11/2/2009

Support for Use of Management Practices in the 

National Emission Standards for Hazardous Air 

Pollutants Standards for Reciprocating Internal 

Combustion Engines at Area Sources 

in Rural Locations

Prepared by

David W. Heinold, CCM

AECOM Environment

Westford, Massachusetts

Prepared on behalf of

American Petroleum Institute

1220 L Street, NW

Washington, DC 20005-4070

November, 2009
Support for Rural RICE NESHAP 1 11/1/2009

SUMMARY

On March 5th, 2009 EPA proposed National Emission Standards for Hazardous Air 

Pollutants (NESHAP) from existing stationary Reciprocating Internal Combustion 

Engines (RICE) with a site rating of less than or equal to 500 brake horsepower located at 

major sources, existing non-emergency engines with a site rating >500 HP at major 

sources, and existing stationary RICE of any power rating located at area sources. 

The rule will have major cost consequences for the oil and gas industry, which operates 

RICE primarily in rural, sparsely populated areas. These operations are primarily in 

twelve western states where most of America’s oil and natural gas is produced. EPA’s 

cost-benefit assessment for the proposed rule did not consider areas where exposure and 

risk from hazardous air pollutants from RICE is extremely limited. To help fill this void, 

a review and technical analysis have been conducted. The principal findings of this 

assessment are as follows:

1. A review of the Clean Air Act confirmed that EPA has been directed to develop 

emission control programs for area sources of Hazardous Air Pollutants in urban 

locations, but does not require or suggest similar programs for area sources in rural 

locations. Thus, by applying the same requirements for RICE (natural gas and diesel 

fired) in urban and rural areas, EPA is disregarding Congressional intent in these 

Clean Air Act provisions and the precedent established by EPA for other area source 

NESHAP, such as Subpart HH for dehydration units associated with oil and gas 

production, where separate requirements were established for area sources in both 

urban and rural locations.

2. Oil and gas industry RICE are predominantly located in remote areas. Engines less 

than 500 HP are typically used for wellsite compression, while those greater than 500 

HP are typically found at compressor stations and natural gas processing plants. 

Major sources of HAP in the oil and gas industry mainly utilize engines greater than 

750 horsepower. These major sources comprise many of the gas plants and some of 

the compressor stations. The rural population density of the aforementioned twelve 

states where oil and natural gas production takes place is very low, averaging only 6 

residents per square mile. In contrast, the average urban population density in those 

states is 2,800 per square mile, about 400 times greater.

3. The chief Hazardous Air Pollutant from natural gas-fired RICE is formaldehyde. The

primary health concern from formaldehyde is cancer from prolonged inhalation. A 

dispersion modeling assessment accounting for the variability of meteorological 

conditions throughout the U.S. indicates that potentially health-significant 

concentrations associated with typical RICE are limited to within a few hundred 

meters of the source. This analysis is highly conservative. It does not account for the 

following:

a. The limited half-life of formaldehyde in air

b. Applying an upper-limit estimate of the cancer potency of formaldehyde
Support for Rural RICE NESHAP 2 11/1/2009

c. Using highly protective significant risk level of one chance in million

d. Assuming continuous operation with 70 years continuous exposure

Overlaying the rural population data onto the dispersion modeling results in an 

estimate that typical RICE, on average, does not result in significant formaldehyde 

exposure to a single individual.

4. Based on these considerations, the RICE NESHAP should follow the precedent 

established for National Emission Standards for Hazardous Air Pollutants for Source 

Categories from Oil and Natural Gas Production Facilities, promulgated in 2007, 

whereby, due to the low risk of exposure in rural areas, only sources in or near urban 

areas are required to apply costly controls and suitable management practices should 

be utilized at area sources in rural locations. 
Support for Rural RICE NESHAP 3 11/1/2009

 

1 INTRODUCTION

On March 5th, 2009 EPA proposed the NESHAP from existing stationary RICE with a 

site rating of less than or equal to 500 HP located at major sources, existing nonemergency engines with a site rating >500 HP at major sources, and existing stationary 

RICE of any power rating located at area sources. 

The focus of the present white paper is the application of NESHAP for RICE associated 

with crude petroleum and natural gas production, which are primarily area sources 

located in rural areas. For RICE, formaldehyde is the chief HAP of concern. The legal 

basis for establishing NESHAP for area sources stems from the 1990 Clean Air Act 

Amendments (CAAA), where the law requires that EPA evaluate exposure and risk in 

urban areas and, if warranted, develops regulatory programs to reduce risk in urban areas. 

The current RICE NESHAP proposal disregards the precedent established by EPA for 

other area source NESHAP, such as Subpart HH for dehydration units associated with oil 

and gas production, where separate requirements were established for area sources 

located in urban and rural locations.

This white paper is organized as follows: 

Section 2 provides background regarding the Clean Air Act and its intention that 

EPA address HAP risk for area sources in urban areas.

Section 3 characterizes RICE associated with oil and gas production and the rural 

nature of their environment.

Section 4 characterizes the low, insignificant, level of HAP exposure typically 

associated with RICE.

Section 5 provides support for the use of Management Practices on area sources 

in rural locations that take into account capital and operating cost and other 

factors as allowed by the Clean Air Act. 
Support for Rural RICE NESHAP 4 11/1/2009

2.0 AIR TOXICS REQUIREMENTS UNDER THE CLEAN AIR 

ACT FOR AREA SOURCES PERTAINING TO OIL AND GAS 

PRODUCTION FACILITIES 

According to the Clean Air Act Amendments (CAAA) of 1990, Section 112(k) 

establishes the basis for EPA to regulate hazardous air pollutants (HAPs) emitted from 

area sources (i.e., non-major stationary sources) in urban areas. For the purposes of this 

discussion in this white paper, the definition of urban is the one used in NESHAP Subpart 

HH and includes both urban areas (UA) plus a 2-mile buffer and urban clusters (UC) with 

a population greater than 10,000. Both UA and UC are terms defined by the U.S. Census 

Bureau (http://www.census.gov/geo/www/ua/ua_2k.html). An urban area consists of 

densely settled areas with a population of at least 50,000 people and an urban cluster 

consists of densely settled territory with at least 2,500 people.

The intent is to reduce health risk due to HAPs in urban areas is clearly stated in Section 

112(k) (1):

Congress finds that emissions of hazardous air pollutants from area sources 

may individually, or in the aggregate, present significant risks to public health 

in urban areas. Considering the large number of persons exposed and the 

risks of carcinogenic and other adverse health effects from hazardous air 

pollutants, ambient concentrations characteristic of large urban areas should 

be reduced to levels substantially below those currently experienced. 

(underlining added for emphasis)

Section 112(k) (2) outlines a requirement that EPA carry out research in urban areas:

...conduct a program of research with respect to sources of hazardous air 

pollutants in urban areas and shall include... ambient monitoring...analysis to 

characterize the sources...and...atmospheric transformation and other factors 

which can elevate health risks. (underline added for emphasis)

Section 112(k) (3) outlines requirements for an EPA urban HAP emissions program for 

area sources: 

...comprehensive strategy to control emissions of hazardous air pollutants 

from area sources in urban areas...identify not less than 30 hazardous air 

pollutants which, as the result of emissions from area sources, present the

greatest threat to public health in the largest number of urban areas... As 

part of such strategy the Administrator shall provide for ambient monitoring 

and emissions modeling in urban areas as appropriate to demonstrate that 

the goals and objectives of the strategy are being met. (underline added for 

emphasis)

Finally, Section 112(k) (4) encourages state and local air pollution agencies to also 

develop strategies:

...to reduce risks from emissions by area sources within a particular urban 

area. (underline added for emphasis)
Support for Rural RICE NESHAP 5 11/1/2009

It is clear from the language of Section 112(k) that the statutory federal mandate to 

develop and implement MACT standards for area sources relates to urban areas. The 

unstated implication is that rural area sources, due to their relatively low rate of HAP 

emissions, generally do not pose a significant health risk over a wide area and, therefore, 

are not of concern in sparsely populated areas. 

In addition to Section 112(k), CAA Section 112 (n) (4) (B) specifically states HAP 

emission standards for oil and gas production wells and associated equipment, could be 

regulated as an area source category, but only for facilities in urban areas: 

4) Oil and gas wells; pipeline facilities.-

(B) The Administrator shall not list oil and gas production wells (with its 

associated equipment) as an area source category under subsection (c), 

except that the Administrator may establish an area source category for oil 

and gas production wells located in any metropolitan statistical area or 

consolidated metropolitan statistical area with a population in excess of 1 

million, if the Administrator determines that emissions of hazardous air

pollutants from such wells. (underline added for emphasis)

This paragraph clearly shows congressional intent not to cover oil and gas production

equipment located at area sources in rural locations.

Another pertinent section of the CAA, Section 112(d)(5), allows EPA to establish more 

flexible emission control requirements for area sources. Specifically, Section 112(d)(5)

indicates that an area source NESHAP can be based on Generally Available Control 

Technologies (GACT) rather than Maximum Achievable Control Technology (MACT):

Alternative standard for area sources.- With respect only to categories and 

subcategories of area sources listed pursuant to subsection (c), the

Administrator may, in lieu of the authorities provided in paragraph (2) 

and subsection (f), elect to promulgate standards or requirements applicable 

to sources in such categories or subcategories which provide for the use of

generally available control technologies or management practices by such 

sources to reduce emissions of hazardous air pollutants. (underline added for 

emphasis)

In 2007 EPA applied this section of the CAA in establishing management practice 

standards for dehydrators in rural areas under NESHAP Subpart HH. Provisions of this 

section are equally applicable to RICE. 

 

In summary, the conclusion derived from this review of CAA Sections 112(d)(5), 112(k), 

and 112 (n) (4) (B), is that RICE NESHAP (both natural gas and diesel fueled) should 

differentiate between area sources in urban locations and area sources in rural locations. 

By not allowing for more flexible emission control approaches such as management 

practices in rural areas, the RICE NESAP as proposed by EPA runs counter to 

Congressional intent as expressed in the CAAA of 1990. 
Support for Rural RICE NESHAP 6 11/1/2009

3.0 RICE IN OIL AND GAS PRODUCTION

According to the Regulatory Impact Analysis (RIA) for Existing Stationary Reciprocating 

Internal combustion Engines (RICE) NESHAP (RTI, 2009) the industries affected most 

by the proposed rule, include the following:

Electric power generation, transmission, and distribution

Oil and gas extraction (including marginal wells)

Pipeline transportation of natural gas

General medical and surgical hospitals

Irrigation sets and welding equipment

The focus of this section of the white paper is to highlight the characteristics of oil and 

gas industry (production, gathering, and gas processing, hereafter referred to as the Oil 

and Gas Industry, OGI) as it pertains to the need to differentiate between urban and rural 

areas in the RICE rule. A common characteristic of energy production is that sites are 

generally located in unpopulated or sparsely populated areas.

A review of state oil and natural gas production rankings by the Energy Information 

Administration (http://tonto.eia.doe.gov/state/state_energy_rankings.cfm) indicate that 

the following twelve states combined account for over two-thirds of the oil production 

and over three quarters of the natural gas production in the U.S. (listed alphabetically): 

Alaska, California, Colorado, Louisiana, Kansas, Montana, New Mexico, North Dakota, 

Oklahoma, Texas, Utah and Wyoming. The balance of U.S. energy production is mostly 

from platforms located in federal waters.

Figure 1 shows how urban/urban clusters are geographically distributed in the 48 

contiguous states and Alaska. This map, which was created by plotting GIS shape files 

provided by the U.S. Census bureau, indicate that the population in these twelve states is 

concentrated in small areas. 

Table 1 provides a comparison of the area size and the population density for urban and 

rural areas within these energy producing states. Only a very small portion of these states 

are classified as urban areas or urban clusters. Excluding Alaska, the population in 

urban/urban clusters range from 2000 to 4000 people per square mile, whereas rural 

population density ranges from about 2 to 30 people per square mile. For Alaska, which 

has a small population relative to the other states, both the urban/urban cluster population 

density and rural population density are much lower. 

The ratio of the rural and urban population densities is an indicator of the relative 

population exposure associated with HAP emissions from a given source in each of these 

states. The comparison of urban and rural population density indicates that the likelihood 

of significant exposure from an area source would be at least 70 times lower rural areas 

than in urban areas and most likely hundreds of times lower. Table 1 indicates that, on 

average, the rural exposure would be about 400 times less than urban exposure, ranging 

from about 67 times less in Louisiana to about 1100 times less in Wyoming. 
Support for Rural RICE NESHAP 7 11/1/2009

This rural-urban population analysis reinforces why Sections 112(k) and 112 (n)(4)of the 

Clean Air Act Amendments are well founded by requiring that EPA establish programs 

to reduce HAP emissions from area sources only in urban areas. The drastic difference 

between rural and urban areas as it pertains to the RICE NESHAP is two-fold:

1) In urban areas, there is more likely to be a number of area sources in close proximity, 

the degree to which significant exposure from multiple area sources overlap is greater 

than in rural areas, where the separation between area sources is typically much greater.

2) Due to the population density in urban areas being several orders of magnitude higher 

in urban and urban cluster areas than in rural areas, many more people could be exposed 

to significant HAP concentrations in urban areas than in rural areas. 

Table 1. Population Distribution in the States with Highest Oil and Gas Production

States with Most 

OGI Facilities

Rural 

Land Area* 

(sq miles)

Urban/Urban 

Cluster Land 

Area 

(sq miles)

Percent 

Urban/Urban 

Cluster Land 

Area

Rural 

Population

Urban/Urban 

Cluster 

Population

Urban/Urban 

Cluster 

Population 

Denisty 

(per sq mile)

Rural Area 

Population 

Density 

(per sq mile)

Urban/Rural 

Population 

Density Ratio

Alaska 570,114 1,837 0.3% 215,675 411,257 224 0.4 592

California 148,059 7,900 5.1% 1,886,536 31,985,112 4,049 12.7 318

Colorado 102,467 1,251 1.2% 668,076 3,633,185 2,904 6.5 445

Kansas 80,961 854 1.0% 767,749 1,920,669 2,249 9.5 237

Louisiana 41,905 1,657 3.8% 1,223,311 3,245,665 1,959 29.2 67

Montana 145,296 256 0.2% 414,317 487,878 1,906 2.9 668

New Mexico 120,612 744 0.6% 455,545 1,363,501 1,833 3.8 485

North Dakota 68,849 127 0.2% 283,242 358,958 2,826 4.1 687

Oklahoma 67,526 1,141 1.7% 1,196,091 2,254,563 1,976 17.7 112

Texas 254,789 7,008 2.7% 3,647,539 17,204,281 2,455 14.3 171

Utah 81,478 666 0.8% 262,825 1,970,344 2,958 3.2 917

Wyoming 96,936 164 0.2% 172,438 321,344 1,959 1.8 1101

Totals 1,778,992 23,605 1.3% 11,193,344 65,156,757 2,760 6.3 439

Source for state-wide data: http://www.statemaster.com/graph/peo_pop_den-people-population-density

States > 2 million bbl/yr, 2/3 of US http://tonto.eia.doe.gov/state/state_energy_rankings.cfm?keyid=28&orderid=1

States > 300 billionbbl/yr, 3/4 of US http://tonto.eia.doe.gov/state/state_energy_rankings.cfm?keyid=29&orderid=1

Note: Estimated from star-sied and rural data. Source for rual data: http://www.demographia.com/db-usa-staterural.htm Source for state-wide data: http://www.statemaster.com/graph/peo_pop_den-people-population-density

Note: Estimated from star-sied and rural data. Source for rual data: http://www.demographia.com/db-usa-staterural.htm

A review of the types of RICE that are typically present at OGI is provided in Table 2, 

which provides EPA’s estimates and estimates provided by the oil and gas industry. 

Based on industry (OGI) estimates, more than 85% of all RICE are less than 500 HP, 

comprised primarily of four-stroke rich-burn (4SRB) and two-stroke lean-burn (2SLB). 

In the Section 4 of this white paper, consistent with this distribution of RICE, six 

examples of RICE have been selected to assess potential exposure to significant HAP 

emissions. (See Appendix B for assumptions used to develop OGI estimates.) 
Support for Rural RICE NESHAP 8 11/1/2009

Figure 1 Distribution of Urban Areas (Red) and Rural Areas (Yellow) According to 

the 2000 U.S. Census Source: http://www.census.gov/geo/www/ua/ua_2k.html
Support for Rural RICE NESHAP 9 11/1/2009

Table 2 Estimated Distribution of RICE for Major and Area Source Categories

RICE at Major Sources

Horse Power (HP) EPA Estimate in Rulemaking OGI Estimate

HP 

Range

Average 

HP

Number 

of 

Affected 

Existing 

SI 

Engines 2SLB 4SLB 4SRB 

Number 

of 

Existing 

SI 

Engines 2SLB 4SLB 4SRB 

25-50 37.5 55,409 5,973 25,521 22,806 6,926 762 0 6,164

50-100 75 15,055 1,623 6,934 6,197 1,882 207 0 1,675

100-175 138 27,553 2,970 12,691 11,341 3,444 379 0 3,065

175-300 238 7,662 826 3,529 3,154 958 105 0 852

300-500 400 4,249 458 1,957 1,749 531 58 27 446

500-600 550 N/A N/A N/A N/A 265 29 125 111

600-750 675 N/A N/A N/A N/A 89 10 42 37

>750 1000 N/A N/A N/A N/A 7,427 817 3,491 3,119

Major Total 109,928 11,850 50,633 45,246 21,523 2,368 3,684 15,471

RICE at Area Sources

Horse Power (HP) EPA Estimate in Rulemaking OGI Estimate

HP 

Range

Average 

HP

Number 

of 

Affected 

Existing 

SI 

Engines 2SLB 4SLB 4SRB 

Number 

of 

Existing 

SI 

Engines 2SLB 4SLB 4SRB 

25-50 37.5 83,114 8,960 38,282 34,210 131,597 14,476 0 117,121

50-100 75 22,583 2,434 10,402 9,295 35,756 3,933 0 31,823

100-175 138 41,330 4,455 19,037 17,011 65,439 7,198 0 58,241

175-300 238 11,493 1,239 5,294 4,731 18,197 2,002 0 16,196

300-500 10,091 1,110 505 8,477

300-600 450 9,559 1,030 4,403 3,934

500-600 5,044 555 2,371 2,118

600-750 675 1,071 115 493 441 1,696 187 797 712

>750 1000 11,141 1,201 5,132 4,586 11,141 1,226 5,236 4,679

Area Total 180,291 19,435 83,042 74,208 278,962 30,686 8,908 239,368

Total Engines 290,219 31,285 133,675 119,454 300,485 33,053 12,593 254,839

Diesel fueled RICE are most commonly utilized in Alaska and for emergency purposes. 

Due to available resources, diesel fueled RICE were not included in the analysis in 

Sections 4 & 5, but the same principles apply. 
Support for Rural RICE NESHAP 10 11/1/2009

4 ESTIMATION OF POTENTIAL SIGNIFICANT HAP EXPOSURE 

FROM RICE AT OIL AND GAS INDUSTRY FACILITIES

An analysis was conducted to evaluate the likelihood that emissions from RICE engines 

could result in HAP concentrations at levels of health concern. It is well-established that 

the HAP of most interest for natural gas combustion in RICE is formaldehyde, because 

this substance, although emitted in very small quantities, is classified as a probable 

human carcinogen. As described in detail in Appendix A, the potential for exposure to 

significant concentrations of formaldehyde was evaluated as follows:

1. Emissions and release parameters were estimated for a sample of OGI RICE,

ranging from 115 to 440 HP, represented by three sizes of two-stroke lean-burn, 

two sizes of four-stroke rich-burn, and one size of four-stroke lean-burn RICE. 

These example RICE were chosen as being representative of the population of 

OGI engine types represented in Table 2. Worst case formaldehyde emission data 

was used to calculate a conservative estimate of formaldehyde emissions.

2. Because ambient concentrations associated with emission sources are highly 

dependent on meteorological conditions, dispersion modeling was conducted for 

each example RICE by applying 145 meteorological data sets from stations 

distributed throughout the U.S. This resulted in a problematic assessment of how 

the average formaldehyde concentration would decrease with distance from each 

example RICE.

3. U.S. EPA guidance was applied to estimate the average concentration of 

formaldehyde that could be considered significant from a health risk perspective. 

For inhalation of carcinogens, the determination of whether a concentration level 

is considered significant is based on the following two factors: 

1) The level of individual lifetime cancer risk that is deemed by EPA to be 

significant

2) The Unit Risk Estimate (URE, cancer risk per microgram per cubic 

meter concentration associated with lifetime exposure). 

EPA has generally applied a range of one chance in ten thousand (1 x 10-4) to one 

chance in a million (1 x 10-6) as significant risk levels in various regulatory 

programs. EPA presently lists two widely varying URE’s for formaldehyde, and 

the determination of the most appropriate URE is under review. Applying the two 

UREs and the outer boundaries of the significant risk levels results in four 

possible formaldehyde concentrations that could be considered to be significant: 

0.077 μg/m3, 7.7 μg/m3, 180 μg/m3 and 18,000 μg/m3. For this assessment only 

the most conservative significant concentration (the lowest value) was 

quantitatively evaluated since the modeled concentrations did not exceed the 

higher three possible significant concentration levels.
Support for Rural RICE NESHAP 11 11/1/2009

4. For each example, the results of each of 145 runs (corresponding to the 145 

meteorological data sets) were compiled and the average formaldehyde 

concentration at various radial distances (ranging from 100m to 50 km) was 

computed. Based on these concentrations, the distance at which the average 

concentration is equal to the formaldehyde significance level (0.077 μg/m3) was 

determined. Based on the 145 runs, the likelihood that the significant 

concentration would exceed a given distance was determined. To focus on OGI 

RICE, this probability assessment was repeated based on the modeling results for 

the 41 meteorological stations located within the twelve states listed in Table 1.

 

5. The distances computed in Step 4 were then used to estimate the area around a 

RICE within which formaldehyde concentrations could be considered significant. 

The number of people who could reside within this area was then estimated by 

assuming that nobody is likely to reside within about 1⁄4 mile of a RICE and 

applying the average population per square mile listed in Table 1 to the rest of the 

significant concentration area. 

Findings of this assessment can be summarized as follows:

Dispersion modeling of example RICE for 145 meteorological data sets 

representative of the entire U.S. indicated that only the lowest formaldehyde 

significant risk concentration estimate of 0.077 μg/m3 would be exceeded at 

distances beyond 100 m. Thus, modeling indicates the potential for significant 

off-site risk only if the most conservative definition of significant risk (1 x 10-6) 

and for the higher URE are applied.

The average radius to significant formaldehyde concentrations (based on the 50th

percentile of the 145 meteorological data sets nationwide) range from about 100 

m to 390 m among the six examples of OGI RICE. The upper limit radius (based 

on one percentile distances) ranges from about 110 m to 440 m among the six 

examples of OGI RICE.

Applying the average rural population density of 6.3 person/square mile and a 

significance radius of 390 m, less than one person would be expected to reside 

within the area with significant formaldehyde concentrations for RICE located in 

rural areas. Given the low population density in rural areas and the OGI’s 

observation that people are seldom located near rural O&G facilities, the level of 

risk posed by RICE in rural areas is negligible.
Support for Rural RICE NESHAP 12 11/1/2009

 

5.0 RATIONALE FOR ESTABLISHING LESS RIGOROUS RICE 

NESHAP REQUIREMENTS FOR RURAL AREAS

5.1 Basis in the Clean Air Act and Existing Regulations 

The review and analyses conducted in this white paper supports the position that RICE 

located in rural areas do not pose significant health risk and the stringent and potentially 

costly controls of area sources in rural areas will not result in a health benefit. Therefore, 

requiring less stringent emissions standards in rural areas than in urban areas is entirely 

consistent with Section 112 of the Clean Air Act Amendments, which directs EPA to 

reduce risks associated with area sources in urban areas, where ambient exposure and risk 

are generally above established significance levels. As discussed below, there is 

substantial statutory basis and regulatory basis for establishing NESHAP for RICE with 

different requirements in rural and urban areas.

The precedent for differentiating urban and non-urban sources under NESHAP has been 

established in the National Emission Standards for Hazardous Air Pollutants for Source 

Categories from Oil and Natural Gas Production Facilities, promulgated in 2007 

72 Fed. Reg. 26 (January 3, 2007) 40 CFR 63, subpart HH (hereinafter, "ONG GACT

Rule"). In that rule, dehydration units located in non-urban areas are subject to less 

rigorous emission control requirements than sources in urban areas. In this recent 

rulemaking, the Agency recognized the need to set different standards for urban and rural 

sources for three key reasons: 

(1) Many such units are located in remote rural areas; 

(2) The location of the units, both now and in the future, is and will be associated with the 

location of a natural resource rather than the need to serve a population center; and 

(3) The units are located at sites that are typically not managed by a large staff and, in 

many cases, are completely remote and checked only periodically.

In this context, EPA judged that the limited health risk in rural areas from that set of 

sources did not justify the burden of unnecessary emission controls that would not 

materially reduce health risks in rural areas. Based on the present assessment it is evident 

that the same conclusion applies to the RICE NESHAP. These same three considerations 

apply to about the same degree to OGI RICE, many of which are located in the same 

remote areas. The vast majority of OGI engines are located at rural, unmanned sites that 

are visited only infrequently by operation and maintenance staff.

5.2 High Cost and Low Benefit of Add-On Controls for Area Sources 

For the purposes of differentiating between urban and rural areas, the definition used in 

NESHAP, Subpart HH (a.k.a. the ONG GACT Rule) of "UA plus offset and UC" 
Support for Rural RICE NESHAP 13 11/1/2009

(referred to as urban area in this white paper) is also appropriate for RICE NESHAP. 

For the ONG GACT Rule, Urbanized Areas with a population greater than 50,000 are 

expanded to also include any area within 2 miles from the boundary of an Urban Area, 

and an Urban Cluster has a population greater than 10,000. In the ONG GACT Rule, 

EPA applied MACT type controls only within urban areas. The result was a rule with a 

total annual cost of $883,000 to reduce emissions in an area that housed an estimated 80 

million people. If EPA extended these controls to rural areas (outside of the UA plus 

offset and UC), then the ONG GACT Rule would have an additional annual cost of $38 

million, but would have covered an area that only housed an estimated 12 million people. 

(see 72FR29)

The rationale for this segregation of requirements is not only based on the absence of 

significant exposure in rural areas but the high cost of unnecessary controls for remote 

OGI facilities. The high cost and low environmental benefit make after-treatment 

controls such as oxidation catalysts for rural RICE unreasonable. Table 3 provides a 

similar examination of the cost and population covered for the proposed RICE rule as 

was calculated for the ONG GACT. The result of applying MACT-type controls to urban 

areas results in a total annual estimated cost of $288 million and will reduce emission in 

an area housing 11 million people. Extending the MACT type controls to rural areas 

would cost an additional $2,588 million, while only 251,000 people live in the area.

This indicates that cost estimates used by EPA severely underestimate the beyond-theMACT-floor cost per ton reduction for area sources. Taken together, the control costs for 

applying NSCRs or oxidation catalysts to sources in rural areas, without any real 

environmental benefit, are unreasonable and exceeds EPA's estimate by up to several

orders of magnitude. Therefore, EPA should follow the ONG GACT Rule in deciding 

that costly control in rural areas is unwarranted. EPA concluded that controls in rural 

areas were not warranted because, "This analysis showed that the overall cost of 

controlling units outside [urban areas] was much higher for a lower population." (72 

FedReg 29.) The same conclusion is appropriate for RICE.
Support for Rural RICE NESHAP 14 11/1/2009

Table 3 HAP Cost Effectiveness Calculations Based on Estimated Distribution of 

RICE Type and Horsepower

Rural 

Engine 

Count

Total 

Annual 

Cost 

($/engine)

Total TCC 

($Million)

Total 

Population 

Affected

Rural 

Engine 

Coung

Total 

Capital 

Cost 

($/engine)

Total TCC 

($Million)

Total 

Population 

Affected

25-50 14,476 13,028 $23,992 $312.57 13,028 1,448 $23,992 $34.73 579,028

50-100 3,933 3,540 $23,992 $84.93 3,540 393 $23,992 $9.44 157,328

100-175 7,198 6,478 $23,992 $155.43 6,478 720 $23,992 $17.27 287,932

175-300 2,002 1,802 $23,992 $43.22 1,802 200 $23,992 $4.80 80,068

300-500 1,110 999 $25,871 $25.85 999 111 $25,871 $2.87 44,402

500-600 555 499 $32,971 $16.46 499 55 $32,971 $1.83 22,192

600-750 187 168 $32,971 $5.54 168 19 $32,971 $0.62 7,461

>750 1,226 1,103 $32,971 $36.37 1,103 123 $32,971 $4.04 49,020

2SLB 

Total 30,686 27,617 $680.36 27,617 3,069 $75.60 1,227,432

25-50 0 0 $6,365 $0.00 0 0 $6,365 $0.00 0

50-100 0 0 $6,365 $0.00 0 0 $6,365 $0.00 0

100-175 0 0 $6,365 $0.00 0 0 $6,365 $0.00 0

175-300 0 0 $6,365 $0.00 0 0 $6,365 $0.00 0

300-500 505 454 $6,365 $2.89 454 50 $6,365 $0.32 20,183

500-600 2,371 2,133 $6,365 $13.58 2,133 237 $6,365 $1.51 94,822

600-750 797 717 $6,365 $4.57 717 80 $6,365 $0.51 31,880

>750 5,236 4,713 $6,365 $30.00 4,713 524 $6,365 $3.33 209,451

4SLB 

Total 8,908 8,018 $51.03 8,018 891 $5.67 356,335

25-50 117,121 105,409 $6,019 $634.46 105,409 11,712 $6,019 $70.50 4,684,859

50-100 31,823 28,641 $6,019 $172.39 28,641 3,182 $6,019 $19.15 1,272,928

100-175 58,241 52,417 $11,636 $609.92 52,417 5,824 $11,636 $67.77 2,329,634

175-300 16,196 14,576 $13,757 $200.52 14,576 1,620 $13,757 $22.28 647,822

300-500 8,477 7,629 $16,638 $126.93 7,629 848 $16,638 $14.10 339,070

500-600 2,118 1,907 $16,638 $31.72 1,907 212 $16,638 $3.52 84,734

600-750 712 641 $16,638 $10.66 641 71 $16,638 $1.18 28,489

>750 4,679 4,211 $16,638 $70.07 4,211 468 $16,638 $7.79 187,169

4SRB 

Total 239,368 215,431 $1,856.68 215,431 23,937 $206.30 9,574,706

Grand 

Total 278,962 251,066 $2,588.07 251,066 27,896 $287.56 11,158,473

4 Stroke Rich Burn Engines

HP Range Engine 

Count

Rural Engine Data Urband Engine Data

2 Stroke Lean Burn Engines

4 Stroke Lean Burn Engines
Support for Rural RICE NESHAP 15 11/1/2009

5.3 Alternative Standards Should be Considered for RICE Located in 

Rural Areas 

Based on the considerations outlined above, EPA should classify rural RICE as a separate 

subcategory of engines that utilize Management Practices such as maintenance work 

practices (as proposed for other engines not equipped with catalyst) in lieu of catalysts. 

Following the Clean Air Act, EPA should consider the high cost of control and low 

emission reduction benefit for area sources in rural locations. Alternative requirements 

for the various natural gas-fired engine subcategories at area sources in rural locations

should consider factors such as: whether the facility is manned; emissions (e.g., engine 

size); feasibility of control and cost of control,. In addition, when considering the 

potential benefits of HAP emission control for natural gas-fired engines, it would be 

appropriate for EPA to consider the implications of the ongoing review of the 

formaldehyde URE. 

The establishment of alternative, more flexible requirements for area sources are firmly 

established in Section 112(d)(5) of the Clean Air Act, where Generally Available Control 

Technologies (GACT) may be applied, rather than Maximum Achievable Control 

Technology (MACT).

Alternative standard for area sources.- With respect only to categories and 

subcategories of area sources listed pursuant to subsection (c), the

Administrator may, in lieu of the authorities provided in paragraph (2) 

and subsection (f), elect to promulgate standards or requirements applicable 

to sources in such categories or subcategories which provide for the use of

generally available control technologies or management practices by such 

sources to reduce emissions of hazardous air pollutants. (underline added for 

emphasis)

Based on the exposure assessment and cost-effectiveness calculations developed in this 

white paper, it is apparent that RICE in rural areas qualify for this consideration. The risk 

analysis conducted for this white paper indicates that the average range of risk due to 

formaldehyde emissions is very limited and perhaps non-existent if the updated EPA 

OAQPS URE is more realistic than the historical IRIS value. Therefore, staying

consistent with the Clean Air Act Amendments, methods of control should be established 

that are commensurate with the potential risk and associated limited benefits. 

In this light it is important to point out that aspects of the required controls under the 

proposed NESHAP are not practical to apply to remote, rural settings. For example,

because the sites often have no electrical supply and are typically unmanned, catalyst 

systems which require routine attention are not viable. Therefore, management practices 

that help ensure efficient operation are much more viable in than emission limits for 

RICE in rural areas.
Support for Rural RICE NESHAP 16 11/1/2009

REFERENCES

RTI, 2009. Regulatory Impact Analysis (RIA) for Existing Stationary Reciprocating 

Internal combustion Engines (RICE) NESHAP, prepared for U.S. EPA OAQPS by RTI 

International, February (htttp://www.epa.gov/ttn/atw/rice/ricepg.html)

Innovative Environmental Solutions, 2009. Reciprocating Internal Combustion Engine 

National Emission Standard for Hazardous Air Pollutants (RICE NESHAP) Proposed 

Revisions – Emission Control Dost Analysis for “Above the Floor” Emission Controls 

for Natural-Gas Fired RICE, prepared for INGAA and API, October.
Support for Rural RICE NESHAP 17 11/1/2009

APPENDIX A

ESTIMATE OF SIGNIFICANT LONG-TERM FORMALDEHYDE 

CONCENTRATIONS ASSOCIATED WITH TYPICAL RICE

The region surrounding a RICE to which HAP emissions could contribute significantly to 

health risk is limited due to its relatively low rate of emissions. The following analysis 

demonstrates that for typical RICE located in rural areas, there is virtually no risk to the 

general population.

The off-site exposure associated with HAP emissions from low-level point sources, such 

as RICE, depend primarily on the following factors:

Emissions: HAP emission rate of HAPs;

Release characteristic: stack height and inner diameter, exhaust temperature and 

flow rate and presence of nearby structures; and

Dispersion meteorology: wind speed, wind direction, temperature, and 

atmospheric turbulence intensity.

It is well-established that the HAP of most interest for natural gas combustion in RICE is 

formaldehyde, because this substance, although emitted in very small quantities, is 

classified as a probable human carcinogen. Formaldehyde emissions and stack 

parameters are listed in Table A-1 for six natural gas engines (three sizes of two-stroke 

lean-burn, two sizes of four-stroke rich-burn, and one size of four-stroke lean-burn). 

Because these emissions are primarily based on vendor guarantees, they are likely to 

represent upper limits. 

Table A-1 Emissions and Stack Parameters for Typical RICE

Manufacturer Model OEM HP Type

Formaldehyde 

Emission Rate

(g/sec)

Stack 

Diameter

 (m)

Stack 

Height

(m)

Exit 

Velocity

(m/sec)

Exit 

Temperature

(degrees K)

Ajax DPC - 115 115 2SLB 0.021 0.30 4.83 5.63 500

Ajax DPC - 280 280 2SLB 0.036 0.30 4.83 13.13 516

Cooper Bessemer GMB-4VTF 440 2SLB 0.022 0.20 4.57 28.71 533

Caterpillar G3306NA 145 4SRB 0.011 0.10 2.44 23.81 783

Waukesha F1197G 225 4SRB 0.003 0.10 4.57 25.00 811

Waukesha F18GL 400 4SLB 0.024 0.30 4.57 30.00 811

Data Source: vendor specifications

These six typical natural gas fired spark ignited engines were evaluated to estimate the 

potential area of significant HAP exposure. For this assessment, it is conservatively 

assumed that a person continuously resides at a fixed location for 70 years and that RICE 

operate continuously at 100% load over that extended period. Annual average 

formaldehyde concentrations were modeled and the lifetime inhalation cancer risk was 

then calculated by multiplying the modeled annual average formaldehyde concentrations 

by the EPA URE. A URE represents an upper limit estimate of the lifetime inhalation 
Support for Rural RICE NESHAP 18 11/1/2009

cancer risk assuming continuous exposure to a modeled concentration for 70 years, for a 

person with a 70 kg body weight and an inhalation rate of 20 m3/day. In extrapolating 

toxicology data in developing a URE, EPA’s objective is apply conservative assumptions 

such that application of the URE is likely to overestimate the actual risks associated with

exposure to a specified long-term average concentration. The assessment of inhalation 

risk due to formaldehyde is complicated in that EPA has publicly listed two URE values 

for formaldehyde. EPA’s Prioritized Chronic Dose-Response Values (6/12/2007)

compiled by the Office of Air Quality Planning and Standards (OAQPS) lists a value of 

5.5 x 10-9(m3/μg). EPA’s Integrated Risk Information System (IRIS) lists a value of 1.3 

x 10-5(m3/μg). The IRIS value was established in 1991 and is presently under review. 

EPA has labeled the much less stringent formaldehyde URE than the value published 

IRIS as the “best science” in its most recent National Air Toxics Assessment.

The Residual Risk provisions of the Clean Air Act set a de minimis incremental lifetime 

cancer risk of 1 x 10-6. In evaluating the significance of incremental individual lifetime 

cancer risk, EPA generally applies a range of 1 x 10-6to 1 x 10-4. For instance, EPA’s 

Superfund (CERCLA) site cleanup program sets a goal of one-in-a-million (1 x 10

-6

) 

excess risk of cancer as the point of departure. If that goal cannot be met, after 

consideration of nine balancing criteria, cancer incidence risk levels of no more than 

about one in ten thousand (1 x 10-4) are considered acceptable. 

For this assessment, two conservative assumptions were used to estimate the modeled 

annual average formaldehyde concentration that would correspond to significant cancer 

risk; EPA’s lower-limit risk threshold of 1 x 10-6and EPA’s upper-limit URE of 

1.3 x 10-5(m3/μg). Taken together these conservative factors indicate that an annual 

average formaldehyde concentration exceeding 0.077 μg/m3could be significant. If the 

OAQPS URE is applied with the 1 x 10-6risk level, the significant formaldehyde 

concentration would be 180 μg/m3. If the 1 x 10-4risk level were applied, the significant 

formaldehyde concentrations would increase by a factor of 100 to 7.7 μg/m3and 18,000 

7.7 μg/m3, respectively for the IRIS and OAQPS URE. 

As noted, the potential off-site risk of typical OGI RICE depends on the dispersion 

meteorology. To evaluate this variability, dispersion modeling has been conducted 

applying AERMOD in the rural mode for each of the six sets of RICE source parameters 

and the 145 sequential one-year meteorological data sets (each representing a single year 

for over 145 locations). This meteorological data has been downloaded from EPA’s 

Human Exposure Model (HEM3) system website. Figure A-1 identifies the 

meteorological data surface stations in the contiguous United States and Alaska that were 

used in this assessment (two stations, one in Hawaii and one in Puerto Rico were used but 

are not shown in the map). The analysis conservatively assumed that formaldehyde 

disperses as a passive tracer and does not degrade in the atmosphere. However, when 

released formaldehyde is removed from the atmosphere by direct photolysis and 

oxidation by photochemically produced hydroxyl radicals, the estimated formaldehyde 

half-life ranges from about 1.6 – 19 hours. By not accounting for this degradation, the 

analysis is conservative for distances beyond a few kilometers, making it less likely that 

significant concentrations from multiple sources would overlap.
Support for Rural RICE NESHAP 19 11/1/2009

Figure A-1 Location of Meteorological Stations Provided by EPA’s HEM-3 

Database 
Support for Rural RICE NESHAP 20 11/1/2009

Rings of 36 radial receptors, spaced at 10o, were used to estimate annual average 

concentrations starting from 100 m to 50 km from the sources. The average formaldehyde 

concentration as a function of distance was then computed for each meteorological data 

set. 

For this conservative analysis, applying the IRIS URE and 1 x 10-6risk level, significant 

average formaldehyde concentrations extend to beyond 100 m for all of the example 

RICE. The average distance to which significant formaldehyde concentrations extended 

was computed for each RICE source for all 145 meteorological data sets and also for the 

meteorological stations located in the 12 states with the most OGI activity. The results 

of the assessment are shown in Figure A-2 and Figure A-3, respectively. These are 

probability plots for each RICE indicates the likelihood, based on meteorological 

variability, that the significant formaldehyde associated with each engine would exceed 

the significance level at given distance. 

0 %

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

100 200 300 400 500 Probablity (%) Based on 145 Meteroological Data Sets

Distance (m)

Likelihood of Significant Annual Formaldehyde 

Concentrations for Example RICE 

225 HP 4-Cycle RICE

400 HP 4-Cycle RICE

145 HP 4-Cycle RICE

440 HP 2-Cycle RICE

115 HP 2-Cycle RICE

280 HP 2-Cycle RICE

Figure A-2 Probability Plot Based on All Meteorological Stations of Significant 

Formaldehyde Concentrations based on the IRIS URE and the 1 x 10-6 Risk Level 
Support for Rural RICE NESHAP 21 11/1/2009

0 %

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

100 200 300 400 500 Probablity (%) Based on 41 Meteroological Data Sets

Distance (m)

Likelihood of Significant Annual Formaldehyde Concentrations

for Example RICE in the 12 States with Highest Oil and Gas Production

225 HP 4-Cycle RICE

400 HP 4-Cycle RICE

145 HP 4-Cycle RICE

440 HP 2-Cycle RICE

115 HP 2-Cycle RICE

280 HP 2-Cycle RICE

Figure A-3 Probability Plot of Significant Formaldehyde Concentrations based on 

the IRIS URE and the 1 x 10-6 Risk Level and 41 Meteorological Stations in the 12 

Oil and Gas Producing States (see figure

From these figures it can be deduced that it is not possible for a significant formaldehyde 

concentration level for a typical RICE to exceed 500 m. The typical range (at the 50th

percentile level) would be from about 100 m to 400 m. When the OAQPS URE and/or 

the 1 x 10-4risk level is considered significant, formaldehyde concentrations do not 

extend to the closest model receptor (100 m) for any meteorological data set. 

The number of people potentially exposed to significant formaldehyde concentrations can 

be estimated by combining the modeling results with the population density data. For 

this assessment, it is assumed that the area within 1⁄4 mile of a RICE is uninhabited and 

that the population density in the surrounding area is representative of the 12 oil and gas 

states. Given a significant formaldehyde distance of 390 m corresponding to the 50th

percentile value for the 280 HP two-stroke RICE in Figure A-3. The potential off-site 

exposure area within this radius would be 0.14 square mile. Multiplying this area by the 

average population density 6.3 persons per square mile for rural areas, the result is an 

estimate that 0.87 of a person could be exposed significant formaldehyde concentrations. 

In typical urban areas with a population density of about 2,760 persons per square mile, 

375 persons could be exposed to significant formaldehyde concentrations. Because this 

estimate is based on the lowest range of significance and the higher IRIS URE, these 

potential exposure estimates are conservatively high. 
Support for Rural RICE NESHAP 22 11/1/2009

The conclusion that can be drawn from this analysis is that in rural locations where most 

oil and gas production exists, it is unlikely that even a single person lives sufficiently 

close to a RICE to be exposed to significant formaldehyde concentrations. In addition, 

given the OGI’s observation, OGI facilities are typically very remote. People seldom 

reside within several hundred meters of an oil and gas facility. It can be concluded that 

the level of risk posed by OGI RICE in rural areas is negligible. 
Support for Rural RICE NESHAP 23 11/1/2009

APPENDIX B

TABLE 2 & 3 ASSUMPTIONS

Table 2 - Estimates of the Distribution of RICE at Located Major Sources and 

Areas Sources

a. EPA estimated SI engine counts are from Melanie King’s document (titled RICE NESHAP 

population assumptions.doc) supplied to Jeff Adams by email on May 19, 2009. These 

engine counts included all engines except engines >500 HP at MACT major sources.

b. EPA calculated the numbers in this document using the assumptions outlined in Docket 

Document # EPA-HQ-OAR-2005-0030-0014.

c. The 300 – 600 HP range for area sources could be divided into 300 – 500 HP and 500 –

600 HP ranges by assuming those ranges were at the same ratio as the same 

horsepower ranges for major sources. 

d. Engine counts for engines >500 HP at MACT major sites was calculated by using area 

source engine counts for engines >500 HP, dividing them by 60% to get total engines, 

and multiplying by 40% to get engines at major sources;

e. Assumed that 90% of O&G RICE are located in rural areas (as defined by NESHAP, 

Subpart HH) as a best estimate. This assumption is based on engine inventories of 

several operators. As a comparison, the Subpart HH, GACT rule estimated that 2172 out 

of 2222 total dehydrators were in rural areas, which equates to 97.75% of dehydrators in 

rural areas. O&G engines are located within the same O&G fields as the dehydrators. 

However, the recently developed Barnet Shale Field is predominately in the Dallas/Fort 

Worth Urban area. This would increase the percentage of urban area engines 

significantly. 

f. EPA’s estimate that 40% of RICE are located at major sources of HAPs (with 60% at 

area sources) is probably accurate for engines over 750 HP. It should be noted that 

many of these major sources have become synthetic minor sources by the installation of 

catalytic controls with federally enforceable permit conditions. Although these are 

technically not major source engines, they have been left in this category because 

catalytic controls have been installed, thus the cost of controls have already been 

incurred. 

g. Very few engines less than 750 HP are located at major sources of HAPs, because these 

sites typically have electricity that can support these smaller energy demands and electric 

motors are preferred by industry over RICE. The RICE that are at major sources and 

under 750 HP are predominately emergency RICE. Assumed that 95% of RICE under 

750 HP are at area sources. 

h. EPA’s estimate that SI engines are 11% - 2SLB, 47% - 4SLB, and 42% - 4SRB is a good 

estimate for engines over 500 HP. For engines under 500 HP, there are no known 4SLB 

engines used in the O&G industry. RICE <500 HP are predominately 4SRB with a few 

2SLB. Our best estimate for <500 HP is 89% - 4SRB and 11% - 2SLB.

Table 3 - HAP Cost Effectiveness Calculations Based on Estimated Distribution of 

RICE Type and Horsepower

Total Capital Cost and Total Annual Cost are from the tables in the Appendices to “Reciprocating Internal 

Combustion Engine National Emission Standard for Hazardous Air Pollutants (RICE NESHAP) Proposed 

Revisions – Emission Control Cost Analysis Background for “Above the Floor” Emission Controls for 

Natural Gas-Fired RICE”

a) 2 Stroke Lean Burn engine Total Annual Cost comes from Appendix O, Row 69

b) 4 Stroke Lean Burn engine Total Annual Cost comes from Appendix L, Row 31

c) 4 Stroke Rich Burn engine Total Annual Cost comes from Appendix 1 Rows 37
Support for Rural RICE NESHAP 24 11/1/2009

d) Population per engine is based on one person/engine for rural areas and 400 people/engine for 

urban areas as discussed at the end of Section 4.