate the potential impacts of chemical releases.

The Toxics Release Inventory (TRI) Environmental Indicators Methodology (Abt Assoc., 1992) is

intended to evaluate TRI releases and derive a value to indicate the overall impacts of those releases by

all facilities and to each environmental medium. Annual calculations of the indicator numbers allow a

comparison of potential TRI impacts from year to year.

The UT ranking system (Davis et. al., 1993) has been modified to allow a comparative assessment of

the potential hazard or risk posed by aggregate TRI chemical releases and transfers from an entire state

or from facilities (Kincaid and Bartmess, 1993). The modified system was used to assess, on a relative

basis, the potential impacts from chemical releases for the five states with greatest releases (Tennessee,

Texas, Louisiana, Indiana, and Ohio) and is being used to score TRI releases from chemical production

facilities.
12

The New Pollutant Standards Index (EPA, 1978) was developed for use by local and state air

pollution control agencies to provide a simple method for reporting daily air pollutant concentrations

and to inform the public about the potential health effects associated with these concentrations. The

index is calculated from a measured air pollutant concentration and its national ambient air quality

standard, and is characterized with terms such as "good" or "unhealthful".

Another aspect of impact assessment is within the field of life cycle assessment (LCA). The methods

for conducting life cycle impact assessment are currently being developed (SETAC, 1993). Chemical

ranking and scoring methods with a focus on impact assessment have the potential to be applied to this

emerging technology. LCA is discussed further in section 2.3.7.

2.3.2 Human Health Criteria and Endpoints

In order to assess the potential or actual hazard associated with a particular chemical substance,

chemical ranking and scoring systems may include criteria for scoring the toxicity of a chemical to

terrestrial mammals, non-mammalian terrestrial species, aquatic organisms, and/or plants. The toxicity

of chemicals to these organisms is used to evaluate potential effects to human health and the

environment. Acute, subchronic and chronic mammalian toxicity data are often used as surrogates for

human health effects. Ecological effects typically include acute, subchronic and chronic toxicity to

terrestrial mammals, non-mammalian terrestrial species, aquatic organisms and plants. Although some

ranking and scoring systems include bioaccumulation and/or persistence in the environmental effects

category, these are discussed within the framework of this paper as factors affecting exposure.

Overview of Human Health Effects

Human health effects include many responses in humans caused by chemical exposure. Epidemiological

data may be used to characterize health effects, but laboratory mammalian toxicity data are most often

included in chemical ranking and scoring systems. The numbers and types of endpoints used to assess

potential health effects vary significantly.

Criteria for evaluating health effects often include toxicity resulting from varying durations of exposure. 

Those effects resulting from acute, sub-chronic and chronic exposure are commonly included in

chemical ranking and scoring systems. A brief description of each of these types of effects is listed

below: 

Acute Effects. Acute toxicity tests usually involve a single dose and a 14-day observation period. 

These tests are most commonly conducted on the mouse or rat, but other species, such as the dog or

rabbit, may be used. Acute effects are often characterized by lethality, commonly reported as the

mammalian median lethal dose or concentration (LD50 or LC50). This is the dose or concentration

required to elicit lethality in 50% of the animals tested. Non-lethal acute effects are sometimes included

as well. Skin or eye irritation and sensitization are examples of such effects. Routes of administration

commonly preferred include oral, dermal and inhalation exposure. Some systems use data from other

routes of exposure in the absence of preferred data.
13

Sub-chronic Effects. The most common test duration is 90 days for these tests, but the exposure time

may vary. The main goal of these studies is to determine the no-observed-effect-level (NOEL) and to

identify the specific organs affected after repeated doses of the test substance. These tests are usually

conducted on the rat or the dog by an oral route of administration, but other species and routes may be

used (Klaassen and Eaton, 1993). 

Chronic Effects. These are long-term (longer than 3 months) studies designed to assess the cumulative

toxicity of chemicals (Klaassen and Eaton, 1993). Chronic test data are often utilized in chemical

scoring systems. Chronic health effects may be evaluated on the basis of a wide variety of toxicological

endpoints. Carcinogenicity, mutagenicity, teratogenicity, reproductive toxicity, and other chronic toxic

effects are often included in chemical scoring systems. General chronic effects may be characterized by

RQs, reference doses (RfDs), threshold planning quantities (TPQs), the minimum effective dose

(MED), or other measures.

A brief description of chronic effects often assessed in chemical ranking and scoring is provided below:

Carcinogenicity. Chemical carcinogens are substances which cause cancer in humans or other animals. 

Carcinogenicity is often evaluated on the basis of weight-of-evidence classifications developed by the

EPA and/or strength-of-evidence classifications by the International Agency for Research on Cancer

(IARC). Weight-of-evidence classification considers all long-term animal and relevant human studies

as well as metabolism, pharmacokinetic and mechanistic information, structure-activity relationships

(SARs) and other studies involving biochemical or physiological function. Strength-of-evidence

classification may refer to the magnitude of conviction about the results of an experiment. For example,

the National Toxicology Program (NTP) classifies each carcinogenic bioassay according to the amount

and type of data from an experiment (Scala, 1993). The strength-of-evidence scheme developed by

IARC excludes mechanistic information relating to the relevance to humans of bioassay data which

show evidence of carcinogenesis in animals. This scheme does not consider all relevant data. For

these reasons, the IARC classification scheme is considered to be based on strength-of-evidence rather

than weight-of-evidence (Ashby et. al., 1990).

Sometimes one or both of the EPA and IARC classifications are combined with a measure of potency,

usually the slope factor (q1

*

) or the ED10 in chemical ranking and scoring systems. Potency refers to the

relationship between the dose and the response. The slope factor is an upper bound estimate of the

probability of a response per unit intake of a chemical over a lifetime. It is used to estimate the

probability of an individual developing cancer resulting from a lifetime exposure to a carcinogen (EPA

1989b). This is the potency factor normally used in EPA risk assessments. The ED10 is the estimated

dose associated with a lifetime cancer risk increase of 10 percent. This is used in establishing RQs

under CERCLA Section 102.

Mutagenic Effects. Mutagenesis occurs when chemicals cause changes in the genetic material which

can be transmitted during cell division (Klaassen and Eaton, 1993). Several procedures, both in vitro
14

and in vivo, have been developed to test chemicals for possible mutagenicity. Tests for mutagenicity

are often used to screen for potential carcinogenesis because the initiation of chemical carcinogenesis is

believed to be a mutagenic occurrence (Klaassen and Eaton, 1993). Like carcinogenicity, mutagenicity

is often evaluated according to weight-of-evidence classification which is sometimes combined with a

measure of potency and/or severity. Mutagenic effects may also be of interest in terms of genetic

alterations in the next generation.

Reproductive Toxicity. This refers to the occurrence of adverse effects, resulting from exposure to

chemical or physical agents, on the male or female reproductive system. These may include effects on

fertility, gestation, or lactation, among others (Klaassen Eaton, 1993). These effects are usually scored

according to some type of weight-of-evidence and/or potency and severity. 

Teratogenic Effects. Teratogenicity occurs when exposure to some chemical or physical agent induces

defects during the development of an organism from conception to birth (Klaassen and Eaton, 1993). 

Teratogenicity is sometimes considered separately from other reproductive effects. Likewise, these

effects are often scored on the basis of weight-of-evidence and/or some measure of potency and/or

severity.

Neurotoxicity. This includes adverse effects on the nervous system caused by chemical exposure which

may be structural and\or functional and may include behavioral changes and learning disabilities.

Several chemical ranking systems include toxicity scoring criteria based not on specific toxicological

endpoints, but on indices that combine or scale one or more toxicological endpoints, such as:

• RQ: A hazardous substance, under CERCLA, released in a quantity above this amount

must be reported to the National Response Center, the State Emergency Response

Commission and the Local Emergency Planning Committee;

• RfD: An estimated daily exposure to the human population that is likely to be without

appreciable risk or adverse effects in a lifetime, expressed in mg/kg/day; and

• TPQ: The amount of an extremely hazardous substance present at a facility above

which the emergency planning notification must be provided to the State Emergency

Response Commission and the Local Emergency Planning Committee.
15

Endpoints Used for Human Health Effects

Member countries of the OECD have undertaken the investigation of approximately 600 high

production volume (HPV) chemicals. A data set has been established which is necessary to provide an

initial screening of health and environmental risks. The Screening Information Data Set (SIDS) is the

minimum information needed for deciding whether or not a HPV chemical should be considered of low

current concern, considered for further information gathering or testing, or a candidate for further

review with possible action to reduce risks posed by the chemical (Auer, 1992). This represents an

effort to standardize, on an international basis, the basic experimental data needed to characterize the

potential health and environmental effects of chemicals.

The SIDS database (Auer, 1992) includes the following toxicological data:

• acute toxicity;

• repeated dose toxicity;

• genetic toxicity (two endpoints, generally point mutation and chromosomal aberrations);

and

• reproductive toxicity (including fertility and developmental toxicity).

The endpoints used to score human health and ecological effects are presented in Appendix C and in

Table 2. (The use of criteria and endpoints is discussed further in section 3.2.1.) Although most

chemical ranking and scoring systems evaluated were designed with human health protection in mind,

most of them do not clearly distinguish between endpoints included to assess human health from those

used to characterize toxicity to other terrestrial mammals. Often a score for 'general toxicity' is

determined. The manner in which systems evaluate health effects varies dramatically among the many

different systems. As can be seen, the specific endpoints selected to represent health effects are

numerous.

The Nordic System (Gjøs et. al., 1989) scores health effects based on data pertaining to acute

mammalian toxicity, irritation, sensitization, general toxicity, genotoxicity, carcinogenicity and

reproduction damage/teratogenicity.

The EPA DfE Use Cluster Scoring System (EPA, 1993) assigns a human health hazard ranking for

chemicals. This system assigns a score on the basis of criteria such as the RfD, RQ, or the TPQ,

among others. The carcinogenic properties of each chemical are also scored and the higher of the two

scores is selected as the human health hazard potential score. 
16

TABLE 2. Endpoints Used for Scoring Environmental and Human Health Effects

Endpoint System Number for Method Using this Endpoint a

Plant Toxicity (8 systems)b

 phytotoxicity in higher plants (qualitative) 28

 qualitative effects (algae) 28, 47

 aquatic, terrestrial IC50 17

 EC50 6, 22, 25, 47, 49

 NOAEL/NOAEC 6, 25, 49

Aquatic Toxicity (26 systems)

 AWQC (acute or chronic) 1, 9, 42, 45

 AALAC (ambient aquatic life advisory

 concentration)

1, 9

 NOAEL 1, 6, 10, 25, 49

 acute LC50 1, 2, 3, 4, 5, 6, 9, 10, 11, 12, 17, 19, 20, 22, 24, 25, 28, 29, 34,

35, 39, 49

 acute LD50 17

 acute EC50 3, 4, 10, 11, 12, 17, 19, 20, 22, 24, 28,

 subchronic or chronic EC50 6, 25, 34, 49

 subchronic or chronic MATC 6, 22, 25, 34, 49

 NOEL/NOEC 20, 24, 29

 aquatic toxicity RQ 41, 42

 population growth rate (PGR) (algae) 5

 lethality (LET) (algae) 5

 acute and prolonged fish, Daphnia toxicity 47

Terrestrial (non-mammal) Toxicity (8 systems)

 subchronic or chronic effects 6

 acute LC50 22

 acute LD50 10, 22, 28

 chronic NOAEL 10

 severity and effective dose 22

 subchronic or chronic NOEL 25, 49

 acute toxicity (bird, earthworm) 47
TABLE 2. Endpoints Used for Scoring Environmental and Human Health Effects

Endpoint System Number for Method Using this Endpoint a

17

 unspecified 11

Mammalian Toxicityc(19 systems)

 acute LD50 2, 3, 4, 6, 7, 10, 11, 12, 17, 19, 20, 24, 25, 28, 29, 34, 35, 49

 acute LC50 2, 3, 4, 6, 7, 11, 12, 17, 24, 25, 28, 35, 49

 subacute NOEL/NEL 19, 20, 28

 subchronic or chronic NOAEL 6

 skin or eye irritation 19, 28

 skin or eye sensitization 19, 28

 severity and effective dose 24

 subchronic or chronic NOEL 25, 49

 subchronic or chronic LOAEL 17

 subchronic or chronic LDlo and TDlo 34

 acute, subchronic, and chronic toxicity (rat) 47

General Ecological Effects (7 systems)

 ecological disruption 10

 ecosystem recovery potential 23

 ecological effects benchmarks 32

 AWQC 43

 aquatic toxicity RQ 43

 team rank chemical category 43

 ecotoxicity 44

 ecological effects RQ 45

 effect on fertility (mammals, birds, plants, etc) 47

Systemic (Non-carcinogenic) or General Health Effects (30 systems)

 acute LD50 and LC50 9, 29, 37

 NOAEL and LOAEL 1, 43

 LOEL 7

 10-day health advisory (HA) 5

 max. concentration level 8
TABLE 2. Endpoints Used for Scoring Environmental and Human Health Effects

Endpoint System Number for Method Using this Endpoint a

18

 severity and effective dose 10

 number of "highly toxic" chemicals 16

 potency, relative potency 18, 23

 other toxic effects 35

 type of effect 35, 36, 37, 38

 unspecified 46

 chronic MED 1, 2, 35

 RfD/RfC 1, 5, 7, 9, 23, 33, 42, 43, 45, 48, 50

 chronic toxicity severity rating (Rv) 2

 systemic mammalian toxicity 10

 irritation 11, 47, 51

 sensitization 11, 47, 51

 general acute or chronic toxicity 5, 8, 11, 33, 36, 44, 51

 chronic ADI 27, 32

 RQ (acute, chronic, cancer) 33, 41, 42, 43, 45

 NAAQS 40

 TPQ 42, 43, 45

 neurotoxicity 29, 44

 Toxic Substances Control Act chemical scoring

 system category (TSCA CSSC)

43

 HWQC 43

 chemical category human toxicity estimate

 (CCHTE)

43

 human health structure activity rank (HHSATR) 43

Carcinogenicity/Mutagenicity/Genotoxicityc (36 systems)

carcinogenicity

 weight, amount, or type of evidence;

 or probability

1, 2, 4, 6, 7, 9, 10, 17, 20, 22, 24, 25, 29, 33, 35, 39, 43, 44, 49

 potency or slope factor (q1* or CPF) 1, 9, 42, 43, 50

 ED10 1, 9, 10, 33, 35

 qualitative potency or effects 2, 3, 4, 23, 25, 34, 48, 49
TABLE 2. Endpoints Used for Scoring Environmental and Human Health Effects

Endpoint System Number for Method Using this Endpoint a

19

 CERCLA hazard rank 7

 carcinogen (yes or no) 11, 20, 47

 URF (unit risk factor) 5, 7, 45

 UCR 27

 cancer potency RQ 41, 43

 EEC guidance 28

 unspecified endpoint 36, 51

 number of carcinogens 16

mutagenicity

 weight, amount, or type of evidence 1, 4, 17, 22, 51

 severity or dose 1

 effects 3, 4, 34

 EEC guidance 28

 bacterial or short-term tests 19, 20

 mutagen (yes or no) 29, 47

genotoxicity

 weight, amount, or type of evidence 1, 17, 24

 severity or dose 1

 genotoxin (yes or no) 11

Developmental/Reproductive Toxicity (19 systems)

 unspecified 36, 44

 weight, amount, type of evidence 1, 10, 22, 24, 51

 severity, dose 1, 10

 teratogenic effects 6, 34

 effective dose 6, 17, 22, 25, 49

 LOEL 7

 developmental or reproductive toxin (yes, no) 11, 29, 39, 47

 teratogen, EEC guidance 28

 teratogenesis and fetotoxicity 38
TABLE 2. Endpoints Used for Scoring Environmental and Human Health Effects

Endpoint System Number for Method Using this Endpoint a

20

Physical Hazard and Other Properties (15 systems)

 ignitability, flash point 2, 11, 22, 28, 35, 37

 boiling point 2, 11, 28, 35, 51

 reactivity, explosivity 2, 11, 22, 28, 35, 37

 corrosivity 22

 odor/taste and appearance 22

 pH 22

 molecular weight 28, 48

 melting point 28

 relative density 28

 vapor pressure 28, 48, 51

 surface tension 28

 water solubility 28, 48

 fat solubility 28

 oxidizing properties 28

 ecological risk reduction potential (ERRP) 43

 human health risk reduction potential

 (HHRRP)

43

 radionuclide dose factors 15, 48

 welfare effects (recreation, aesthetics etc.) 23

 gross alpha 16

 Sax toxicity rating 14, 16

 presence on CAA Amendments list 42

 low Kow 51

 other dangers not covered by specific 

 criteria

12

(a) refer to Table 1 for the corresponding systems

(b) the number of systems listed in parentheses indicates how many of the evaluated systems consider some

endpoint in the listed category

(c) criteria could apply to ecological and/or human receptors, depending on the system. Often, this distinction is not

clear in the system documentation.
21

The CERCLA HRS (EPA, 1990) evaluates human health effects based on acute toxicity and two

categories of chronic toxicity, which include cancer and non-cancer toxicological responses. The noncancer score is based on the RfD, and the score for carcinogenicity is based on the q1

*

. The higher of

the two chronic toxicity scores is the score assigned for human toxicity. Acute toxicity data are used to

score human toxicity only in the absence of RfD and q1

*

 data.

The Ontario MOE Candidate Substances List for Bans or Phase-outs system (Socha et. al., 1992)

includes mammalian acute lethality, sublethal effects on mammals and chronic effects such as

teratogenicity, genotoxicity/mutagenicity and carcinogenicity for evaluating human health effects.

Not only is there a wide variation of endpoints selected for assigning a score or scores in the area of

human health effects, but the preference given to these endpoints may vary as well. For example, the

HRS (EPA, 1990) prefers chronic toxicity data over acute data, whereas many other scoring systems

include both. Often, the purpose or application of the system affects the choice of criteria to be

considered.

Potency and Severity of Effects

Potency. The dose required to elicit a toxic effect is referred to as potency. Data regarding the potency

of chemicals in test organisms are more frequently used to assess acute rather than chronic effects to

terrestrial and aquatic plants and animals. For acute toxicity endpoints such as lethality, the dose

required to elicit the effect (i.e. potency) is commonly reported (e.g. LC50). Data regarding the potency

of chemicals required to elicit chronic effects such as mutagenicity or carcinogenicity, however, are

often limited. 

Many ranking and scoring systems assign scores, particularly for carcinogenicity, based only on the

weight of evidence that the chemical causes the effect in humans. Examples include: 

• Michigan CMR (MDNR, 1987);

• German Existing Chemicals of Environmental Relevance List I and II (BUA;

Behret, 1989a,b); and

• The MOE Candidate Substance List for Bans or Phase-outs (Socha et. al., 1992).

It is helpful to distinguish between a highly potent carcinogen and a moderately potent one, but the

availability of potency data is limited. A few systems include both weight-of-evidence and potency

data. For example, GWU (Foran and Glenn, 1993) assigns a score according to a matrix which

includes a weight-of-evidence classification as well as the potency factor (1/ED10) that has been used to

assign reportable quantities for carcinogens by the CERCLA Section 102 program. In the EPA DfE

Use Cluster Scoring System (EPA, 1993) weight-of-evidence is combined with the reportable

quantity potency factor or the q1

*

. The CERCLA HRS (EPA, 1990) assigns values based on weightof-evidence and the q1

*

 or an estimate based on the ED10.
22

Severity. Severity is a function of both the type and magnitude of the effect. The type of effect (e.g. a

specific biochemical effect) reflects the mechanism of action as well as the target organ of a particular

chemical. The magnitude of the effect (e.g. the percent change from normal of the biochemical effect)

reflects the dose-response properties associated with a chemical (Environ, 1986). 

Currently, there is no widely accepted method for incorporating severity into chemical ranking and

scoring systems and many systems do not include measures of severity at all. This may yield misleading

results. For example, two chemicals may have exactly the same NOEL of 0.3 mg/kg/day for

neurotoxicity. Assigned scores based on the NOEL would be the same for these two chemicals, even

though at similar doses one may cause a temporary narcotic effect and the other long-term irreversible

brain damage. Below are examples of two systems that do include severity when scoring substances:

The Toxic Substances Control Act (TSCA) System (O'Bryan and Ross, 1988) includes severity in

several parameters. For example, for nonlethal acute toxicity, the dose score is multiplied by the

severity score to arrive at a final score in this category. The severity of effects are scored according to

whether the effect is life-threatening or severe, moderately serious, mild, or no effects are observed at

high doses.

The Michigan CMR (MDNR, 1987) also includes severity in several parameters. For example, a

score for "other toxicity" (chronic or subchronic) to terrestrial animals includes severity ranging from

adverse effects to severe effects. Examples of each effect classification are provided to guide decisions

in response to the question of how severe is the effect. For example, "moderate effects" include effects

such as degenerative or necrotic changes with no apparent decrement of organ functions; or reversible,

slight changes in organ function. 

2.3.3 Criteria and Endpoints for Environmental Effects

Overview of Environmental Effects

Ecological effects resulting from chemical exposure may occur in populations of organisms from many

trophic levels. These include both terrestrial mammalian and non-mammalian species, aquatic

organisms, and plants. Most ranking systems evaluate ecological effects through the use of toxicity test

data. Typically, surrogate species are selected for use in the evaluation of the potential environmental

hazards of chemicals. 

When including toxicity information in a chemical ranking and scoring system, it is important to specify

the following information regarding the endpoints to be considered (EPA, 1989d):

• organism tested or observed;

• nature of the effect;

• concentration or dose needed to produce the effect;

• duration of exposure needed to produce the effect; and 
23

• environmental conditions under which the effects were observed.

Similar to tests used for evaluating human health effects, the durations for testing chemicals of ecological

concern include acute, sub-chronic and/or chronic exposure periods. Observations include lethality as

well as sub-lethal effects. Lethal doses directly disrupt important physiological functions and result in

death. Sub-lethal toxicity may have long-term physiological or behavioral impacts on a population.

Terrestrial effects. Terrestrial organisms commonly considered by chemical ranking and scoring systems

to be important receptors include mammals, non-mammals and plants. Most scoring systems do not

make a distinction between mammalian toxicity and human health effects. At least two of the chemical

ranking and scoring systems examined use the same endpoint for human health effects as was used to

characterize terrestrial mammalian toxicity (rodent acute toxicity). Terrestrial non-mammalian data are

usually derived from tests on avian species such as the mallard duck or ring-neck pheasant. Data for

terrestrial plants are sometimes included, but such data are limited. Other organisms may be included

as surrogates, but unfortunately, there is not a large data base of information regarding the effects of

chemicals on most terrestrial organisms. 

Aquatic effects. The aquatic receptors of interest in most chemical ranking and scoring systems include

non-mammalian aquatic organisms and plants. The most common surrogates for evaluating the toxic

potential of chemicals include fish, Daphnia and algae. There is a fairly large data base of information

regarding the toxicity of chemicals to these organisms. The majority of available data are from shortterm toxicity tests performed on these species, but in reality there is often greater concern regarding the

effects of long-term exposure and effects. Additionally, the types of organisms most often tested are

not necessarily those about which there is the most concern. Due to standardized test methods and

data availability, however, it is reasonable to include these commonly tested organisms in chemical

ranking and scoring systems. 

The endpoints most frequently included in ranking and scoring systems for aquatic toxicity are the LC50

and the EC50. The LC50 describes lethality and the EC50 describes the concentration at which there are

observed effects in 50% of the test population. The effects may be behavioral or physiological and may

indicate immobilization or changes in growth and reproduction, among others. Other endpoints, such as

the no-observable-adverse-effect-level (NOAEL) or regulatory standards such as ambient water

quality criteria (AWQC) may be included.

In the absence of data, there are over fifty high-quality quantitative structure-activity relationships

(QSARs) for estimating biological effects in these three organisms (Clements, 1993). QSARs are

discussed further in section 2.2.5.

Endpoints used for Environmental Effects
24

Endpoints used for environmental effects for each system are presented in Appendix C and are

summarized in Table 2. (The use of criteria and endpoints is discussed further in section 3.2.1.) The

following examples illustrate some of the types of data used to evaluate ecological effects:

The Schmallenberg System (Klein et. al., 1988; Weiss et. al., 1988) was developed by a group of

French and German officials and scientists and uses many endpoints to assess ecological effects,

including acute, subacute and chronic effects in algae, plants, earthworms, birds and fish.

The GWU system (Foran and Glenn, 1993) includes acute and chronic toxicity to aquatic organisms as

well as terrestrial and avian species.

The EPA Toxics Release Inventory Environmental Indicators Methodology (Abt Assoc., 1992)

scores ecological effects based on a matrix of aquatic toxicity and bioaccumulation. Aquatic toxicity is

scored according to the LC50, life cycle/chronic NOAEL, or other measures such as the Acute or

Chronic Ambient Water Quality Criteria (AWQC). Bioconcentration is based on the water solubility,

log Kow, or bioconcentration factor (BCF). 

The EPA DfE Use Cluster Scoring System (EPA, 1993) scores ecological hazard based on the

AWQC (acute and chronic) or Aquatic Toxicity Reportable Quantities.

The Michigan Critical Materials Register (MDNR, 1987) scores acute and chronic effects on

aquatic organisms (fish, invertebrates, amphibians), acute and chronic effects on terrestrial animals, and

plants.

The SIDS database (Auer, 1992) includes the following ecotoxicological data:

• acute toxicity to fish;

• acute toxicity to daphnids;

• acute toxicity to algae; and

• terrestrial toxicity (if significant exposure in terrestrial environment or if aquatic toxicity

testing is not possible).

2.3.4 Measures of Exposure

As stated earlier, chemical ranking and scoring systems span a wide range of complexity. The simplest

level is to use toxicity data alone. When exposure is considered, the systems become more complex. 

In fact, the level of complexity is primarily a function of the degree of sophistication used to estimate

exposure. In a quantitative risk assessment, a dose to each receptor from each potential pathway is

estimated on a site-specific basis using measured or modeled concentrations for each potentially

contaminated environmental medium. This approach represents a high degree of sophistication and is

not used in most chemical ranking and scoring systems. In a simpler approach, production or use

volume or TRI emission data are often used as surrogates for dose. An intermediate level of
25

sophistication might employ a multimedia fate and transport model to estimate the distribution of

chemicals in the environment based on release data.

Many factors affect the potential for exposure to a chemical. These factors generally relate to

properties of the chemical, characteristics of the human or environmental receptors of concern, amount

of the chemical available for exposure, and the behavior (fate, transport) of the chemical in the

environment. Appendix C and Table 3 present the measures of exposure used by the ranking systems

evaluated. This is also discussed further in section 3.2.1. Examples of the use of these various

measures for exposure follow.

The WMS Scoring System (Könemann and Visser, 1988; Timmer et. al., 1988) assigns scores for

environmental exposure according to use volume, percentage release to the environment, degradation in

air, soil and/or water, relative occurrence in these media and bioconcentration. Exposure via products

is also scored, and this includes use patterns, exposure frequency and intensity of exposure. 

The GWU system (Foran and Glenn, 1993) assesses exposure according to the bioaccumulation,

persistence, and release or production volume. 

The Michigan CMR (MDNR, 1987) scores chemical exposure according to bioaccumulation,

persistence, and a few physicochemical properties such as flammability, reactivity, and corrosivity.

The Effluent Monitoring Priority Pollutants List (EMPPL) (Environment Ontario, 1987, 1988)

includes environmental persistence, bioaccumulation and detection in the environment.

The German Beratergremium für Umweltrelevante Altstoffe (BUA) System (Behret, 1989a,b)

includes bioaccumulation, persistence and production volume.

The EPA Toxics Release Inventory Environmental Indicators Methodology (Abt Assoc., 1992)

uses facility-specific data and generic fate, transport, and exposure models to estimate a "surrogate

dose", or the amount of chemical an individual might be exposed to. A separate evaluation is

conducted for each release pathway allowing comparisons across media. The level of uncertainty is

included in the scoring for exposure. Exposure of aquatic life is obtained by estimates of the ambient

water concentration.
26

Table 3. Endpoints Used for Scoring Exposure

Endpoint System Number for Method Using this Endpoint a

Degradation or Transformation Potential (25 Systems)b

 degradability in air, water, soil and/or sediment

 (t 1/2)

3, 4, 6, 9, 10, 20, 21, 22, 24, 25, 34, 48, 49

 biodegradation 8, 19, 47

 transformation 24, 45

 oxidation 34

 photolysis 9, 35, 47

 hydrolysis 8, 9, 29, 34, 35

 BOD 1/2 life, BOD5/COD, BOD28, BOD/ThOD 11, 12, 28, 29, 35

 volatilization half-life 9

 log Kow 9, 43

 qualitative degree of persistence/

 expert judgement

16, 43

 source existence (yrs) 16

 EPA persistence rating scale 14

Mobility/Partitioning (30 Systems)

 adsorption (Kd, Koc) 8, 21, 34, 45

 water solubility 1, 9, 11, 21

 log Kow (P, Pow) 1, 3, 4, 6, 9, 11, 12, 19, 20, 21, 22, 24, 25, 28, 34, 43, 47, 49,

51

 BCF 1, 3, 4, 6, 9, 12, 17, 20, 22, 24, 25, 29, 39, 43, 49

 BAF (some systems consider BCF=BAF) 10, 17, 22, 34

 molecular weight (regarding bioconcentration) 11, 51

 bioconcentration (yes/no) 45

 vapor pressure 21

 initial partitioning/transport 19, 24, 45, 47, 48, 51

 evaporation 22

 volatilization 9, 45

 site-specific characteristics 16, 32

 leaching potential 21

 environmental transfer factors (e.g. soil to plants) 48
Table 3. Endpoints Used for Scoring Exposure

Endpoint System Number for Method Using this Endpoint a

27

 environmental spread/degree of mobility 

(qualitative)

28, 16

Estimated Dose, Environmental Occurrence, Concentration, or Releases (37 Systems)

Dose estimate

 human exposure potential (mg/kg/day) 1

 theoretical daily dose 2

 estimated daily intake 32

chemical concentration, frequency or occurrence

 exposure to contaminant or medium containing

 contaminant

2

 exposure via migration pathway 9

 projected levels in various media/compartments 19, 46, 47,

 estimated ambient concentrations 1, 7, 23

 frequency of occurrence/application 2, 14, 21, 23, 34

 occurrence in air/soil/water 3, 4, 20, 27, 39

 measured concentrations 15, 26, 32, 40, 50

 concentration of chemical introduced 21 

Use, production amounts, release or emission data 

 number of sites of discharge/use 34, 43

 emission source data 36, 45

 emissions estimates (rate,type,release height,

 temp.,flow rate,vent diameter)

36

 TRI releases/transfers 5, 29, 42, 46

 national emissions data (various sources) 7

 release to environment (annual) 10, 24, 34, 41, 43, 44, 45

 production volume 10, 11, 12, 19, 21, 24, 44, 51

 import volume 11, 12, 51

 waste volume 16

 use volume 10, 20, 43

 quantity on the market 28

 release reduction potential 43
Table 3. Endpoints Used for Scoring Exposure

Endpoint System Number for Method Using this Endpoint a

28

Exposure Frequency or Intensity (Receptor Characteristics) (11 Systems)

 use pattern 19, 20, 51

 population size/number potentially exposed 1, 23, 24, 27, 28, 32, 43

 land use 32

 distance from site 32

 presence of critical environments 32

 intensity of exposure 20, 23, 24

 length of exposure 23

 probability of exposure 24

 frequency of exposure 20, 23

 plurality of exposure 28

 geographical extent of exposure (ecological) 23

(a) refer to Table 1 for the corresponding systems

(b) the number of systems listed in parentheses indicates how many of the evaluated systems consider some

endpoint in the listed category
29

The SIDS database (Auer, 1992) includes the following data to assess sources and levels of exposure:

• production ranges (metric tons per annum);

• categories and types of use;

and environmental fate and pathways:

• aerobic biodegradability;

• abiotic degradability (hydrolysis and photodegradation by estimation); and

• estimates of environmental fate, pathways and concentrations (including Henry's Law

constant, aerosolization, volatilization, soil adsorption and desorption).

2.3.5 Missing Data, Data Selection Approach

Due to the extensive time and resources required to perform thorough testing of each of the thousands

of chemicals in commerce, the available data necessary for complete assessment of the hazards of these

chemicals are limited. Therefore, chemical ranking and scoring systems must somehow work in the

absence of complete data sets for every chemical. An overview of the approaches for assigning values

to criteria or endpoints used by the ranking systems evaluated can be found in Appendix C. Four

general approaches were identified:

1. Assign one endpoint per criteria being assessed and estimate missing data (e.g. use oral

LD50 for rats for acute human toxicity criteria). In the UT system (Davis et. al., 1993) there is only one

endpoint specified for each criterion, and missing data are estimated using QSARs, surrogate

chemicals, or other structure-activity relationships (SARs).

2. Choose data from a hierarchy of endpoints, listed in order of preference, based on data

quality, appropriateness of test, etc. The Chemical Use Cluster Scoring Methodology (EPA,

1993) uses a data hierarchy based on data quality for scoring the toxicity components. Preference is

given to high quality data as specifically defined in the document. Data of medium or low quality may

be used in the absence of high quality data. When scoring non-carcinogens, the RfD and RQ are

classified as "high quality" data whereas chronic and sub-chronic NOAELs are "medium quality". If

multiple data are available within the high quality classification, then the data yielding the highest score

(i.e. most conservative or health-protective) is used.

3. Choose the most conservative value from a pool of different endpoints. The GWU System

(Foran and Glenn, 1993) is an example of a system where a group of endpoints could be considered

for a given criterion, and the data yielding the most conservative (health-protective) result are chosen. 

If multiple data are available for a particular criterion (e.g. acute mammalian toxicity), then the data

resulting in the highest score are used. One drawback that should be noted for such a system is that it

does not encourage further testing of compounds, because it is unlikely that a score for a chemical can

be lowered by filling data gaps.
30

4. Assign cutoff (or trigger) values to a large number of criteria and select a chemical if one or

a specified number of the criteria are met. Several systems designed to select chemicals for

regulatory action or for further study do not provide an overall score for a chemical. Rather, a chemical

is selected if it meets certain criteria. Examples of this type of system include the Michigan CMR

(MDNR, 1987), where a chemical is selected for regulatory action if it receives a certain score; and the

Candidate Substance List for Bans or Phase-Outs (Socha et. al., 1992), where certain cut-off values

are used to determine if a substance is to be considered as "persistent", "bioaccumulative" and/or

"toxic". The primary list consists of substances considered to be all three of these.

For approaches where required data are not available, it is necessary to somehow estimate the missing

data. SARs, QSARs, default values, and/or ad hoc expert judgement have been used in the absence of

experimental data. SARs and QSARs are often used in many chemical scoring and ranking systems to

estimate chemical properties for which data are lacking. SARs qualitatively relate known data from a

chemical with similar molecular or molecular fragment structure, and therefore similar expected

biological activity or other properties, to the chemical with missing data. QSARs are obtained primarily

through regression analysis which results in mathematical equations representing correlations. Chemical

structure or physicochemical properties, such as molecular weight and octanol-water partitioning

coefficients, are typically compared with chemical toxicity or fate (Hermans, 1989). The limitations of

these methods must be weighed against the impact of missing data on the results of a chemical ranking

or scoring system.

Examples of systems that use SARs QSARs for predicting the effects of chemical exposure are

discussed below.

The Schmallenberg system (Klein et. al., 1988; Weiss et. al., 1988) defines the minimum data set

required for priority-setting. Data on mutagenesis and acute aquatic toxicity are essential and may be

estimated by QSARs when necessary. Other important missing data can be estimated from QSARs,

but any information, even if it is semi-quantitative in nature is considered.

The German (BUA) scoring system (Behret, 1989a,b) assigns negative scores where data are

unavailable. These scores are based largely on SARs. The use of negative scores occurred for only a

few chemicals evaluated by this system.

The UT system (Davis et. al., 1993) requires a quantitative assessment for many of the endpoints

included in the algorithm. This is obtainable through the use of expert judgement, SARs or QSARs for

estimation.

The EPA DfE Use Cluster System (EPA, 1993) allows a score for human health hazard potential to

be assigned by a structure activity team. Ecological hazard potential may be scored according to

QSAR concentrations of concern.
31

Another approach to filling data gaps results in the assignment of default values in the absence of data

or reliable QSARs or SARs. The European Communities (EC) system (van de Zandt and van

Leeuwen, 1992) uses default values and flags the data to identify when default values are used.

A few systems use expert judgement to give qualitative or semi-quantitative scores to chemicals (e.g.

high, med., low). An example of a system using this approach is the EPA Unfinished Business Report

(Morgenstern et. al., 1987). The project was described as being based on "informed judgement". 

Quantitative information available within the EPA was collected on the various environmental problems

addressed, but because the qualitative judgement was extensive, the study results were not considered

to be scientifically "reproducible". 

2.3.6 Aggregation and Weighing

The majority of the chemical ranking and scoring systems reviewed include human health and

environmental effects as well as exposure parameters. The manner in which these parameters are

combined to give an overall ranking or risk categorization varies tremendously and significantly effects

the final result. Scores may be added, multiplied, divided or not combined at all. Scores may be

weighted differently for human versus environmental effects (e.g. Screening Methodology for

Pollution Prevention Targeting, EPA, date unknown), acute versus chronic effects (e.g. EPA, 1990)

or exposure via water versus exposure via air (e.g. Klein et. al., 1988; Weiss et. al., 1988). Examples

of various aggregation and weighing methods follow.

Chemical selection in the Michigan CMR (MDNR, 1987) is based on combinations of scores in the

various parameters. For example, selection for the Register may occur if a chemical scores a "5" in two

or more criteria. These criteria include all of the health, environmental and exposure parameters where

none are considered more important than the others.

The Nordic system (Gjøs et. al., 1989) selects chemicals with a score of "high" in an effects category

(health or environmental) regardless of the exposure score or chemicals with an effect score of

"medium" and an exposure score of "high".

The WMS Scoring System (Könemann and Visser, 1988; Timmer et. al., 1988) calculates ten scores

for ten different combinations of exposure and effects. Examples of such combinations include 1)

Exposure via air and general toxicity for mammals, carcinogenicity, and mutagenicity; and 2) Exposure

via products and general toxicity for mammals, carcinogenicity, and mutagenicity.

In the MOE Candidate Substance List for Bans and Phase-outs (Socha et. al., 1992) substances are

selected on the basis of combinations of scores in toxicity, persistence and bioaccumulation. For

example, a substance is placed on the primary list if it scores a "10" in any one of the seven toxicity

categories and has a persistence of a half-life of 50 days and it has a bioconcentration factor of more

than 500.
32

In the ATSDR Priority List (ATSDR, 1992) scores are assigned for each of three criteria: frequency

of detection at National Priorities List (NPL) sites, toxicity, and potential for human exposure. The

total score is simply the sum of the scores assigned for each of these parameters.

Once scores are obtained for each of the various exposure and effects parameters, chemicals may be

placed into categories, such as selected or non-selected chemicals, or they may be ranked in order of

priority. 

Categorization may be accomplished by determining an overall concern score of high, medium or low,

or by selecting chemicals according to combinations of numerical scores in the exposure and effects

categories. For example:

The Schmallenberg system (Klein et. al., 1988; Weiss et. al., 1988) scores chemicals and places them

in one of three categories: immediate action, observation, or no action.

The GWU Method (Foran and Glenn, 1993) assigns scores of high, medium or low in several toxicity

categories, release and production, persistence, and bioaccumulation. A chemical is selected as a

candidate for Sunsetting if 1) it scores "high" in any toxicity category and "high" in release and

production (excluding pesticides) or 2) it scores "high" in any acute or chronic toxicity category and

"high" in persistence or bioaccumulation (including pesticides).

The WMS Scoring System (Könemann and Visser, 1988; Timmer et. al., 1988) categorizes chemicals

into three areas of priority by plotting an image point on a two-dimensional diagram. The image point is

determined after each chemical has been assigned a rank for exposure and effects.

The MOE Candidate Substances List for Bans or Phase-outs (Socha et. al., 1992) assigns numerical

scores for environmental transport, persistence and bioaccumulation as well as seven categories for

toxicity. Specific combinations of scores in these categories result in the inclusion of the chemical on the

primary or secondary list for appropriate action. 

The scoring systems which are designed to rank chemicals often include an algorithm for combining

individual scores for toxicity and exposure in order to determine an overall score which represents a

relative indicator of hazard for each chemical. These

ranking systems then prioritize chemicals in order of their potential hazard to human and environmental

health. For example:

The UT Method (Davis et. al., 1993) uses an algorithm which includes both additive and multiplicative

parameters to determine an overall hazard value for each chemical. The chemical rank indicates its

hazard relative to the other chemicals that are scored. 
33

The ATSDR Method (ATSDR, 1992) places 275 substances in order of hazard potential based on the

total score. The total score is obtained by the following formula:

Total score = NPL frequency + Toxicity + Potential for human exposure

The EPA DfE Use Cluster Scoring System (EPA, 1993) arrives at an overall chemical score as

follows:

Chemical score = Human risk reduction potential+Ecological risk reduction potential+EPA interest level

The average chemical score for the entire cluster is then added to the cluster score for pollution

prevention potential to give an overall cluster score which forms the basis for prioritizing clusters.

Whether a method results in categorization or ranking of chemical substances will depend on the

specific goals of the system. Systems developed for regulatory objectives frequently categorize

chemicals by selecting those that pose the greatest potential threat for regulation and/or monitoring. 

Systems for prioritization for other purposes may rank chemicals in order to focus research or other

activities on those chemicals highest on the list.

2.3.7 Other Issues

Other issues that can be considered in chemical ranking and scoring include:

• indirect ecological impacts, such as greenhouse gases, acid rain, eutrophication, and

stratospheric ozone depletion;

• effect of chemical mixtures;

• application to life cycle assessment, specifically, life cycle impact assessment; and

• social or economic impacts.
34

Indirect Ecological Impacts

The release of chemicals into the environment can have direct effects on human health and the

environment, but indirect effects may result as well. Certain substances are known to affect abiotic

components of the environment and ultimately result in large scale ecological disruption. For example, a

few substances (e.g. carbon dioxide) are considered "greenhouse gases" because they are believed to

have the potential to change the global climate. Other examples of chemicals that may cause these

indirect impacts are sulfur and nitrous oxides, which contribute to acid rain, chlorofluorocarbons

(CFCs), which contribute to stratospheric ozone depletion, and nitrous oxides and volatile organic

compounds (VOCs) which contribute to tropospheric (low atmosphere) ozone formation. Another

example is eutrophication of surface waters caused by excess nutrient inputs.

Most chemical ranking and scoring systems do not include these types of indirect impacts as scoring

criteria. The hazards posed by substances causing these effects have not been quantified in such

manner that they can be easily included. This is not to say that the risks are not significant or that they

should be ignored in chemical scoring. Ayres (1993), in an analysis of the relationship between natural

resources and economic growth, concludes by asserting that "those activities that are most likely to

interfere with natural climatological and nutrient cycling processes" should be of primary importance to

both economic development and environmental protection policy.

Foran and Glenn (1993) list several reasons why there is no mechanism currently available for

predicting or quantifying large scale ecological disruption. First, due in part to a lack of understanding

of how hazardous compounds affect the structure and function of ecosystems it is difficult to predict the

adverse impacts at an ecosystem or global level. Additionally, hazardous substances may interact with

components of ecosystems, but due to a lack of understanding of the fate and transport characteristics

of these substances, their effects on ecosystems are difficult to predict. It is important to have

information about the use and release patterns of a chemical in order to predict large-scale ecological

disruption, but this information is not generally available when a chemical is created and tends to change

over time. 

The GWU system (Foran and Glenn, 1993) considers evidence for ecological disruption when

selecting candidates for Sunsetting. The data available are mostly derived from studies of effects in

natural ecosystems, but other chemical-specific studies may be considered as well. Laboratory studies

or theoretical evaluations that suggest ecological disruption potential must be supported by predicted

use and release patterns of the substance. 

The TRI Environmental Indicators Methodology (Abt Assoc., 1992) has not yet included scoring

criteria for these effects, but it notes a major EPA project aimed at determining the risks associated

with CFCs and their alternatives. The model is complicated but could perhaps be used to evaluate the

risks posed by the emissions of these substances. Methods are also suggested for developing an

indicator for tropospheric ozone. This would involve identifying all VOCs reported in the TRI that are

considered to be ozone precursors and to sum the releases of these compounds from each facility. 
35

Alternatively, policy changes which would require reporting of VOCs and nitrous oxides (NOx) as a

group would help in the development of an indicator. The developers of the system are also examining

models designed to determine the exposure resulting from particle deposition to evaluate the risks

posed by airborne chemicals which are deposited into other media, such as surface water. 

Because chemical ranking and scoring systems have focused on direct toxicity to humans and other

organisms in the environment, indirect, large-scale impacts such as those discussed above have rarely

been taken into account. Impact assessment approaches have been developed, and the challenge will

be to incorporate these with the direct toxicological impacts. The field of life cycle assessment,

discussed below in section 2.3.7.3 is one place where this challenge is being addressed.

Effect of Chemical Mixtures

The majority of toxicity tests are conducted for single chemicals, usually at high doses. Organisms in

the environment, however, are more typically exposed to low levels of several chemicals rather than to

one chemical at a time. Chemicals may interact with one another to elicit effects that are different from

the combined effects of the individual chemicals. The effects may be additive, where the total effects

approximately equal the sum of the individual chemical effects; synergistic, where the total effects are

greater than the additive effects; or antagonistic, where the total effects are less than the additive effects. 

Other types of chemical interaction can also occur that result in unanticipated effects. There are limited

data regarding the effects of chemical mixtures. One data source developed by the EPA (MIXTOX)

contains summary information from and literature citations on studies of toxicological interactions,

primarily for binary mixtures (EPA, 1992).

The chemical ranking and scoring systems evaluated do not consider the effects of chemical mixtures. 

Because they are not usually site-specific, they are designed to evaluate the general potential harm from

substances on a chemical by chemical basis without regard to the effects of mixtures. It is difficult to

predict the effect of chemical mixtures on humans and environment. If incorporation of mixture toxicity

into chemical ranking and scoring systems is a goal, however, there is some evidence to suggest that

real world situations involving many organic chemicals are likely to result in approximate additive

toxicity (McCarty and MacKay, 1993). 

Life Cycle Assessment

Recently, much attention has been focused on the methodology of LCA, a holistic approach to

evaluating the human health and environmental burdens associated with a product or process life cycle. 

LCAs are tools for evaluating the effects on the environment associated with products, processes or

activities (SETAC, 1993). A full LCA includes a quantitative inventory of resource and energy inputs

and pollutant outputs and some form of impact assessment. A life cycle impact assessment is a process

for assessing the potential and actual effects of environmental loadings identified in the inventory

(SETAC, 1993). Chemical ranking and scoring could become an essential element in the development

of tools for assessing the impacts to health and the environment from chemical releases throughout the

life cycle of products.
36

LCA has several uses, although its uses are more limited if some form of impact assessment is not

included. LCA can be used for internal product improvements, for designing new products, for setting

public policy on products and materials, and for environmental labeling. Clearly, the different uses of

LCA create different needs for impact assessment. An LCA used for internal product improvement,

for example, might simply use the inventory component and operate on a "less-is-best" approach. An

LCA used for setting public policy on materials or products would need to include some framework for

assessing and comparing the significance of environmental releases and resource and energy use of the

different products and materials being compared.

One of the systems evaluated was developed for life cycle impact assessment. The EPS-EnviroAccounting Method (Steen and Ryding, 1992) was developed to assess the health and ecological

effects associated with the entire life-cycle of a product, process, or activity. The main objective of the

EPS Method is to provide one overall economic measure of resource depletion and potential health

and environmental impacts throughout a life cycle.

In this system, values are assigned to impacts on the environment in terms of five 'safe-guard subjects'

(human health, biodiversity, production, resources & aesthetic values) according to willingness to pay to

restore them to normal status. Emissions, use of resources, and other human activities are then valued

according to their estimated contribution to the changes in these safeguard subjects. The information on

environmental impacts originates from LCA-based inventory of the materials/process under study.

Economic Considerations

Chemical ranking and scoring systems are usually designed to be fairly quick and simple, limiting the

time and resources required for assessment to a minimum. It is usually beyond the scope of these

systems to determine potential economic impacts caused by releases of a particular chemical.

One system that does include economic considerations is the EPS Enviro-Accounting system (Steen

and Ryding, 1992) which is intended for life cycle impact assessment. Impacts on five safeguard

subjects are valued on a relative scale which is based on the willingness to pay for avoiding undesired

effects. Environmental load units (ELUs) which are standard monetary values, are assigned accordingly

to score impacts. As another example, the Unfinished Business Report (Morgenstern et. al., 1987)

considers monetary estimates of damage when categorizing environmental problems as they relate to

welfare effects.
37

Social Values

Economic impacts can be included under the broader category of social welfare or social values. 

SETAC (1993, p. 20) outlines a variety of "social welfare impact categories"; some of these issues or

impacts possibly relevant to chemical ranking and scoring include:

• demographic impacts, including fertility and mortality, morbidity, migration;

• sociopolitical impacts, including legal, governmental;

• social impacts, including quality of life;

• community impacts, including land use, physical appearance of community, community

satisfaction;

• sociocultural impacts, including social justice, aesthetics, environmental values; and

• psychosocial impacts, including anxiety or stress.

Only a few systems include social considerations, per se, in the scoring or selection criteria. One

example, The Michigan Critical Materials Register (MDNR, 1987) includes aesthetic considerations

such as taste, odor and appearance as criteria for scoring chemicals within one parameter. The

Unfinished Business Report (Morgenstern et. al., 1987), although not specifically a chemical ranking

system, ranks issues related to social welfare such as recreation, losses in aesthetics and non-user

values. The EPS Enviro-Accounting method (Steen and Ryding, 1992) includes aesthetic values as

one of the five safeguard subjects for which impacts are evaluated.
38

SECTION 3: DISCUSSION

3.1 RISK ASSESSMENT PRINCIPLES APPLIED TO CHEMICAL RANKING AND

SCORING

Chemical ranking and scoring systems are typically intended to be fairly simple and quick methods for

determining the health and environmental hazards posed by the use and release of chemical substances. 

Although not intended to provide a quantitative assessment of risk, the majority of the systems reviewed

do rely on the basic principles of risk assessment for chemical ranking and scoring. 

Chemical risk is a product of both toxicity and exposure. Most chemical ranking and scoring systems

include measures of both toxicity and exposure and, in this way, are similar to quantitative risk

assessment methods. The major difference is the extent to which the exposure assessment is

performed.

Exposure assessment in a quantitative risk assessment involves an analysis of contaminant releases,

identification of exposed populations, identification of all potential exposure pathways, and estimation of

exposure point concentrations and contaminant intakes. The exposure assessment results in an estimate

of the magnitude, frequency and duration of actual or potential human exposures through various

pathways expressed as a total dose. None of the chemical ranking and scoring systems reviewed

include a detailed site-specific quantitative risk assessment, although the (TRI) Environmental

Indicators Methodology (Abt Assoc., 1992) approaches this level of detail.

The system developed by Jones et. al. (1988) is one of the few systems that does not include any

exposure assessment. The stated purpose of the system, however, is to estimate only the relative

toxicological potency for hazardous substances rather than overall hazard. Another example is the

method developed by the EPA (1993b) for demonstrating the relative hazard of emissions under

section 112(g) of the Clean Air Act. As required, offsetting emission decreases must be considered

"more hazardous" than emission increases. For categorizing emissions, toxicity criteria alone are

considered. The Sax Toxicity Ratings (Sax and Lewis, 1989) is also based only on chemical properties

posing a physical hazard and toxicological characteristics. The other systems reviewed include some

measure of exposure, although the endpoints used to characterize exposure vary significantly.

The final step in a baseline risk assessment is risk characterization. Here, chemical toxicity data are

combined with potential exposure levels for the receptors of interest at a site to arrive at a quantitative

estimate of the risk that receptors will suffer adverse effects. Such site-specific characterization is not

generally performed in chemical ranking and scoring, but most of the systems reviewed do combine

toxicity and exposure in some manner to score, select, or prioritize chemicals. As discussed in Section

2.3.6, the manner in which toxicity and exposure are combined for an overall assessment also varies

significantly. 
39

3.2 SIMILARITIES AND DIFFERENCES AMONG SYSTEMS REVIEWED

3.2.1 Most Common Effects and Exposure Endpoints Used 

The effects and exposure categories specified by the systems were difficult to compare. They were

divided, as best could be done, into categories of commonly recognized criteria for presentation in

Tables 2 and 4 and in Appendix C. More information is available in Appendix A , where the criteria,

subcriteria, and endpoints as defined by each system are outlined. Some criteria and endpoints were

commonly used in many systems while others were unique to a particular system. The most commonly

listed endpoints for major classes of effects criteria include:

• carcinogenicity, mutagenicity, genotoxicity, most often characterized by the weight,

type, or amount of evidence that a chemical would elicit that effect;

• systemic (non-carcinogenic) or general health effects, most commonly characterized

by chronic or subchronic Rfd or RfC values;

• aquatic toxicity, most often quantified by acute LC50 and EC50 data;

• mammalian toxicity, most often quantified by acute LD50 and LC50 data;

• developmental/reproductive toxicity, again most often measured by the weight, type,

or amount of evidence;

• physical hazard, most often characterized by ignitability, boiling point, and reactivity;

• plant toxicity, most commonly measured by EC50 data;

• terrestrial non-mammalian toxicity, most often characterized by acute LD50; and

• general ecological effects, with no specific endpoint used by more than one system.

The most commonly listed endpoints for major classes of exposure criteria include:

• degradation or transformation potential: most commonly measured by half-life in the

environment and some type of BOD data;

• mobility and partitioning, most often characterized by Kow and BCF;

• estimated dose, environmental occurrence, concentration, or amount released,

most commonly measured by annual releases to environment and production volume;

and

• exposure frequency or intensity, relates to potential receptors and usually is measured

by population size or number potentially exposed.

The classes of criteria are listed in both cases in decreasing order from the most to the least often

included as components of the evaluated systems.

3.2.2 Ranking versus Categorization

As mentioned in Section 2.3.6, there are two major types of chemical ranking and scoring systems;

those that rank chemicals and those that categorize chemicals. Examples of categories that may be

used include substances of high, medium or low concern or selected chemicals, non-selected chemicals
40

and chemicals for further review. Ranking systems, however, usually derive overall scores and rank

chemicals relative to one another based on these scores. 

In developing a consensus framework for chemical scoring, a decision must be made whether to use the

system to rank chemicals, select or categorize chemicals, or rank and select chemicals. To a certain

extent, this decision depends on the purpose of the system, but one disadvantage for a system that only

categorizes or selects chemicals is that those selected substances cannot be further prioritized. For

example, suppose such a system were used to choose a subset of chemicals that are considered to be

of high concern from a regulatory perspective. If 200 chemicals meet the criteria to be considered high

concern, there may need to be a means of further prioritizing these substances before regulatory policy

decisions could be made. van de Zandt and van Leeuwen (1992) suggest that a ranking system is

preferable to classification methods for this reason. 

Three of the systems reviewed assign chemical scores which are not used to rank or categorize the

chemicals. For example, the TRI Environmental Indicators Methodology (Abt Assoc., 1992) scores

TRI chemicals to reflect the impacts of the releases and transfers. These numbers can then be

compared from one year to the next to monitor changes in impacts. Chemicals are not ranked or

selected. 

Several systems assign chemical scores and rank the substances according to relative hazard. The

priority list of substances prepared by the ATSDR (1992) is the result of ranking and scoring, in order

of priority, chemicals commonly found at NPL sites. 

Chemical ranking may be used for selecting chemicals for further review, regulatory action or some

other activity. An example is the Ranking System for the Clean Water Act (Poston and Prohammer,

1985; Cornaby et. al., 1986) in which chemicals were ranked and then selected as candidates for

possible inclusion on or deletion from the Priority Pollutant List. There are seven systems reviewed

which result in both ranking and selection of chemicals. 

Thirteen of the systems evaluated resulted in the selection or categorization of chemicals, mostly for

regulatory purposes (ten systems), without first ranking them. An example of such a system is that

developed by ICF (1990) for the EPA which selects chemicals as "High Priority Pollutants" if certain

exposure and effects criteria are met. Another example is the Michigan CMR (MDNR, 1987) which is

derived from scoring chemicals for various exposure and effects criteria. Chemicals are then selected

for inclusion in the Register if they receive a score a "5" (the maximum score) in two or more criteria or

score an additive level of "15" or greater for all criteria. Chemicals are chosen for more detailed

scrutiny if other similar criteria are met. 

3.2.3 Quantitative versus Qualitative Endpoints

Chemical ranking and scoring systems are basically quantitative in nature, oftentimes resulting in ranking

or selection based on numeric cut-off values for various criteria. The assignment of scores for specific

endpoints, however, is not always based on quantitative data. Many of the systems reviewed include a
41

combination of both qualitative and quantitative endpoints. Particularly for chronic health effects, such

as carcinogenicity, qualitative data are essential. A large number of the systems which include

carcinogenicity as a scoring criterion utilize weight-of-evidence and/or strength-of-evidence information. 

Sometimes, as in the GWU system (Foran and Glenn, 1993), these qualitative data are combined with

quantitative potency estimates where available. Qualitative data are also used frequently in the absence

of quantitative data for certain criteria. For example, the BUA system (Behret, 1989a,b) assigns

negative scores where qualitative information is utilized in the absence of a desired quantitative

endpoint. When there is no data such as the Kow or bioconcentration factor (BCF) regarding

bioaccumulation, then a score of -1 is assigned if there is no suspicion of bioaccumulation or a -2 if

bioaccumulation potential is suspected. Qualitative data, such as SARs, are frequently used to fill data

gaps.

3.2.4 Assigning Scores to Endpoint Data

For those systems where there is agreement on which particular endpoint to use, there still exists a

variety of approaches to assign corresponding numerical scores. To illustrate this, BCF is used as an

example of an endpoint included in several systems and scored in a variety of ways. Table 4 presents

the numerical scores assigned to BCF or log BCF by eight systems. For comparison purposes, all

BCF data are converted to log BCF and the scores are normalized on a scale of 0 to 1 corresponding

to the minimum and maximum value assigned by the system. This information is also presented

graphically in Figure 1.

A few items are worthy of note. First, there is a noticeable lack of agreement; the eight systems

represented here use seven different schemes to assign values to BCF data. Second, the majority of

systems use some kind of step function to assign values to endpoint data (i.e. if log BCF is between 3

and 4, then assign a value of x). One exception is the UT system, which uses a continuous linear

function with upper and lower bounds. The systems that use step functions use different numbers of

steps. The BUA system has only two; it simply assigns a minimum score if the log BCF is less than 2

and a maximum score if it is greater than 2. Several others use five steps. Finally, it can be seen that

different systems assign varying levels of significance to BCF values. The BUA system considers a log

BCF of 2.1 to be of maximum significance, while, for this same BCF value, the Michigan CMR assigns

a relatively low value and the UT system assigns a mid-range score.

Similar evaluations could be useful for all endpoints included in a chemical ranking and scoring system,

but are beyond the scope of this report.
42

TABLE 4. Bioconcentration Factor (BCF) Scores for Several Systems

System name

(System no.)

BCF Log BCF Score Normalized

score a

Candidate Substance List for Bans or

Phase-outs (6);

Effluent Monitoring Priority Pollutant

List (EMPPL) (25);

Canadian Accelerated

Reduction/Elimination of Toxics

(ARET) Scoring Protocol (49)

>15,000

500 - 15,000

20 - 500

<20

>4.2

2.7 - 4.2

1.3 - 2.7

<1.3

10

 7

 4

 0

1.0

0.7

0.4

0

Michigan Critical Materials Register

(MCMR) (22)

>100,000

10,000 - 99,999

1,000 - 9,999

100 - 999

<1 - 99

>5

4 - 5

3 - 4

2 - 3

<2

S(sufficient)

5

3

1

0

1.0 b

0.71

0.43

0.14

0

WMS Scoring System (20) >3

1.5 - 3

<1.5

2

1

0

1.0

0.5

0

Chemical Scoring System for Hazard

and Exposure Identification (TSCA)

(24)

 >1000

200 - 1,000

100 - 200

10 - 100

<10

>3

2.3 - 3

2 - 2.3

1 - 2

<1

9

7

5

3

0

1.0

0.78

0.56

0.33

0

TRI Environmental Indicators

Methodology (1)

>10,000

 1,000 - 10,000

100 - 1,000

10 - 100

1 - 10

<1

>4

 3 - 4

2 - 3

1 - 2

0 - 1

<0

50,000

5,000

500

50

5

0.5

1.0

.79

.57

.36

.15

0

Existing Chemicals of Environmental

Relevance (BUA) I & II (3,4)

>100

<100

>2

<2

2

0

1.0

0

UT Method (29) >4

1 - 4

<1

2.5

0.5logBCF+0.5

1

1.0

(calc)

0.4

(a) normalized on a scale from 0 to 1 by dividing each score by the maximum score for that system

(b) normalized assuming a maximum score of 7
43

Figure 1
44

3.3 GENERAL STRENGTHS AND WEAKNESSES OF EVALUATED SYSTEMS

A detailed critique of all fifty-one systems reviewed is beyond the scope of this report. Instead, a

general discussion is presented for consideration either in choosing an existing system, or in developing

a new or hybrid system. Van de Zandt and van Leeuwen (1992) suggest that a priority setting system

should be:

• quick to use as a screening tool to identify priority substances for further review;

• systematic and computerized;

• transparent, with methodologies described in detail for clarity to the user

• flexible;

• accurate, avoiding too many false positives or negatives and working with available

data to give consistent results;

• based on a scientifically justifiable framework and generally accepted methodologies;

and

• based on exposure and effects, with emphasis on long-term effects.

These aspects, along with a few additional issues we have identified, are discussed below:

Ease of use, complexity. The EPA chemicals of potential concern selection process is simple and

straight forward when concentration data are available. The required toxicity data, however, are

readily available for only a limited number of chemicals. The TRI indicators methodology is very

complex and amounts to practically conducting a quantitative risk assessment at every facility that

reports TRI releases. The time and resources required is an important deciding factor in choosing or

developing a method. Also, the mathematical or mechanical processes of scoring and ranking should

be straight forward enough so that the system is transparent to the user. It should easy to understand

the results given a basic knowledge of the chemicals and data used.

Flexibility. A system should have enough flexibility so the user can augment or simplify it, add or

subtract components, add new data as they become available, or change the weighing for various

criteria while the system maintains its integrity. The UT system is an example of a relatively flexible

system.

Availability of data. Many systems require data that either are not readily available or frequently require

estimation methods. As much of the required data as possible should be peer-reviewed. The Use

Cluster Scoring System (EPA, 1993) draws on a number of possible endpoints, arranged in a

hierarchy for high, medium and low quality data. This data selection approach allows the user to draw

from a much wider range of data, and should result in fewer instances of missing data.

Comparability to other systems. Ranking and scoring results, especially when regulations are involved,

should be comparable across regulatory programs and across industries. Ideally, they should also be
45

comparable on an international basis. One example where international comparability could be

important is where trade issues are involved. Three of the evaluated systems (the Canadian Substance

List for Bans or Phase-outs, the EMPPL system, and the ARETS Scoring Protocol) use the same

scoring criteria, based on the MOE scoring criteria, although their selection processes differ slightly. 

Otherwise, there is little consistency among the evaluated systems.

Number of chemicals the existing system has been demonstrated on. Some systems may seem like a

good idea, but may disintegrate under the pressure of a large number of chemicals. A few of the

evaluated systems were presented only in theory, with no demonstration using real chemical data. 

Others have been demonstrated or used on hundreds of chemicals. For example, over 700 chemicals

were ranked by the CERCLA 104 Priority List system (ATSDR, 1992), and van de Zandt and van

Leeuwen (1992) addressed 2000 HPV chemicals in the Priority Setting of Existing Chemical

Substances system. 

Applicability to various classes of chemicals. Ions, metals and radionuclides pose problems that are

very different from organic compounds. For example, many QSAR or SAR estimation methods do not

apply to inorganic chemicals. The CERCLA HRS system (Hallstedt et. al., 1986) was intended only

for ranking organic compounds, but has been modified to include radionuclides in the site ranking

process (Hawley & Napier, 1985).

Reproducibility or subjectiveness of scoring methods. Results from a panel of experts or when

qualitative estimations are used may not be reproducible at different times or when applied by different

users. For example, the WMS Scoring System relies an a panel of experts to score chemicals; if used

in another application, different experts could arrive at different scores for the same chemicals. In

addition, a system should be based on scientific principles and an understanding of risk assessment

methodologies.

Completeness. All potential and important effects should be included in a system. The following

components should be included, or the decision not to include a component should be made clear:

• the purpose and application of the ranking and scoring system;

• the human health criteria and endpoints included;

• the criteria and endpoints included for environmental effects;

• whether measures of exposure are included;

• the data selection approach and handling of missing data;

• the use of aggregation and weighing of different health and environmental impacts;

• methods of accounting for chemical potency and severity of effects; and

• inclusion of other impacts or issues.
46

SECTION 4: CONCLUSIONS

This review of various chemical ranking and scoring systems demonstrates that there is currently no

consensus regarding an appropriate framework for evaluating adverse impacts to human health and the

environment from exposure to chemical substances. There is much variation in the degrees of

sophistication, the types and numbers of endpoints incorporated, the data selection approach and the

manner of weighing and/or combining scores to arrive at a final assessment.

Clearly, the purpose for which the ranking or scoring tool is intended will affect these factors. Although

the intended purpose of a ranking or scoring system can influence its design, it would be useful to

develop a standard framework for chemical ranking and scoring that is flexible enough to be adapted

for most purposes. A standardized system for ranking and scoring chemicals is desirable for several

reasons. It could assist business and regulatory agencies to target their pollution prevention efforts;

enable environmental policy and business decisions to be made on similar, objective, and scientificallybased methods; provide consistency nationally across regulatory programs as well as internationally;

make use of and build on previous experience gained from the development of existing systems; allow

new information to be incorporated while maintaining system integrity; and save on time and expense

incurred by continuing to develop new systems from scratch with every new application.

Due to the large variation among ranking systems in terms of criteria, endpoints, data selection

approaches, and scoring methods, one logical way to approach a consensus on methods would require

the following steps (illustrated in Figure 2):

• Develop a consensus on the overall framework for chemical ranking and scoring,

including its component parts: criteria, endpoints, data selection approach, and scoring

formula;

• Work toward agreement on the component parts, such as:

- which criteria should be included in any chemical ranking and scoring system

- which endpoint(s) should be used to measure or score that criteria

- what data selection approach should be used to select or estimate data for the

criteria

- how should criteria scores be weighted and combined to reach an overall score

or rank for each specific chemical (if an overall score per chemical is the goal)
47

Figure 2
48

- what level of sophistication in exposure estimation is appropriate - from total

amounts produced, used or released, to multimedia environmental fate models,

to site-specific models including estimates of dose over time

• Reassemble these elements into the agreed-upon framework, providing flexibility to use

some or all of the components for specific purposes.

The process of developing the consensus framework should involve all of the significant stakeholders -

government agencies, chemical manufacturers and users, environmental and consumer groups, and

academic researchers. This will create greater acceptance of the results.

Given the current interest in chemical ranking and scoring, and the many potential applications, an effort

to develop a consensus framework for chemical ranking and scoring would be an important

contribution to reducing chemical risk.
49

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