Impact of Revised Human Health Benchmarks

on Solvent Wipes

Risk-Based Mass Loading Limits (RB-MLLs)

April 2012

Office of Resource Conservation and Recovery

U.S. Environmental Protection Agency

Prepared with the support of RTI International

EPA Contract Number EP-W-09-004
1

Introduction

To support a proposed change in the regulatory status of disposed solvent-contaminated wipes and laundry 

sludges, EPA developed and performed a risk analysis in 2009 to characterize the potential risks associated with 

the disposal of wipes and laundry sludge in nonhazardous waste landfills. See the original risk document in the 

docket for this rule (Risk-Based Mass Loading Limits for Solvents in Disposed Wipes and Laundry Sludges 

Managed in Municipal Landfills, October 2009). Under this analysis, we estimated the risk-based mass loading 

limit (RB-MLL) for the allowable amount of solvent that can be managed in an unlined or composite-lined 

landfill in any year that, if contained on disposed wipes or in laundry sludge, would not exceed EPA’s risk criteria 

if these materials were disposed of in nonhazardous waste landfills. 

EPA recently finalized new health benchmarks for two solvents, tetrachloroethylene (also called 

perchloroethylene, or PCE) and trichloroethylene (TCE). The purpose of this document is to present the 

recalculated risk estimates and RB-MLLs for these two solvents derived using the revised health benchmarks. 

Revised Health Benchmarks

Table 1 provides a comparison between the revised IRIS benchmarks and those used in the 2009 assessment. 

Table 1. Comparison of Benchmarks applied in 2009 Analysis to Revised Benchmarksa

Constituent CASRN Source

RfD (mg/kg-d) RfC (mg/m3) CSFo (per mg/kg-d) URF (per μg/m3)

Value Ref Value Ref Value Ref Value Ref

Tetrachloroethylene127-18-4 2009 Value 1.0E-02 IRIS 3.0E-01 ATSDR 5.4E-01 CalEPAb 5.9E-06

7.1E-7b CalEPAb

Current 

IRIS Value

6.0E-03 IRIS(r) 4.0E-02 IRIS(r) 2.1E-03 IRIS(r) 2.6E-07 IRIS(r)

Trichloroethylene

79-01-6 2009 Value none NA 6.0E-01 CalEPA 1.3E-02 CalEPA 2.0E-06 CalEPA

Current 

IRIS Value

5.0E-04 IRIS(r) 2.0E-03 IRIS(r) 4.6E-02 IRIS(r) 4.1E-06 IRIS(r)

a IRIS(r): Final revised IRIS values. (September 2011, February 2012)

U.S. EPA (Environmental Protection Agency). 2011. Integrated Risk Information System (IRIS) for Trichloroethylene 

(CASRN 79-01-6). Washington, DC: National Center for Environmental Assessment, Office of Research and 

Development. http://www.epa.gov/iris/subst/0199.htm.

U.S. EPA (Environmental Protection Agency). 2012. Integrated Risk Information System (IRIS) for 

Tetrachloroethylene (Perchloroethylene) (CASRN 127-18-4). Washington, DC: National Center for Environmental 

Assessment, Office of Research and Development. http://www.epa.gov/iris/subst/0106.htm b In 2009, two estimates were provided for tetrachloroethylene cancer risks to reflect the high and low ends of the range of 

benchmarks.

As shown in Table 1, the revised benchmarks include both noncancer and cancer endpoints. The benchmarks for 

noncancer endpoints include the reference dose (RfD), which is applied in evaluating ingestion and dermal 

exposures, and the reference concentration (RfC), for inhalation exposures. The benchmarks for cancer endpoints

include the oral cancer slope factor (CSF), which is used to evaluate ingestion and dermal exposures, and the unit 

risk factor (URF), for inhalation exposures. No RfD was available in 2009 for trichloroethylene, but one is now

available to support this current effort. 
2

Revised Risk Estimates

Using the revised human health benchmarks shown in Table 1, we performed modeling to estimate risk or HQ 

and RB-MLLs for tetrachloroethylene and trichloroethylene. Cancer risk is directly proportional to the oral CSF 

or URF. Noncancer HQ is indirectly proportional to the RfC or RfD.

Cancer Risk 

Cancer risks were calculated using Equations 1 to 3. For the oral pathway, the lifetime excess cancer risk 

estimates were based on the lifetime average daily dose (LADD) and the oral CSF as follows: 

Riskoral = LADD ×CSFo (Equation 1)

where

Riskoral = lifetime incremental cancer risk from the oral exposure route (unitless)

LADD = lifetime average daily dose (mg/kg-d)

CSFo = oral cancer slope factor (mg/kg-d)-1.

For the groundwater dermal contact pathway, the lifetime excess cancer risk estimates were based on the dermal 

absorbed dose (DAD) and the oral CSF: 

Riskdermal = DAD ×CSFo (Equation 2)

where

Riskoral = lifetime incremental cancer risk from the oral exposure route (unitless)

DAD = dermal absorbed dose (mg/kg-d)

CSFo = oral cancer slope factor (mg/kg-d)-1

.

For inhalation of ambient air and inhalation of indoor air during showering, lifetime excess cancer risk estimates 

were calculated as follows:

A T

C U RFE R is k a ir

a ir

× × × = 100

(Equation 3)

where

Riskair = lifetime incremental cancer risk from the air exposure route (unitless)

Cair = concentration in air (mg/m3)

URF = inhalation unit risk factor ([μg/m3

]

-1

)

1000 = conversion factor (μg/mg)

ED = exposure duration (yr)

AT = averaging time (yr).

Noncancer Hazard

Noncancer hazards were calculated using Equations 4 to 6. Noncancer hazard from drinking water ingestion was 

characterized based on the average daily dose (ADD) during the period of exposure and the RfD: 

RfD

ADD

HQ dw

oral = (Equation 4)
where

HQoral = oral hazard quotient (unitless)

ADDdw = average daily dose from consumption of drinking water (mg/kg-d)

RfD = reference dose (mg/kg-d).

Noncancer hazard from dermal exposure via showering or bathing was calculated as follows: 

RfD

DAD HQdermal = (Equation 5)

where

HQoral = oral hazard quotient (unitless)

DAD = average daily dose from consumption of drinking water (mg/kg-d)

RfD = reference dose (mg/kg-d).

Noncancer hazard from air inhalation was calculated using an RfC:

R fC

C H Q a ir

a ir= (Equation 6)

where

HQair = air hazard quotient (unitless)

Cair = air concentration (mg/m3

)

RfC = reference concentration (mg/m3).

Tables 2 through 7 present the recalculated 50th percentile and 90th percentile risk results for the 

different pathways (air and groundwater), for both child and adult, for unlined and composite-lined 

landfills, and for noncancer and cancer risk. In the case of the combined risks, the driving pathways are 

identified in parentheses in the tables (i.e., DW indicates drinking water and SH indicates shower 

inhalation).1

The results are presented in tables similar in format to the tables in Section 4 of the original 

risk document (Risk-Based Mass Loading Limits for Solvents in Disposed Wipes and Laundry Sludges 

Managed in Municipal Landfills, October 2009). As noted in the titles of Tables 2 through 7, these tables 

correspond to Tables 4-1 through 4-6, respectively, in the 2009 report.

Table 2. Revised Ambient Air Risk Estimates at the Unitized Mass Loading Rate (1 kg/year): Adult

(updates to Table 4-1)

Constituent

50th Percentile 90th Percentile

NL Composite NL Composite

CANCER Risk Risk Risk Risk

Tetrachloroethylene 2.4E-12 2.5E-12 2.0E-11 2.1E-11

Trichloroethylene 3.7E-11 3.9E-11 3.1E-10 3.3E-10

NONCANCER HQ HQ HQ HQ

Tetrachloroethylene 1.3E-06 1.4E-06 9.1E-06 9.5E-06

Trichloroethylene 2.7E-05 2.9E-05 1.9E-04 1.9E-04

 1 The analysis produced separate risk distributions for each pathway, as well as one for risk combined across 

pathways. The results shown are the 50th or 90th percentile of these different distributions. As a result, the 

combined risk may not exactly equal the sum of the separate pathway risks.
1

Table 3. Revised Ambient Air Risk Estimates at the Unitized Mass Loading Rate (1 kg/year): Child 

(updates to Table 4-2)

Constituent

50th Percentile 90th Percentile

NL Composite NL Composite

CANCER Risk Risk Risk Risk

Tetrachloroethylene 1.0E-12 1.1E-12 9.5E-12 9.8E-12

Trichloroethylene 1.6E-11 1.7E-11 1.5E-10 1.5E-10

NONCANCER HQ HQ HQ HQ

Tetrachloroethylene 1.5E-06 1.6E-06 9.9E-06 1.0E-05

Trichloroethylene 3.0E-05 3.2E-05 2.0E-04 2.1E-04

Table 4. Summary of Revised 50th Percentile Solvent Wipes at the Unitized Mass Loading Rate 

(1kg/year) Adult Groundwater (updates to Table 4-3)

Constituent

Drinking Water Shower Inhalation Dermal Combined DW/SH/DM

NL Composite NL Composite NL Composite NL Composite

CANCER Risk Risk Risk Risk Risk Risk Risk Risk

Tetrachloroethylene 2.5E-10 6.4E-20 5.9E-10 1.2E-19 6.1E-11 1.3E-20 1.0E-09(DW) 2.1E-19 (SH)

Trichloroethylene 6.3E-09 1.6E-18 9.8E-09 2.2E-18 2.0E-09 4.6E-19 2.1E-08 (SH) 5.4E-18 (SH)

NONCANCER HQ HQ HQ HQ HQ HQ HQ HQ

Tetrachloroethylene 1.4E-04 3.3E-14 3.5E-04 7.1E-14 3.2E-05 6.4E-15 NA NA

Trichloroethylene 1.7E-03 3.6E-13 7.6E-03 1.6E-12 5.3E-04 1.1E-13 NA NA

SH = Driving pathway is shower inhalation.
2

Table 5. Summary of Revised 90th Percentile Solvent Wipes at the Unitized Mass Loading Rate 

(1kg/year) Adult Groundwater (updates to Table 4-4)

Constituent

Drinking Water Shower Inhalation Dermal Combined DW/SH/DM

NL Composite NL Composite NL Composite NL Composite

CANCER Risk Risk Risk Risk Risk Risk Risk Risk

Tetrachloroethylene 1.5E-08 3.1E-11 3.9E-08 7.2E-11 3.3E-09 7.1E-12 5.8E-08 (SH) 1.3E-10 (SH)

Trichloroethylene 3.6E-07 6.0E-10 6.5E-07 9.4E-10 1.0E-07 1.8E-10 1.2E-06 (DW) 1.9E-09 (DW)

NONCANCER HQ HQ HQ HQ HQ HQ HQ HQ

Tetrachloroethylene 6.2E-03 1.5E-05 1.9E-02 4.4E-05 1.3E-03 3.5E-06 NA NA

Trichloroethylene 7.6E-02 1.3E-04 4.1E-01 6.9E-04 2.2E-02 4.2E-05 NA NA

DW = Driving pathway is drinking water.

SH = Driving pathway is shower inhalation.

Table 6. Summary of Revised 50th Percentile Solvent Wipes at the Unitized Mass Loading Rate 

(1kg/year) Child Groundwater (updates to Table 4-5)

Constituent

Drinking Water Dermal Bathing Combined DW/DM

NL Composite NL Composite NL Composite

CANCER Risk Risk Risk Risk Risk Risk

Tetrachloroethylene 2.3E-10 5.5E-20 6.4E-11 1.6E-20 3.1E-10 (DW) 7.5E-20 (DW)

Trichloroethylene 5.9E-09 1.1E-18 2.2E-09 4.6E-19 8.6E-09 (DW) 1.7E-18 (DW)

NONCANCER HQ HQ HQ HQ HQ HQ

Tetrachloroethylene 3.0E-04 6.1E-14 8.5E-05 1.8E-14 NA NA

Trichloroethylene 3.8E-03 7.8E-13 1.4E-03 3.3E-13 NA NA

DW = Driving pathway is drinking water.
3

Table 7. Summary of Revised 90th Percentile Solvent Wipes at the Unitized Mass Loading Rate 

(1kg/year) Child Groundwater (updates to Table 4-6)

Constituent

Drinking Water Dermal Bathing Combined DW/DM

NL Composite NL Composite NL Composite

CANCER Risk Risk Risk Risk Risk Risk

Tetrachloroethylene 1.2E-08 2.7E-11 3.3E-09 7.4E-12 1.6E-08 (DW) 3.6E-11 (DW)

Trichloroethylene 3.1E-07 5.4E-10 1.1E-07 1.8E-10 4.4E-07 (DW) 7.3E-10 (DW)

NONCANCER HQ HQ HQ HQ HQ HQ

Tetrachloroethylene 1.3E-02 3.0E-05 3.4E-03 8.6E-06 NA NA

Trichloroethylene 1.7E-01 2.9E-04 5.8E-02 1.0E-04 NA NA

DW = Driving pathway is drinking water.

Revised Risk Estimates and RB-MLLs

As described in Section 5 of Risk-Based Mass Loading Limits for Solvents in Disposed Wipes and 

Laundry Sludges Managed in Municipal Landfills, the RB-MLLs are derived from the 50th and 90th 

percentile risk values as follows:

 We calculate a scaling factor to derive the mass loading rate that corresponds to the cancer risk 

values of interest (1E-4, 1E-5, or 1E-6) and/or an HQ of 1. The scaling factor is simply the ratio 

of the risk/hazard criteria to the risk/hazard estimate at the 50th or 90th percentile. 

 We then multiply the constant mass loading rate (MLR) of 1 kg/yr by the scaling factor at the 

selected percentile. Because the rate is a constant value of 1 kg/yr, the scaling factor is equivalent 

to the MLR at the percentile of interest (i.e., 1 x SF = SF = MLR). The MLR can be derived for 

any percentile of interest; however, the risk-based MLRs were derived at the 90th and 50th 

percentiles of the distribution per EPA policy for this analysis.

 The lowest rate represents that value that will protect at least 90 (or 50) percent of the 

hypothetical individuals (all receptor types) living within 2 km of a municipal solid waste landfill 

for all exposure pathways. This MLR is referred to as the Risk-Based Mass Loading Limit (RBMLL)

 Lastly, we perform a check on the MLR to ensure that solubility limits are not exceeded (e.g., the 

MLR should not result in pure product in the landfill).

Tables 8 and 9 present the noncancer- and cancer-based RB-MLLs, respectively. For comparison, these 

tables also present the values shown in the 2009 risk assessment document. 
4

Table 8. RB-MLLs for Noncarcinogens

Analysis 

Year CAS Chemical

No Liner Composite Liner

MLL (kg/yr)a Limiting Pathwayb MLL (kg/yr)a Limiting Pathwayb

90th Percentile (Table 5-1)

2009

127-18-4 Tetrachloroethylene 1.3E+02 DW-Ch 5.6E+04 DW-Ch

79-01-6 Trichloroethylene 7.3E+02 SH-A 4.4E+05 SH-A

2012

127-18-4 Tetrachloroethylene 7.7E+01 DW-Ch 3.3E+04 DW-Ch

79-01-6 Trichloroethylene 2.4E+00 SH-A 1.4E+03 SH-A

50th Percentile (Table 5-2)

2009

127-18-4 Tetrachloroethylene 5.6E+03 DW-Ch 4.0E+06 S AA-Ch

79-01-6 Trichloroethylene 3.9E+04 SH-A 9.3E+06 AA-Ch

2012

127-18-4 Tetrachloroethylene 3.3E+03 DW-Ch 6.3E+05 S AA-Ch

79-01-6 Trichloroethylene 1.3E+02 SH-A 3.1E+04 AA-Ch

Table 9. RB-MLLs for Carcinogens

Analysis 

Year

Risk 

Threshold

No Liner Composite Liner

MLL 

(kg/yr)a Sol Flaga Limiting Pathwayb MLL (kg/yr)

Sol 

Flaga Limiting Pathwayb

90th Percentile (Table 5-3)

2009

127-18-4: Tetrachloroethylene (high)c

10-4 1.7E+01 Comb(DW)-A 8.2E+03 Comb(DW)-A

10-5 1.7E+00 Comb(DW)-A 8.2E+02 Comb(DW)-A

10-6 1.7E-01 Comb(DW)-A 8.2E+01 Comb(DW)-A

HQ=1 1.3E+02 DW-Ch 5.6E+04 DW-Ch

127-18-4: Tetrachloroethylene (low)c

10-4 2.0E+01 Comb(DW)-A 9.8E+03 Comb(DW)-A

10-5 2.0E+00 Comb(DW)-A 9.8E+02 Comb(DW)-A

10-6 2.0E-01 Comb(DW)-A 9.8E+01 Comb(DW)-A

HQ=1 1.3E+02 DW-Ch 5.6E+04 DW-Ch

79-01-6: Trichloroethylene

10-4 2.2E+02 Comb(SH)-A 1.3E+05 Comb(DW)-A

10-5 2.2E+01 Comb(SH)-A 1.3E+04 Comb(DW)-A

10-6 2.2E+00 Comb(SH)-A 1.3E+03 Comb(DW)-A

HQ=1 7.3E+02 SH-A 4.4E+05 SH-A

2012

127-18-4: Tetrachloroethylene

10-4 1.7E+03 Comb(SH)-A 7.7E+05 Comb(SH)-A

10-5 1.7E+02 Comb(SH)-A 7.7E+04 Comb(SH)-A

10-6 1.7E+01 Comb(SH)-A 7.7E+03 Comb(SH)-A

HQ=1 7.7E+01 DW-Ch 3.3E+04 DW-Ch
5

79-01-6: Trichloroethylene

10-4 8.3E+01 Comb(DW)-A 5.3E+04 Comb(DW)-A

10-5 8.3E+00 Comb(DW)-A 5.3E+03 Comb(DW)-A

10-6 8.3E-01 Comb(DW)-A 5.3E+02 Comb(DW)-A

HQ=1 2.4E+00 SH-A 1.4E+03 SH-A

50th Percentile (Table 5-4)

2009

127-18-4: Tetrachloroethylene (high)c

10-4 9.5E+02 Comb(DW)-A 1.7E+06 AA-A

10-5 9.5E+01 Comb(DW)-A 1.7E+05 AA-A

10-6 9.5E+00 Comb(DW)-A 1.7E+04 AA-A

HQ=1 5.6E+03 DW-Ch 4.0E+06 S AA-Ch

127-18-4: Tetrachloroethylene (low) c

10-4 1.1E+03 Comb(DW)-A 1.5E+07 AA-A

10-5 1.1E+02 Comb(DW)-A 1.5E+06 AA-A

10-6 1.1E+01 Comb(DW)-A 1.5E+05 AA-A

HQ=1 5.6E+03 DW-Ch 4.0E+06 S AA-Ch

79-01-6: Trichloroethylene

10-4 1.3E+04 Comb(SH)-A 5.1E+06 AA-A

10-5 1.3E+03 Comb(SH)-A 5.1E+05 AA-A

10-6 1.3E+02 Comb(SH)-A 5.1E+04 AA-A

HQ=1 3.9E+04 SH-A 9.3E+06 AA-Ch

2012

127-18-4: Tetrachloroethylene

10-4 1.0E+05 Comb(DW)-A 4.0E+07 AA-A

10-5 1.0E+04 Comb(DW)-A 4.0E+06 AA-A

10-6 1.0E+03 Comb(DW)-A 4.0E+05 AA-A

HQ=1 3.3E+03 DW-Ch 6.3E+05 S AA-Ch

79-01-6: Trichloroethylene

10-4 4.8E+03 Comb(SH)-A 2.6E+06 AA-A

10-5 4.8E+02 Comb(SH)-A 2.6E+05 AA-A

10-6 4.8E+01 Comb(SH)-A 2.6E+04 AA-A

HQ=1 1.3E+02 SH-A 3.1E+04 AA-Ch

a S = Based on solubility limit. b Limiting Pathway is denoted as Pathway-Receptor, where: 

AA = ambient air; Comb = combined drinking water, shower inhalation, shower/bath dermal; DM = shower/bath 

dermal; DW = drinking water; SH = shower inhalation. 

For combined (Comb) risks, the associated limiting pathway is provided in parentheses. 

A = Adult, Ch = Child. C In 2009, two estimates were provided for tetrachloroethylene cancer risks to reflect the high and low ends of the 

range of benchmarks.
6

Tables 10, 11, and 12 present the recalculated 50th percentile and 90th percentile RB-MLLs that 

represent the changes to the Executive Summary tables contained in the original risk document. As noted 

in the titles of Tables 10 through 12, these tables correspond to Tables ES-2, ES-3, and ES-4, 

respectively, in the 2009 report.

Table 10. RB-MLLs Based on a Cancer Risk Level of 10-4 or an HQ of 1 for 5 Solvents Analyzed 

Both as Carcinogens and Noncarcinogens (kg/year) [updates to Table ES-2]

Solvent

No Liner Composite

50%ilea 90%ilea 50%ilea 90%ilea

Tetrachloroethyleneb 3.3E+03 N 7.7E+01 N 6.3+E05 N 3.3E+04 N

Trichloroethyleneb 1.3+02 N 2.4E+00 N 3.1E+04 N 1.4E+03 N

a N = based on noncancer risk..

Table 11. RB-MLLs Based on a Cancer Risk Level of 10-5 or an HQ of 1 for 5 Solvents Analyzed 

Both as Carcinogens and Noncarcinogens (kg/year) [updates to able ES-3]

Solvent

No Liner Composite

50%ilea 90%ilea 50%ilea 90%ilea

Tetrachloroethylenec 3.3E+03 N 7.7E+01 N 6.3E+05 N 3.3E+04 N

Trichloroethylenec 1.3E+02 N 2.4E+00 N 3.1E+04 N 1.4E+03 N

a N = based on noncancer risk.

Table 12. RB-MLLs Based on a Cancer Risk Level of 10-6 or an HQ of 1 for 5 Solvents Analyzed 

Both as Carcinogens and Noncarcinogens (kg/year) [updates toTable ES-4]

Solvent

No Liner Composite

50%ilea 90%ilea 50%ilea 90%ilea

Tetrachloroethylenec 1.0E+03 C 1.7E+01 C 4.0E+05 C 7.7E+03 C

Trichloroethylenec 4.8E+01 C 8.3E-01 C 2.6E+04 C 5.3E+02 C

a C = based on cancer risk.

Finally, the tables attached to the end of this document present the updated results for tetrachloroethylene 

and trichloroethylene contained in Appendix A of the original risk document. This appendix presents 

chemical-specific results for a series of percentiles (50th, 75th, 90th, 95th, and 99th) and scenarios that 

represent the theoretical risk distributions based on the unitized mass loading rate of 1 kg /year. Separate 

tables present the data for each solvent (Tables 17a and 17b for tetrachloroethylene and Tables A-19a 

and A-19b for trichloroethylene). Tables labeled with an a present results for the ambient air pathway and 

those labeled b present the results for the groundwater pathway.