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MEMORANDUM

DATE: May 30, 2014

SUBJECT: Estimated Risks and Risk Reductions Associated with the Emissions Controls 

Options Considered in the Ample Margin of Safety Analysis for the Off-Site 

Waste and Recovery Operations Source Category

FROM: Lesley Stobert, EC/R Incorporated

TO: Paula Hirtz, EPA/OAQPS

The purpose of this memorandum is to describe how risks and risk reductions were

estimated for each of the emissions control options considered in the ample margin of safety 

analysis conducted for the Off-Site Waste and Recovery Operations (OSWRO) source category

under the EPA’s Risk and Technology Review (RTR) program. 

1.0 BACKGROUND

As directed by section 112(f)(2) of the Clean Air Act, a baseline risk assessment was 

conducted to assess the health and environmental risks that remain after affected facilities have 

complied with the maximum achievable control technology (MACT) standards for the OSWRO 

source category (40 CFR part 63, subpart DD). The results of these baseline risk assessments 

were analyzed to determine whether more stringent standards must be set. The first step in the 

process of evaluating residual risk is the determination of acceptable risk. It was determined that 

the risks from the OSWRO source category are acceptable, considering the available health risk 

information and factors, including the uncertainties associated with the risks. The second step is 

the determination of whether standards must be further revised in order to provide an ample 

margin of safety to protect public health. The ample margin of safety is the level at which the 

standards must be set, unless an even more stringent standard is necessary to prevent, taking into 

consideration costs, energy, safety and other relevant factors, an adverse environmental effect.

While the baseline risks were determined to be acceptable, in striving to provide the maximum 

feasible protection against risks to health from hazardous air pollutants (HAP), available 

emissions control options that might reduce the risk associated with HAP emissions from the 

source category were then estimated, along with all of the health risks and other health 

information considered in determining risk acceptability, in the ample margin of safety decision 

process. 
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2.0 IDENTIFIED EMISSIONS AND RISK REDUCTION OPTIONS

As part of the RTR, a technology review was conducted that focused on identifying 

developments in practices, processes and control technologies for the emission sources in the 

OSWRO production source category. Through this review, several developments in practices, 

process or control technologies were identified.

1 The control options identified to reduce risks 

are the same as those identified in the technology review. No options were identified that would 

reduce HAP emissions from containers, surface impoundments, oil-water separators, organicwater separators or transfer systems beyond what is currently required in the rule. For process 

vents, tanks and equipment leaks, several additional control options were identified that might 

reduce the risk associated with emissions from the source category. For process vents, one 

emissions control option was identified that would require compliance with a 98 percent 

reduction in HAP emissions rather than the currently required 95 percent reduction. For tanks, 

two emissions control options were identified. Tanks Option 1 requires more stringent control of 

emissions for additional tanks containing liquids with lower vapor pressures, and tanks Option 2 

requires compliance with a 98 percent reduction in HAP emissions rather than the currently 

required 95 percent reduction. For equipment leaks, two emission control options were 

identified. Equipment leaks Option 1 requires compliance exclusively with 40 CFR part 63, 

subpart H, rather than allowing compliance with 40 CFR part 61, subpart V, without the 

connector leak detection and repair (LDAR) requirements of subpart H. Equipment leaks Option 

2 requires compliance with equipment leaks Option 1, plus it includes the connector LDAR 

requirements of subpart H.

3.0 ESTIMATION OF POTENTIAL RISKS AND RISK REDUCTION 

To determine the risk reduction that would be achieved with the implementation of these 

control options, the maximum risks for the source category and the amount of those risks 

attributed to OSWRO process vents, tanks and equipment leaks were examined. The basic 

approach to estimate the risk reduction that would be achieved with the implementation of these 

control options was to first estimate the relationship between the current MACT-allowable 

emissions and the MACT-allowable emissions that would result from the implementation of the 

control options, and then to estimate the potential risk impacts from the differences between 

these emission levels. In general, the risk impact was assumed to be proportional to the 

difference in emission levels. For example, if MACT-allowable emissions after implementation 

of the control option were estimated to be three times lower than the current MACT-allowable 

emissions, it was assumed that risks could be as much as three times lower than the risks

estimated using current MACT-allowable emissions.

 

1 See Technology Review and Cost Impacts for the Proposed Amendments to the Off-Site Waste and Recovery 

Operations Source Category, which is available in the docket for this action.
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The cancer maximum individual risk (MIR) and noncancer target organ-specific hazard 

index (TOSHI) were examined in this analysis, and these values were attributed to a facility for 

which information is not available regarding the processes and emission point types that 

contribute to these maximum risk values. Because it was not possible to precisely determine the 

magnitude of HAP emissions from specific process types and how those emissions relate to the 

risk estimates, it was conservatively assumed that the type of equipment under investigation was 

responsible for the maximum risks. For example, in the assessment of process vents, it was

assumed the maximum risks for were due to process vents. An evaluation of how further controls 

might reduce this risk was then undertaken. Table 1 provides a summary of the OSWRO

inhalation risk assessment results and the estimated risks that would occur with the 

implementation of the identified control options. The sections that follow describe how the levels 

were estimated for each equipment type where alternative control options were identified. 

Table 1. Summary of OSWRO Baseline Inhalation Risks and Estimated Risks After 

Implementation of Identified Control Options

Source Category

Actual MACT-Allowable1

Maximum 

individual 

cancer risk (in 

a million)

Maximum 

chronic noncancer TOSHI

Maximum 

individual 

cancer risk (in 

a million)

Maximum 

chronic noncancer TOSHI

OSWRO 9 0.6 20 1

Control Options

Process Vents option ----- ----- 8 0.4

Tanks Option 1 ----- ----- 1 0.05

Tanks Option 2 ----- ----- 0.4 0.02

EL Option 1 7 0.5 ----- -----

EL Option 2 5 0.3 ----- -----

1Emissions from equipment leaks are estimated to be the same at actual and MACT-allowable emission levels.

3.1 Process Vents

For process vents emissions control option was identified that requires compliance with a 

98 percent reduction in HAP emissions rather than the currently required 95 percent reduction. In 

this case, 5 percent of the HAP is currently emitted at the MACT-allowable level, and the control 

option would require that only 2 percent of the HAP could be emitted at the MACT-allowable 

level. Therefore, emissions would be reduced by 60 percent with this control option (i.e., 5

percent emitted reduced to 2 percent emitted for a difference of 3 percent, and 3/5=0.6=60%). To 

assess the maximum potential for risk reduction that could result from this process vent control 

option, it was assumed that the maximum risks are due to emissions from a process vent with 

emissions controlled at 95 percent and that the emissions and resulting risks would be reduced by 

60 percent. As shown in Table 1, the maximum cancer MIR at the MACT-allowable emissions 
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level is 20-in-1 million, and the maximum non-cancer TOSHI at the MACT-allowable emissions 

level is 1. Applying the emissions reduction factor to the cancer MIR value results in a MIR of 8-

in-1 million (i.e., 20*(1-0.6) = 8), and application of this factor to the non-cancer TOSHI value 

results in a TOSHI of 0.4 (i.e., 1*(1-0.6) = 0.4). 

3.2 Tanks

For tanks, two emissions control options were identified. Option 1 requires Level 2 

control of emissions for additional tanks containing liquids with lower vapor pressures, where 

these tanks are currently required to use Level 1 emission controls. “Level 2” control essentially 

requires one of five options:

(1) A fixed roof tank equipped with an internal floating roof; 

(2) A fixed roof tank equipped with an external floating roof; 

(3) A tank with a vapor-tight cover and vented through a closed-vent system to a control 

device that has an efficiency of 95 percent or more; 

(4) A pressure tank; or 

(5) A tank inside a permanent total enclosure (PTE) that is vented through a closed-vent 

system to an enclosed combustion control device. 

“Level 1” control generally requires a fixed roof with closure devices. To assess the maximum 

potential for risk reduction that could result from this tank control option, it was assumed the 

maximum risks are due to emissions from a tank not currently subject to Level 2 controls and 

that a 95 percent reduction in emissions and resulting risks would occur from this tank with the 

implementation of this option. As shown in Table 1, the maximum cancer MIR from the 

OSWRO source category at the MACT-allowable emissions level is 20-in-1 million, and the 

maximum non-cancer TOSHI at the MACT-allowable emissions level is 1. Applying the 

emissions reduction factor to the cancer MIR value results in a MIR of 1-in-1 million (i.e., 

20*(1-0.95) = 1), and application of this factor to the non-cancer TOSHI value results in a 

TOSHI of 0.05 (i.e., 1*(1-0.95) = 0.05). 

Tank control Option 2 requires compliance with Option 1, plus it requires a 98 percent

reduction rather than a 95 percent reduction in HAP emissions from tanks that are vented 

through a closed-vent system to a control device or are located in a PTE that is vented through a 

closed-vent system to a control device. To assess the maximum potential for risk reduction that 

could result from this tank control option, it was assumed that the maximum risks for the 

OSWRO source category are due to emissions from a tank not currently subject to Level 2 

controls and that a 95 percent reduction in emissions and resulting risks would occur from the 

implementation of control Option 1. It was further assumed that this tank would be required with 

tank control Option 2 to reduce HAP emissions by 98 percent. In this case, emissions would be 
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reduced by 60 percent with tank control Option 2 over tank control Option 1 (i.e., 5 percent

emitted reduced to 2 percent emitted for a difference of 3 percent, and 3/5=0.6=60%). As 

discussed above, the maximum cancer MIR from the OSWRO source category at the MACTallowable emissions level after implementation of tank Option 1 is 1-in-1 million, and the 

maximum non-cancer TOSHI at the MACT-allowable emissions level after implementation of

tank Option 1 is 0.05. Applying the emissions reduction factor of tank control Option 2 to this

cancer MIR value results in a MIR of 0.4-in-1 million (i.e., 1*(1-0.6) =0.4), and application of 

this factor to this non-cancer TOSHI value results in a TOSHI of 0.02 (i.e., 0.05*(1-0.6) = 0.02). 

3.3 Equipment Leaks

The OSWRO MACT standards currently require compliance with either 40 CFR part 61, 

subpart V, or 40 CFR part 63, subpart H, to control emissions from equipment leaks. While 

many provisions of these two rules are the same or similar, 40 CFR part 63, subpart H requires 

the use of a more stringent leak definition for valves in gas and vapor service and in light liquid 

service, pumps in light liquid service, and connectors. For equipment leaks, two emissions 

control options were identified. Equipment leaks control Option 1 (EL Option 1) requires 

compliance exclusively with 40 CFR part 63, subpart H, without the connector LDAR 

requirements of subpart H. To assess the maximum potential for risk reduction that could result 

from this equipment leaks control option, several assumptions were made. For instance, it was

assumed that the maximum risks for the OSWRO source category are due to emissions from 

equipment leaks. It was also assumed that since emissions from equipment leaks are estimated to 

be the same at actual and MACT-allowable emission levels, the risks due to equipment leaks at 

the MACT-allowable level are the same as risks due to equipment leaks at actual emissions 

levels. It was additionally assumed, based on the analysis of estimated baseline equipment leak 

emissions (i.e., compliance with 40 CFR part 61, subpart V) in the technology review referenced 

above, that half of the equipment leak emissions causing the maximum risks are from nonconnector components (i.e. pumps and valves), and the other half are from connectors. This 

analysis also showed that a HAP emissions reduction of 23% would be achieved with EL Option 

1 over the baseline emissions. As shown in Table 1, the maximum cancer MIR from the 

OSWRO source category at the MACT-allowable emissions level for equipment leaks is 9-in-1 

million, and the maximum non-cancer TOSHI at the MACT-allowable emissions level for 

equipment leaks 0.6. Applying a 23 percent emissions reduction factor to the cancer MIR value 

results in a MIR of 7-in-1 million (i.e., 9*(1-0.23) = 7), and application of this factor to the noncancer TOSHI value results in a TOSHI of 0.5 (i.e., 0.6*(1-0.23) = 0.5).

Equipment leaks control Option 2 (EL Option 2) requires compliance with EL Option 1, 

plus it includes the connector LDAR requirements of 40 CFR part 63, subpart H. To assess the 

maximum potential for risk reduction that could result from this equipment leak control option, 

the same assumptions described for EL Option 1 were made. Based on the analysis of estimated 

equipment leak emissions and the reductions that would be achieved with EL Option 2 in the 

technology review referenced above, a HAP emissions reduction of 31 percent would be 
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achieved with EL Option 2 over that of EL Option 1. As discussed above, the maximum cancer 

MIR from the OSWRO source category at the MACT-allowable emissions level including EL 

Option 1 is 7-in-1 million, and the maximum non-cancer TOSHI at the MACT-allowable 

emissions level including EL Option 1 is 0.5. Applying the emissions reduction factor of EL

Option 2 to this cancer MIR value results in a MIR of 5-in-1 million (i.e., 7*(1-0.31)=5), and 

application of this factor to this non-cancer TOSHI value results in a TOSHI of 0.3 (i.e., 0.5*(1-

0.31) = 0.3).