Page B-1

Technical Support Document for U.S. EPA’s Proposed Rule for Addition of a 

Subsurface Intrusion Component to the Hazard Ranking System

Appendix B: Chemical Factor Analysis

Table B-1:

Table B-2:

Table B-3:

Draft Subsurface Intrusion HRS Factor Values and Benchmarks for Volatile 

Substances Hazardous Substance Inhalation Unit Risks (IURs) 

Hazardous Substance Reference Concentrations (RfCs) 
Page B-2

Chemical Factor Analysis

As part of the development of the proposed Hazard Ranking System (HRS) addition, EPA has adopted a 

procedure for using Inhalation Unit Risk (IUR) and Reference concentration (RfC) values in determining the 

human toxicity factor and for assigning HRS benchmarks reflecting non-cancer related risk levels for inhalation 

exposures. Additional information regarding the use of RfCs instead of Reference Doses (RfDs) in the proposed 

HRS addition is detailed in Section 9.0.b of the proposal’s technical support document (TSD).

Tables B-1, B-2 and B-3 (below) contain the information used to develop the procedure for using the IURs and 

the RfCs to assign human toxicity factor values. Table B-1 provides the human toxicity factor value, air gas 

migration factor value (vapor migration potential values), inhalation Reference Dose Screening Concentration 

benchmark and inhalation cancer risk screening concentration benchmarks for a partial list of common volatile 

substances consistent with the 1990 HRS. The IURs and RfCs used in developing the new procedures are 

presented in Tables B-2 and B-3. An explanation of how the human toxicity factor value is determined and used 

in calculating the waste characteristics factor category value can be found in Sections 2.4.1.1 and 5.2.1.2.1 of the 

proposed HRS additions. Air gas migration numbers are provided for the evaluated substances to aid in assigning 

the vapor migration potential value, which is used to determine the likelihood of exposure factor category value 

via potential for exposure, as described in Section 5.2.1.1.2.4 and Table 5-17 of the proposed HRS additions. 

.
Please note, that these are draft HRS factor values and health-based benchmarks that are provided for testing purposes only and are subject to change. Page B-3

Table B-1

Draft Subsurface Intrusion HRS Factor Values and Benchmarks for Volatile Substances

Substance Name CAS Number Toxicity Vapor Migration 

Potential Values

Inhalation RfD 

Screening Conc.

(mg/m3)

Inhalation Cancer 

Risk Screening Conc. 

(mg/m3)

Acenaphthene 83-32-9 10 11 - -

Acetone 67-64-1 1 17 3.2E+1 -

Acrolein 107-02-8 10000 17 2.1E-5 -

Anthracene 120-12-7 10 6 - -

Benzene 71-43-2 1000 17 3.1E-2 3.1E-4

Bromodichloromethane 75-27-4 100 17 - 6.6E-5

Carbon disulfide 75-15-0 10 17 7.3E-1 -

Carbon tetrachloride 56-23-5 1000 17 1.0E-1 4.1E-4

Chlorobenzene 108-90-7 100 17 5.2E-2 -

Chloroform 67-66-3 100 17 1.0E-1 1.1E-4

Cumene 98-82-8 10 17 4.2E-1 -

Dibenzofuran 132-64-9 1000 11 - -

Dibromo-3-chloropropane, 1,2- 96-12-8 10000 11 2.1E-4 1.6E-7

Dibromoethane, 1,2- 106-93-4 10000 17 9.4E-3 4.1E-6

Dichlorobenzene, 1,4- 106-46-7 100 17 8.3E-1 2.2E-4

Dichloroethane, 1,1- 75-34-3 10 17 - 1.5E-3

Dichloroethane, 1,2- 107-06-2 100 17 7.3E-3 9.4E-5

Dichloroethylene, 1,1- 75-35-4 10 17 2.1E-1 -

Dichloroethylene, 1,2-(mixed isomers) 540-59-0 100 17 - -
Please note, that these are draft HRS factor values and health-based benchmarks that are provided for testing purposes only and are subject to change. Page B-4

Table B-1 (cont.)

Draft Subsurface Intrusion HRS Factor Values and Benchmarks for Volatile Substances

Substance Name CAS Number Toxicity Vapor Migration 

Potential Values

Inhalation RfD 

Screening Conc.

(mg/m3)

Inhalation Cancer 

Risk Screening Conc. 

(mg/m3)

Dichloroethylene, 1,2-cis- 156-59-2 1000 17 - -

Dichloroethylene, 1,2-trans- 156-60-5 100 17 6.3E-2 -

Dichloropropane, 1,2- 78-87-5 1000 17 4.2E-3 2.4E-4

Dichloropropene, 1,3- 542-75-6 100 17 2.1E-2 6.1E-4

Ethylbenzene 100-41-4 10 17 1.0E+0 9.7E-4

Fluorene 86-73-7 100 11 - -

Mercury (elemental) 7439-97-6 1000 17 3.1E-4 -

Methyl ethyl ketone (2-butanone) 78-93-3 1 17 5.2E+0 -

Methyl tert-butyl ether (MBTE) 1634-04-4 10 17 3.1E+0 9.4E-3

Methylene chloride 75-09-2 10 17 1.1E+0 5.2E-3

Methylnaphthalene, 2- 91-57-6 1000 11 - -

Naphthalene 91-20-3 1000 11 3.1E-3 7.2E-5

Nitrobenzene 98-95-3 1000 11 9.4E-3 6.1E-5

Pyrene 129-00-0 100 6 - -

Styrene 100-42-5 10 17 1.0E+0 -

Tetrachloroethane, 1,1,2,2- 79-34-5 100 11 - 4.2E-5

Tetrachloroethylene 127-18-4 100 17 2.8E-1 4.1E-4

Toluene 108-88-3 10 17 5.2E+0 -

Trichlorobenzene, 1,2,4- 120-82-1 1000 17 2.1E-3 -

Trichloroethane, 1,1,1- 71-55-6 1 17 5.2E+0 -

Trichloroethane, 1,1,2- 79-00-5 1000 17 2.1E-4 1.5E-4

Trichloroethylene 79-01-6 1000 17 2.1E-3 5.9E-4

Trichlorofluoromethane 75-69-4 10 17 7.3E-1 -

Trichloropropane, 1,2,3- 96-18-4 10000 11 3.1E-4 -

Vinyl acetate 108-05-4 10 17 2.1E-1 -

Vinyl chloride 75-01-4 10000 17 1.0E-1 1.6E-4

Xylene, m- 108-38-3 100 17 1.0E-1 -

Xylene, o- 95-47-6 100 17 1.0E-1 -

Xylene, p- 106-42-3 100 17 1.0E-1 -

Xylenes 1330-20-7 100 17 1.0E-1 -
Page B-5

Table B-2

Hazardous Substance Inhalation Unit Risks (IURs)

Substance Name Inhalation Unit 

Risk (IUR)

Weight-ofEvidence (WOE)

Benzene 2 0.0000078 A

Vinyl chloride (2) 0.0000088 A

1,3-Butadiene 0.00003 A

Bis(chloroethyl)ether (BCEE) 0.00033 A

Coke oven emissions 0.00062 A

Arsenic, inorganic 0.0043 A

Chromium(VI) 0.012 A

Bis(chloromethyl)ether (BCME) 0.062 A

Benzidine 0.067 A

Nickel refinery dust 0.00024 A

Nickel subsulfide 0.00048 A

Trichloroethylene 0.000004 B

Chloroform 0.000023 B

Hexachloroethane 0.000004 B1

Carbon tetrachloride 0.000006 B1

Nitrobenzene 0.00004 B1

Acrylonitrile 0.000068 B1

Acrylamide 0.0001 B1

Chloroprene 0.0003 B1

1,2-Dibromoethane (2) 0.0006 B1

Cadmium 0.0018 B1

Beryllium and compounds 0.0024 B1

Formaldehyde 0.000013 B1

Bromoform 0.0000011 B2

Acetaldehyde 0.0000022 B2

2,4,6-Trichlorophenol 0.0000031 B2

Propylene oxide 0.0000037 B2

Aramite 0.0000071 B2

Azobenzene 0.000031 B2

p,p'-Dichlorodiphenyltrichloroethane (DDT) 0.000097 B2

Chlordane (Technical) 0.0001 B2

Polychlorinated biphenyls (PCBs) 0.0001 B2

Toxaphene 0.00032 B2

technical Hexachlorocyclohexane (t-HCH) 0.00051 B2

N-Nitrosopyrrolidine 0.00061 B2

N-Nitroso-di-n-butylamine 0.0016 B2

alpha-Hexachlorocyclohexane (alpha-HCH) 0.0018 B2

Aldrin 0.0049 B2

N-Nitrosodiethylamine 0.043 B2

Hexachlorodibenzo-p-dioxin (HxCDD), mixture of 1,2,3,6,7,8-HxCDD 

and 1,2,3,7,8,9-HxCDD 1.3 B2
Page B-6

Table B-2 (cont.)

Hazardous Substance Inhalation Unit Risks (IURs)

Substance Name Inhalation Unit 

Risk (IUR)

Weight-ofEvidence (WOE)

N-Nitrosodimethylamine 0.014 B2 

1,3-Dichloropropene 0.000004 B2 

Hexachlorobenzene 0.00046 B2 

Heptachlor epoxide 0.0026 B2 

Dichloromethane 0.00000047 B2

Epichlorohydrin 0.0000012 B2

1,2-Dichloroethane 0.000026 B2

1,2-Diphenylhydrazine 0.00022 B2

Heptachlor 0.0013 B2

Dieldrin 0.0046 B2

1,1,1,2-Tetrachloroethane 0.0000074 C

1,1,2-Trichloroethane 0.000016 C

Hexachlorobutadiene 0.000022 C

beta-Hexachlorocyclohexane (beta-HCH) 0.00053 C
Page B-7

Table B-3

Hazardous Substance Reference Concentrations (RfCs)

Substance Name Reference Concentration (RfC)

Acetaldehyde 0.009

Acetonitrile 0.06

Acrolein 0.00002

Acrylamide 0.006

Acrylic acid 1000

Acrylonitrile 0.002

Allyl chloride 0.001

Ammonia 0.1

Aniline 0.001

Antimony trioxide 0.0002

Arsine 0.00005

Benzene 0.03

Beryllium and compounds 0.00002

Bromobenzene 0.06

Bromomethane 0.005

1,3-Butadiene 0.002

Carbon disulfide 0.7

Carbon tetrachloride 0.1

Cerium Oxide and Cerium Compounds 0.0009

Chlordane (Technical) 0.0007

Chlorine dioxide 0.0002

1-Chloro-1,1-difluoroethane 50

2-Chloroacetophenone 0.00003

Chlorodifluoromethane 50

Chloroprene 0.02

Chromium(VI) 0.0001

Chromium(VI) 0.000008

Cumene 0.4

Cyclohexane 6

1,2-Dibromo-3-chloropropane (DBCP) 0.0002

1,2-Dibromoethane 0.009

1,4-Dichlorobenzene 0.8

1,1-Dichloroethylene (1,1-DCE) 0.2

1,2-Dichloropropane 0.004

1,3-Dichloropropene 0.02

Dichlorvos 0.0005

Diesel engine exhaust 0.005

1,1-Difluoroethane 40

N,N-Dimethylformamide 0.03

Epichlorohydrin 0.001

1,2-Epoxybutane (EBU) 0.02

2-Ethoxyethanol 0.2
Page B-8

Table B-3 (cont.)

Hazardous Substance Reference Concentrations (RfCs)

Substance Name Reference Concentration (RfC)

Ethyl chloride 10

Ethylbenzene 1

Ethylene glycol monobutyl ether (EGBE) (2-Butoxyethanol) 1.6

Hexachlorocyclopentadiene (HCCPD) 0.0002

1,6-Hexamethylene diisocyanate 0.00001

n-Hexane 0.7

2-Hexanone 0.03

Hydrogen Cyanide and Cyanide Salts 0.0008

Hydrogen chloride 0.02

Hydrogen sulfide 0.002

Manganese 0.00005

Mercury, elemental 0.0003

2-Methoxyethanol 0.02

Methyl chloride 0.09

Methyl ethyl ketone (MEK) 5

Methyl isobutyl ketone (MIBK) 3

Methyl methacrylate 0.7

Methyl tert-butyl ether (MTBE) 3

Methylene Diphenyl Diisocyanate (monomeric MDI) and polymeric MDI 

(PMDI) 0.0006

Naphthalene 0.003

Nitrobenzene 0.009

2-Nitropropane 0.02

Phosgene 0.0003

Phosphine 0.0003

Phosphoric acid 0.01

Propionaldehyde 0.008

Propylene glycol monomethyl ether (PGME) 2

Propylene oxide 0.03

Styrene 1

1,1,1,2-Tetrafluoroethane 80

Toluene 5

2,4-/2,6-Toluene diisocyanate mixture (TDI) 0.00007

1,1,1-Trichloroethane 5

1,2,3-Trichloropropane 0.0003

Triethylamine 0.007

Vinyl acetate 0.2

Vinyl bromide 0.003

Vinyl chloride 0.1

Xylenes 0.1

Trichloroethylene 0.0005