MEMORANDUM

Date: May 13, 2014

Subject: MACT-Allowable Emissions for the Off-Site Waste and Recovery Operations Source 

Category

From: Lesley Stobert, EC/R Incorporated

To: Paula Hirtz, EPA/OAQPS/SPPD

_________________________________________________________________________________

The purpose of this memorandum is to present analyses related to the determination of 

“MACT-allowable” emissions levels and risks for the Off-Site Waste and Recovery Operations 

(OSWRO) source category being addressed under the EPA’s Risk and Technology Review (RTR) 

program. 

The RTR program addresses statutory requirements in section 112(f) and 112(d)(6) of the 

Clean Air Act (CAA) for federal air toxics regulations. The federal air toxics regulations are National 

Emission Standards for Hazardous Air Pollutants (NESHAP) contained in 40 CFR part 63 (commonly 

referred to as Maximum Achievable Control Technology (MACT) standards). The emissions data 

used for the RTR risk assessment for this category consists of data representative of “actual” emission 

levels. In addition to the risks associated with these actual emission levels, the EPA also considered

the levels of HAP emissions that sources could emit and still be in compliance with the applicable 

MACT regulation. These emission levels are referred to as “MACT-allowable” emissions. The 

purpose of this memorandum is to discuss these MACT-allowable emission levels for the OSWRO 

source category and to relate the potential risks if facilities emitted at these levels.

This memorandum is organized as follows. Section 1 presents the estimated maximum risk 

levels based on the actual emission levels for the source category. Section 2 presents the approach to 

estimating the risks at the MACT-allowable levels. The last section (Section 3) presents a discussion 

of the estimation of the MACT-allowable emission level for the OSWRO MACT standards. Section 3

also includes a presentation of the estimated risks at the estimated MACT-allowable emissions levels.

1.0 Risk Levels Based on Actual Emissions

The estimated maximum inhalation risks for the source category are summarized in Table 1. 

These risks are based on actual emission levels.
2

Table 1. Summary of Estimated Inhalation Risks for the OSWRO Source Category Based On 

Actual Emissions Levels

Source Category

Maximum individual cancer 

risk (in a million)1

Maximum chronic noncancer TOSHI2

Off-Site Waste and 

Recovery Operations 

(OSWRO)

9 0.6

 1 Maximum individual excess lifetime cancer risk.

 2 Maximum target organ specific hazard index (TOSHI). The maximum TOSHI is based on respiratory effects. 

2.0 Approach to Estimating Risks from MACT-Allowable Emission Levels

Detailed MACT-allowable emissions information is not available on an emission pointspecific level. Therefore, modeling was not conducted to estimate the risks from MACT-allowable 

emission levels.

The basic approach to estimate the risks from MACT-allowable emission levels was to first 

estimate the relationship between the actual and MACT-allowable emission levels, and then to 

estimate the potential risk impacts from the differences between actual and MACT-allowable 

emissions. In general, the risk impact was assumed to be proportional to the difference in emission 

levels. For example, if MACT-allowable emissions were estimated to be three times as high as actual 

emissions, it was assumed that risks could be as much as three times as high as those estimated from 

the modeling conducted using actual emissions.

This assumption regarding the linear relationship between emissions and risks is generally 

reasonable for chronic cancer and non-cancer risks. However, using this approach for acute risks 

would likely overestimate the impacts. The acute assessment already has numerous built in 

conservative assumptions. First, the acute assessment assumed a short-term emission rate up to 

10 times that of the average longer term emission rate. Also, the maximum acute risks assume the 

simultaneous occurrence of independent factors that may vary greatly, such as hourly emissions rates, 

meteorology, and human activity patterns. In the assessment using actual emissions, EPA assumed 

that individuals remained for 1 hour at the point of maximum ambient concentration as determined by 

the co-occurrence of peak emissions and worst-case meteorological conditions. These assumptions 

would tend to overestimate actual exposures since it is unlikely that a person would be located at the 

point of maximum exposure during the time of worst-case impact. The addition of another factor to 

account for the MACT-allowable emissions level would further increase the conservativeness of the 

estimated MACT-allowable acute risks. Therefore, this analysis does not discuss acute risks or 

estimate acute risks for the MACT-allowable emissions level.

3.0 Analysis and Results

The OSWRO NESHAP addresses emissions from the following types of emission sources: (1) 

storage tanks, (2) oil-water and organic-water separators, (3) surface impoundments, (4) containers, 

(5) transfer systems, (6) process vents, and (7) equipment leaks. Table 2 summarizes the formats of 

the standards for these emission points.
3

Table 2. Formats of Standards for the OSWRO NESHAP

Emission Source Type Format of Primary Standard

Storage Tanks Equipment Standards or Performance Standard 

(% reduction)

Oil-Water and Organic-Water Separators Equipment Standardsa

Surface Impoundments Equipment Standardsa

Containers Equipment Standardsa

, Performance Standard (% 

reduction) for containers used for waste stabilization

Transfer Systems Equipment Standardsa

Process Vents Performance Standard (% reduction)

Equipment Leaks Work Practice Standards

a As an alternative, the NESHAP allows emissions to be routed through a closed vent system to a control device that 

reduces emissions by 95%

The ability to estimate MACT-allowable emissions for specific emission points, or for specific 

emission point types, is largely dependent on the format of the standards. For equipment standards, 

sources subject to the OSWRO NESHAP are required to install specific equipment. Assuming that the 

equipment is maintained properly and is in good working condition (as required), there would not be a 

difference in the actual emissions level and the level allowed by the standard. Based on data collected 

from facilities through a CAA section 114 information collection request, facilities are complying 

with the equipment standards rather than the alternative standard to vent emissions through a closed 

vent system routed to a control device for oil-water and organic-water separators, surface 

impoundments, containers and transfer systems. Similarly, for emission sources subject to work 

practice requirements, there would be no difference between actual and MACT-allowable emissions, 

provided that facilities are not conducting additional work practices proven to reduce emissions 

beyond those required in the rule. We are not aware of any such situations at facilities in the OSWRO

source category. Therefore, for equipment leaks, oil-water and organic-water separators, surface 

impoundments, containers and transfer systems, which are subject to equipment and work practice 

standards, we believe that the actual emission levels are a reasonable estimation of the levels allowed 

by the standard.

For process vents, the OSWRO NESHAP requires an emissions reduction of 95% of either the 

total organic compounds or the specific HAP listed in Table 1 of the rule. For storage tanks, the 

OSWRO NESHAP requires either the installation of a floating internal or external roof, pressure 

vessel, or fixed roof with emissions to be routed through a closed vent system to a control device 

reducing either the total organic compounds or the specific HAP listed in Table 1 of the OSWRO 

NESHAP by 95%. 

Based on information from the CAA section 114 information collection request, tanks in the 

OSWRO source category are controlled by fixed roofs that are vented to control devices. In addition,

information from the CAA section 114 information collection request shows that emissions 

reductions from some control devices on process vents and storage tanks are 99%. In these cases, 5%

of the uncontrolled amount of HAP would be emitted at the MACT-allowable level, while some

process vents and tanks are actually only emitting 1% (i.e., 99% emissions reduction). The ratio of 

MACT-allowable to actual emissions for these emissions sources is therefore 5/1 = 5. 
4

For facilities where data were available to determine whether an emissions point was a process 

vent, tank or other type of emissions source (i.e., facilities that provided data in response to the CAA 

section 114 information collection request), this ratio of 5 was applied to the process vent and tank 

emissions. For facilities where only NEI data were available, it was not possible to determine whether 

an emissions point was a process vent, tank or other type of emissions source, and the ratio of 5 could 

not be applied to only process vent and tank emissions. For these facilities, this ratio was revised to a 

value that could be applied to all the OSWRO emissions. Based on data from the CAA section 114 

information collection request responses, emissions from process vents and storage tanks contribute 

approximately 50% to the total OSWRO HAP emissions. The ratio of 5 was revised to 2.5 to reflect 

the impact a factor of five applied to 50% of the emissions would have on the total OSWRO 

emissions. This factor of 2.5 was then applied to the total OSWRO emissions for the facilities with 

only NEI data available.

The impacts on the cancer MIR and non-cancer TOSHI were determined based on the 

application of the factors of 5 and 2.5 to the OSWRO emissions. The MIR based on actual emissions 

is 9, which is caused by a facility with only NEI emissions data. The application of a factor of 2.5 to 

the emissions at this facility results in the MIR at the MACT-allowable level of 20.

a The maximum 

TOSHI based on actual emissions is 0.6, which is also caused by a facility with only NEI emissions 

data. The application of a factor of 2.5 to the emissions at this facility results in the maximum TOSHI 

at the MACT-allowable level of 1.

a These values for the MIR and TOSHI were the highest values 

estimated in our analyses, even when considering the application of a factor of 5 to process vent and 

tank emissions at facilities where emission point data were available.

The resulting risks based on the application of the ratios of 5 and 2.5 are shown in the MACTallowable level risks presented in Table 3.

Table 3. Summary of Estimated Inhalation Risks for the OSWRO Source Category Based On 

MACT-Allowable Emissions Levels

Actual MACT-Allowable

Source Category

Maximum 

individual 

cancer risk (in a 

million)

Maximum 

chronic noncancer TOSHI

Maximum 

individual 

cancer risk (in a 

million)

Maximum 

chronic noncancer 

TOSHI

Offsite Waste and Recovery Operations 9 0.6 20 1

 

a The TOSHI at the reported actual emissions level is 0.5978. This value multiplied by 2.5 is 1.49, which rounds down to 

1. The MIR at the reported actual emissions level is 9. This value multiplied by 2.5 is 22.5, which rounds down to 

20.TOSHI and MIR values are presented with one significant digit.