Report to Congressional Requesters

United States Government Accountability Office

GAO

August 2005 AIR POLLUTION

Estimated Emissions 

from Two New 

Mexicali Power Plants 

Are Low, but Health 

Impacts Are Unknown

GAO-05-823 
What GAO Found

United States Government Accountability Office

Why GAO Did This Study

Highlights

Accountability Integrity Reliability

www.gao.gov/cgi-bin/getrpt?GAO-05-823.

To view the full product, including the scope 

and methodology, click on the link above. 

For more information, contact John B. 

Stephenson at (202) 512-3841 or 

stephensonj@gao.gov. 

Highlights of GAO-05-823, a report to 

congressional requesters 

August 2005

AIR POLLUTION

Estimated Emissions from Two New 

Mexicali Power Plants Are Low, but 

Health Impacts Are Unknown 

The estimated emissions from the Sempra and Intergen power plants near 

Mexicali are comparable with similar plants recently permitted in California 

and are low relative to emissions from the primary sources of pollution in 

Imperial County, California, which are dust and vehicles. However, if the 

plants were located in Imperial County, they would be required to take steps 

to improve air quality by reducing emissions from other pollution sources in 

the region, such as paving dirt roads, because the county is not meeting 

certain U.S. air quality standards. 

Although emissions generated from the Sempra and Intergen plants may 

contribute to various adverse health impacts in Imperial County, the extent 

of such impacts is unknown. The Department of Energy (DOE) estimated 

that emissions from these plants may increase asthma hospitalizations by 

less than one per year. However, DOE did not quantify any other asthmarelated impacts, such as emergency room visits or increased use of 

medications, which, although less severe, are likely to occur more often. In 

addition, DOE did not determine whether increased emissions would cause 

other respiratory or cardiovascular problems and the impact of particulate 

matter on particularly susceptible populations. Finally, the potential health 

impacts associated with ozone could be greater than DOE estimated because 

some important data needed for modeling were not available. 

Existing laws and international agreements may not provide adequate 

mechanisms to address adverse health impacts resulting from power plant 

emissions. Policymakers could take some actions, such as requiring plants 

that seek to export electricity to the United States to use specified emission 

controls. While this action would have benefits, it would also have costs, 

such as possibly reducing energy supplies available to Southern California. 

Long-term policy options include the development of a binational pollution 

reduction program or a trust fund to provide grants and loans to support air 

quality improvement projects. However, substantial efforts on both sides of 

the U.S.-Mexico border would be required to establish the legal and 

management framework necessary for such programs to be effective. 

Map of the U.S.-Mexico Border Region Near the Sempra and Intergen Power Plants 

Imperial Valley

Source: GAO presentation of DOE data.

Salton Sea

United States

Mexico

El Centro

Calexico

Colorado River

Mexicali

Yuma

New River

Mexico

United States

Baja

California

Norte

Mexico

Location of Sempra and Intergen power plants

Power plants emit pollutants that 

have been linked to various 

negative health effects. In 2003, 

two new power plants, owned by 

Sempra Energy and Intergen, began 

operations 3 miles south of the 

U.S.-Mexico border near Imperial 

County, California. The county 

does not meet some federal and 

state air quality standards and may 

be further impacted by the 

emissions from these plants. 

Although these plants export most 

of the electricity they produce to 

the United States, they are not 

currently required to meet any U.S. 

or California emissions standards. 

GAO was asked to determine (1) 

how emissions from the two plants 

compare with emissions from 

recently permitted plants in 

California and emissions from 

sources in Imperial County, and 

what emissions standards they 

would be subject to if they were 

located in Imperial County; (2) the 

health impacts of emissions from 

the plants on Imperial County 

residents; and (3) options available 

to U.S. policymakers to ensure that 

emissions from these plants do not 

adversely affect the health of 

Imperial County residents. 

In commenting on a draft of this 

report, DOE disagreed with our 

characterization of the limitations 

of their assessment of the health 

impact of pollution from the 

Sempra and Intergen power plants. 

We believe we have portrayed the 

limitations of this assessment 

accurately. 
Page i GAO-05-823 Mexicali Power Plants 

Letter 1

Results in Brief 4 

Background 7 

Emissions from the Mexicali Plants Are Comparable to New Plants 

in California, but Offset Requirements Would Apply in Imperial 

County 12 

Emissions Generated by the Sempra and Intergen Power Plants 

May Contribute to Various Adverse Health Impacts in Imperial 

County, but the Extent of Such Impacts Is Unknown 23 

Policymakers Have Limited Options to Ensure That Emissions 

from the Sempra and Intergen Power Plants Do Not Adversely 

Affect the Health of Residents in Imperial County 29 

Conclusions 33 

Agency Comments and Our Evaluation 34 

Appendix I Objectives, Scope, and Methodology 37 

Appendix II Comments from the Department of Energy 41 

GAO Comments 44 

Appendix III GAO Contact and Staff Acknowledgments 46 

Tables 

Table 1: Key Power Plant Pollutants and Potential Health Impacts 12 

Table 2: Average Emissions from the Sempra and Intergen Power 

Plants Based on Third-party Testing 13 

Table 3: Comparison of Estimated Sempra and Intergen Plant 

Emissions and the Emission Limits of Recently Permitted 

Power Plants in California 14 

Table 4: Summary of Annual Emission Estimates for the Sempra 

and Intergen Power Plants Using Three Alternative 

Operating Assumptions (Tons per Year) 15 

Table 5: Estimated Annual Average Emissions in 2004 for Imperial 

County, California (Tons per Year) 19 

Contents 
Page ii GAO-05-823 Mexicali Power Plants 

Table 6: Comparison of NOX Emission Rates from the Sempra and 

Intergen Plants and Power Plants in Imperial County and 

Baja California 20 

Table 7: Potential Offsets in Imperial County, California, Identified 

by the Imperial County Air Pollution Control District 22 

Table 8: Imperial County Air Quality Compared with National 

Standard for PM10 (1997-2003) 27 

Figures 

Figure 1: Map of the Sempra and Intergen Power Plants and the 

Associated Transmission Lines 8 

Figure 2: Electrical Production at the Sempra and Intergen Power 

Plants 10 

Figure 3: Relative Emissions Contributions by Source Assuming 

the Sempra and Intergen Plants Operated at Maximum 

Allowable Emissions and All Emissions Reached Imperial 

County 18 

Figure 4: Pyramid of Potential Health Impacts for Asthmatics 25 
Page iii GAO-05-823 Mexicali Power Plants 

Abbreviations 

BLM Bureau of Land Management 

CO carbon monoxide 

DOE Department of Energy 

EPA Environmental Protection Agency 

NAAEC North American Agreement on Environmental Cooperation 

NAFTA North American Free Trade Agreement 

NOX nitrogen oxides 

NH3

 ammonia 

PM particulate matter 

PPM parts per million 

VOC volatile organic compounds 

This is a work of the U.S. government and is not subject to copyright protection in the 

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permission from GAO. However, because this work may contain copyrighted images or 

other material, permission from the copyright holder may be necessary if you wish to 

reproduce this material separately. 
Page 1 GAO-05-823 Mexicali Power Plants 

August 12, 2005 

The Honorable Hilda Solis 

Ranking Member 

Subcommittee on Environment and Hazardous Materials 

Committee on Energy and Commerce 

House of Representatives 

The Honorable Bob Filner 

House of Representatives 

In 2003, two new power plants owned by American-based corporations, 

Sempra Energy and Intergen, began operations 3 miles south of the U.S.-

Mexico border near Mexicali, Mexico, and Imperial County, California. 

These modern natural gas-fired plants use advanced pollution control 

technologies; nevertheless, they emit some pollutants such as nitrogen 

oxides and airborne particles, known as particulate matter. Some health 

studies have found that even the smallest incremental increase in the 

amount of particulate matter in the air corresponds with an increase in 

adverse health effects. 

The U.S.-Mexico border region is experiencing significant economic, 

industrial, and population growth, and that growth is expected to 

continue. Imperial County is one of the fastest growing counties in 

California and is expected to double in population by 2025. The county 

does not currently meet some federal and state air quality standards and 

has one of the highest asthma prevalence rates for children ages 1 through 

17 in the state. The increased demand for energy to meet the needs of the 

border region could lead to the construction of additional power plants, 

with the associated potential for increased air pollution and negative 

impacts on public health. 

In the United States, the Clean Air Act establishes the principal framework 

for federal, state, and local efforts to protect air quality. Under this act, the 

Environmental Protection Agency (EPA) establishes health-based air 

standards that the states must meet.1

 EPA has issued air quality standards 

 1

The standards are known as the National Ambient Air Quality Standards. 

United States Government Accountability Office

Washington, DC 20548 
Page 2 GAO-05-823 Mexicali Power Plants 

for six primary pollutants—carbon monoxide, lead, nitrogen dioxide,2

ozone, sulfur dioxide, and two categories of particulate matter (PM2.5 and 

PM10)

3

—that have been linked to a variety of health problems. Agencies 

collect data on the levels of these pollutants to determine if air quality is 

meeting the federal standards. Areas that do not meet these federal 

standards are designated as “nonattainment” areas and are, as a result, 

generally subject to more stringent emission control requirements. 

Imperial County is currently designated as a nonattainment area for PM10

and ozone.4

 New facilities being built within a nonattainment area that are 

expected to generate emissions above a certain threshold may be required 

to provide mitigation measures in the form of emission offsets. These 

offsets are designed to improve air quality in nonattainment areas by 

reducing emissions from other pollution sources in the region. They could 

include, for example, providing funds to update diesel engines or to pave 

dusty dirt roads. 

The Sempra power plant has a single power-generating unit with the 

capacity to produce a total of 650 megawatts of electricity per hour, all of 

which are designated for export to the United States.5

 The Intergen facility, 

which has a total capacity of 1,060 megawatts, is composed of two units: 

one that produces power exclusively for the United States and a second 

that exports up to one-third of its power to the U.S. market. Because the 

plants are located in Mexico, Mexican agencies have the exclusive 

authority to regulate the permitting and construction, as well as the 

emissions resulting from their operation. Mexico regulates the emissions 

of several pollutants from its power plants but requires natural gas-fired 

plants, such as Sempra and Intergen, to report emissions of only nitrogen 

oxides. In part, because of the advanced technology and control 

 2

Nitrogen dioxide (NO2), a common air pollutant, is one of a group of gases collectively 

known as nitrogen oxides, or NOX. The term NOX is used commonly in both the United 

States and Mexico to describe these gases, but NO2

 is sometimes monitored to report on 

the levels of all nitrogen oxide emissions in general. 

3

PM2.5 and PM10, also known as fine and coarse particulate matter, respectively, refer to the 

size of the airborne particles measured at the diameter (in micrometers). 

4

Ozone is formed at ground level by a chemical reaction of various air pollutants, including 

NOX, combined with sunlight. Ozone is a key ingredient in urban smog. 

5

Independent of such factors as time of day, time of year, and geographical location, in 

general, one megawatt of electricity is sufficient to meet the needs of 750 to 1,000 

households for 1 hour. 
Page 3 GAO-05-823 Mexicali Power Plants 

equipment these plants are using, their estimated emissions of NOX are 

significantly lower than the established emission limit in Mexico. 

The electricity generated by these plants is transmitted into the United 

States over electric transmission lines authorized by presidential permits 

issued by the Department of Energy’s (DOE) Office of Fossil Energy.6

These permits are required before electric transmission facilities are 

constructed, operated, maintained, or connected at the U.S.-Mexico 

border.7

 DOE is responsible for reviewing permit applications and 

conducting an environmental evaluation as part of this review. DOE issued 

a final environmental impact statement on the Sempra and Intergen 

transmission lines in December 2004 that included an assessment of the 

potential health impacts associated with emissions from the power plants. 

In this context, you asked us to determine (1) how emissions from the 

Sempra and Intergen power plants compare to emissions from recently 

permitted plants in California and emissions from sources in Imperial 

County, and what emissions standards the plants would be subject to if 

they were located in Imperial County; (2) the health impacts of emissions 

from the power plants on Imperial County residents; and (3) what options 

exist for U.S. policymakers to ensure that emissions from these power 

plants do not adversely affect the health of Imperial County residents. 

To address these objectives, we visited the Sempra and Intergen plants 

near Mexicali, Mexico; interviewed plant representatives, various federal, 

state, and local air quality officials, and other key stakeholders; and 

reviewed relevant documents and studies. To address how emissions from 

the Sempra and Intergen power plants compare to emissions from recently 

permitted plants in California, we used data from emissions tests 

conducted at the plants by third-party contractors. We did this because 

Mexico does not require the plants to report actual emissions of pollutants 

other than nitrogen oxides. We assessed the reliability of the data by (1) 

reviewing documentation of test objectives and quality control procedures 

provided by the third-party contractors who conducted the tests, (2) 

talking with Sempra and Intergen officials to determine the scope and 

 6

On April 13, 2005, the Secretary of Energy transferred the authority to grant presidential 

permits to the Office of Electricity and Energy Assurance. That office has subsequently 

been renamed the Office of Electricity Delivery and Energy Reliability. 

7

Exec. Order No. 10485, 18 Fed. Reg. 5397 (Sept. 9, 1953) amended by Exec. Order No. 

10238, 43 Fed. Reg. 4957 (Feb. 3, 1978). 
Page 4 GAO-05-823 Mexicali Power Plants 

generalizability of the tests, and (3) reviewing reports of actual NOX

emissions submitted to the Mexican government to verify consistency with 

the test results. We determined that the data were sufficiently reliable for 

the purposes of this report. We also obtained the permissible emission 

limits for comparable plants in California. Comparable plants were 

selected by identifying all natural gas power plants of similar size and 

specifications to the Sempra and Intergen plants that were permitted in 

California between 2000 and 2004. Because all California power plants are 

permitted on a case-by-case basis, emissions limits may vary with each 

project. Therefore, we used the entire range of emission limits for the 23 

plants that were identified during our selection process. To determine how 

the emissions from the Sempra and Intergen power plants compared to 

emissions from sources in Imperial County, we utilized the 2004 estimated 

annual average emissions inventory for the county developed by the 

California Air Resources Board, among other things. To address what 

emissions standards the plants would be subject to if they were located in 

Imperial County, we reviewed federal and California regulations for new 

power plants, interviewed EPA, state, and Imperial County air quality 

officials, and reviewed the emission limits and selected permitting 

conditions for power plants located in California. To identify the potential 

health impacts from plant emissions, we reviewed the health assessment 

methodology DOE used in its environmental impact statement, reviewed 

relevant studies, and met with health experts. To determine available 

policy options, we reviewed the Clean Air Act; environmental and trade 

agreements among the United States, Mexico, and Canada; and academic 

research. See appendix I for additional details on our scope and 

methodology. We conducted our work between September 2004 and 

August 2005 in accordance with generally accepted government auditing 

standards. 

The emissions from the Sempra and Intergen power plants near Mexicali 

are comparable to emissions from similar plants recently permitted in 

California and are low relative to emissions from the primary sources of 

pollution in Imperial County—dust and vehicles. However, if the plants 

were located in Imperial County, they would be required, among other 

things, to offset their emissions to help improve regional air quality. Our 

review of emissions test data obtained from Sempra and Intergen indicates 

that estimated emissions from these plants generally fall within a range of 

allowable emission limits identified from 23 plants of comparable size and 

specifications permitted in California between 2000 and 2004. Although 

the Sempra and Intergen plants will cause some increase in regional 

emissions of PM10 and nitrogen oxides (which contribute to ozone 

Results in Brief 
Page 5 GAO-05-823 Mexicali Power Plants 

formation), the primary sources contributing to PM10 and ozone in Imperial 

County are various forms of dust and motor vehicles. In addition, based on 

the amount of energy produced per pound of NOX emissions, these plants 

are cleaner than other major fuel-fired plants operating in Imperial County 

or the border region of Baja California, Mexico. Nevertheless, if the plants 

were located in Imperial County, they would be required to offset their 

emissions because the county is a nonattainment area for PM10 and ozone. 

More specifically, Imperial County air quality rules would require that the 

operators of each plant provide emissions offsets of at least 1.2 tons for 

every ton of emissions released by the plant that contribute to area 

nonattainment status. 

Emissions generated by the Sempra and Intergen power plants, like any 

other source of emissions, may contribute to adverse health impacts in 

Imperial County, but the full extent of such impacts is unknown. In its 

December 2004 final environmental impact statement, DOE estimated an 

increase in asthma hospitalizations in Imperial County of less than one per 

year as the result of increased emissions from the two plants. However, 

DOE did not fully assess the plants’ health impact because it did not 

quantify other asthma-related health impacts, such as emergency room 

visits, physician visits, and increased use of asthma medication, which, 

although less severe than hospitalization, are likely to occur more often, 

according to health experts. Also, the DOE study did not address the 

extent to which increased emissions of particulate matter would cause 

other adverse health impacts, such as other respiratory or cardiovascular 

conditions. In addition, DOE did not analyze the health impacts from 

increased power plant emissions on particularly susceptible populations, 

such as asthmatic children and low-income asthmatic adults. Imperial 

County is one of the poorest counties in California and low-income 

asthmatics adults are more susceptible to health problems, in part, 

because they have less access to health care. Finally, EPA officials are 

concerned about the accuracy of DOE’s modeling of estimated ozone 

increases for its final environmental impact statement because 

comprehensive data on some key factors, such as temperature and relative 

humidity, were not available. According to EPA officials, if the modeled 

estimates of increased ozone are not correct, the impacts on air quality 

from these two plants could be significant, resulting in some adverse 

health impacts that were not reported by DOE. 

Policymakers have limited options to ensure that emissions from the 

Sempra and Intergen power plants do not adversely affect the health of 

residents in Imperial County. The Sempra and Intergen plants are not 

subject to the federal Clean Air Act or the California Clean Air Act and, 
Page 6 GAO-05-823 Mexicali Power Plants 

therefore, are not required to offset their emissions. In addition, relevant 

agreements among the United States, Canada, and Mexico may not provide 

adequate mechanisms to address adverse health impacts resulting from 

emissions from these plants because they only require the countries to 

enforce their own environmental laws, not to implement specific pollution 

control requirements. Nevertheless, policymakers could take some 

actions. For example, the Congress could enact legislation restricting the 

importation of electricity generated by these plants if they do not meet 

certain U.S. emission and offset requirements. While this action would 

have benefits to air quality and health, it would also have costs, such as 

possibly reducing energy supplies available to Southern California. 

Similarly, DOE could modify its regulations to require permit applicants 

seeking to import electricity into the United States from Mexico to employ 

specified emission controls and obtain offsets. However, these two policy 

options may raise trade issues under the North American Free Trade 

Agreement. A third option would be to develop programs that provide 

economic incentives to reduce pollution in the U.S.-Mexico border region. 

Market-based programs, such as EPA’s program to reduce emissions that 

contribute to acid rain, have proven successful elsewhere in the United 

States in reducing emissions. Finally, another potential option is the 

development of a binational clean air trust fund that could provide grants 

and loans to support air quality improvement projects for cities along the 

U.S.-Mexican border. However, developing the legal and regulatory 

framework needed to create these binational programs is likely to require 

substantial time and effort. 

DOE commented on a draft of this report and generally disagreed with our 

characterization of the limitations of the health risk assessment done as a 

part of the environmental impact statement for the Sempra and Intergen 

power plants. Specifically, DOE did not agree with our assertion that it did 

not analyze all of the likely asthma-related and other health impacts of 

increased pollution from the power plants. However, DOE’s environmental 

impact statement analyzed adverse health effects only for asthma 

hospitalization, which is just one in a continuum of adverse health 

impacts. DOE also disagreed with our assertion that it did not analyze the 

potential health impacts of pollution from the Sempra and Intergen power 

plants on susceptible populations in Imperial County. Although DOE said 

that its environmental impact statement included children in its asthma 

hospitalization estimates, asthmatic children are not the only susceptible 

population and asthma hospitalization is not the only potential health 

impact. Finally, DOE did not agree that health impacts from ozone 

formation may be larger than it estimated in its final environmental impact 

statement because of limitations in its ozone modeling analysis. However, 
Page 7 GAO-05-823 Mexicali Power Plants 

in its comments on the final environmental impact statement, EPA said 

that it continues to support off-site mitigation efforts to ensure that there 

is no net increase in air pollution in Imperial County because of the ozone 

modeling limitations. For these reasons, we believe the report accurately 

characterizes the limitations of DOE’s health assessment and have made 

no changes to the report in response to these comments. DOE’s specific 

comments and our detailed responses are presented in appendix II of this 

report. 

The Sempra and Intergen plants are located in close proximity to each 

other near Mexicali, Mexico—an area 3 miles south of the U.S.-Mexican 

border and Imperial County, California (see fig. 1). Final permitting and 

construction for both of the plants and the associated transmission lines to 

the United States began in 2001, and commercial operations commenced 

in July 2003. Fuel for the plants is provided by a 145-mile cross-border 

natural gas pipeline built by Sempra Energy, which began operating in 

September 2002. 

Background 
Page 8 GAO-05-823 Mexicali Power Plants 

Figure 1: Map of the Sempra and Intergen Power Plants and the Associated Transmission Lines 

The Sempra plant, known as Termoelèctrica de Mexicali, consists of one 

natural gas-fired, combined-cycle power-generating unit with a total 

capacity of 650 megawatts. In this type of plant, electricity is produced by 

a combination of gas turbines and steam turbines. Heat from the gas 

turbine exhaust, which would otherwise be released to the atmosphere 

with exhaust gases, is captured and used by a heat recovery steam 

generator to produce steam, which in turn is used by the steam turbine to 

generate additional electricity. The Sempra plant operates with an export 

permit from the Mexican government and produces electricity exclusively 

for export to the United States. The facility is equipped with the latest 

pollution control technologies, including selective catalytic reduction 

Imperial Valley

Transmission lines

Salton Sea

United States

Mexico

El Centro

Calexico

Colorado River

Mexicali

Yuma

New River

Imperial Valley substation

Sempra and Intergen plants

Mexico

United States

Circled area indicates the location of the Imperial Valley substation (U.S.)

and Sempra, Intergen power plants (Mexico).

Source: GAO presentation of DOE data.
Page 9 GAO-05-823 Mexicali Power Plants 

systems to reduce NOX emissions and an oxidizing catalyst system to 

reduce carbon monoxide (CO) emissions.8

The Intergen plant, which consists of two natural gas-fired combined-cycle 

units (collectively known as the La Rosita Power Complex), has a total 

capacity of 1,060 megawatts. The first unit provides two-thirds of its 750 

megawatt capacity to Mexico, with the remaining one-third available for 

export to the United States. The second unit has a generating capacity of 

310 megawatts, all of which is designated for export to the U.S. market. 

(See fig. 2.) Originally, only the second unit was designed to include a 

selective catalytic reduction system, but as of April 7, 2005, all four of the 

combustion turbines within the two units have been equipped with these 

systems to control NOX emissions. 

 8

Selective catalytic reduction is a post-combustion cleaning technology whereby NOX

emissions chemically react with ammonia (NH3

) to produce ordinary nitrogen and water 

vapor. An oxidizing catalyst is similar in concept to catalytic converters used in 

automobiles. The catalyst, normally coated with a metal, such as platinum, is used to 

promote a chemical reaction with the oxygen present to convert carbon monoxide into 

carbon dioxide and water vapor. 
Page 10 GAO-05-823 Mexicali Power Plants 

Figure 2: Electrical Production at the Sempra and Intergen Power Plants 

Although no U.S. emissions requirements apply to these plants, Sempra 

and Intergen required a presidential permit to construct and connect the 

new transmission lines needed at the U.S.-Mexican border to export 

electricity into the United States. Because of the similarities of the 

proposals submitted by the companies, DOE decided to consider them 

together in a single environmental assessment, required as part of the 

Indicates the exhaust heat which the steam turbine uses to produce more power

Power exported to the United States

Power designated for use in Mexico

Combustion turbines Combustion turbines

Combustion turbine

Steam turbine Steam turbine

Steam turbine

170

MW

170

MW

160

MW

160

MW

150

MW

160

MW

160

MW

310

MW

270

MW

Unit 1 Unit 1

Unit 2

Sempra Plant Intergen Plant

2/3 goes

to Mexico

Source: DOE.
Page 11 GAO-05-823 Mexicali Power Plants 

permitting process.9 In December 2001, DOE completed the environmental 

assessment and issued a finding of no significant impact and presidential 

permits for both of the proposed projects. Following these decisions, 

Sempra and Intergen constructed the transmission lines and began 

commercial operations. However, as a result of subsequent litigation, on 

July 8, 2003, the U.S. District Court for the Southern District of California 

instructed DOE to prepare a more comprehensive environmental review, 

which included an assessment of the health impacts from the power plants 

as part of its analysis. DOE’s environmental impact statement was issued 

in final form in December 2004. DOE found that the proposed power 

plants presented a low potential for environmental impacts and published 

a record of decision in the Federal Register on April 25, 2005, authorizing 

presidential permits to be granted for both transmission lines to the 

respective power plants as presently designed. 

The operation of any fuel-fired power plant results in a variety of air 

pollutants. However, because natural gas is a relatively clean fuel, the 

primary emissions of concern from these plants are generally limited to 

nitrogen oxides (which contribute to ozone formation); particulate matter; 

and, in some cases, carbon monoxide. Nitrogen oxide, or NOX, is the 

generic term for a group of highly reactive gases, all of which contain 

nitrogen and oxygen in varying amounts. Ground level ozone, another 

primary pollutant, is not emitted from the plants directly but is formed in 

the presence of sunlight by a chemical reaction between NOX and various 

air pollutants known as volatile organic compounds (VOC). Particulate 

matter refers to dust, dirt, soot, smoke, and liquid droplets directly emitted 

into the air by various sources. Secondary formation of PM can also take 

place by the combination of NOX and ammonia (NH3

).10 Carbon monoxide 

is a colorless and odorless gas that is formed when carbon in fuel is not 

burned completely. These four pollutants have been linked to a variety of 

negative health effects, including, but not limited to, aggravated asthma, 

reduced lung function and other respiratory illnesses, and aggravation of 

heart disease, as well as premature deaths (see table 1). While emissions 

of sulfur dioxide are also a significant concern at some power plants, the 

 9

Because the proposed lines traverse land managed by the U.S. Department of the Interior, 

Bureau of Land Management, they also participated in the environmental assessment. 

10For this reason, ammonia (NH3

) is often included in the review of potential impacts from 

power plants and is subject to emission limits as part of the permitting process conducted 

in California. NH3

 emissions, typically referred to as ammonia-slip, are released from power 

plants as a byproduct of selective catalytic reduction control technology. 
Page 12 GAO-05-823 Mexicali Power Plants 

use of natural gas at the Sempra and Intergen facilities greatly reduces 

sulfur dioxide emissions compared with other fuels such as coal or oil. For 

example, U.S. coal contains an average of 1.6 percent sulfur, and oil 

burned at electric utility power plants ranges from 0.5 percent to 1.4 

percent sulfur; comparatively, natural gas has less than 0.0005 percent 

sulfur. 

Table 1: Key Power Plant Pollutants and Potential Health Impacts 

Pollutant Potential health impact 

Nitrogen oxides • Can irritate the lungs, cause bronchitis and pneumonia, and 

lower resistance to respiratory infections 

• Contribute to the formation of ozone 

Ozone • Triggers a variety of health problems, including aggravated 

asthma, even at very low levels 

• Can cause permanent lung damage after long-term exposure 

• Can contribute to premature death 

Particulate matter • Can aggravate asthma 

• Can cause increases in respiratory problems like coughing and 

difficult or painful breathing 

• Can lead to chronic bronchitis or decreased lung function 

• Can contribute to premature death 

Carbon monoxide • Can cause harmful health effects by reducing oxygen delivery to 

the body’s organs (like the heart and brain) and tissues 

• Can cause chest pains in those with heart disease and other 

cardiovascular effects after repeated exposures 

• High levels can lead to vision problems, reduced ability to work 

or learn, reduced manual dexterity, and difficulty performing 

complex tasks. 

Sources: EPA and the Southwest Consortium for Environmental Research and Policy. 

The emissions from the Sempra and Intergen power plants in Mexicali are 

comparable to emissions from similar plants recently permitted in 

California and are low relative to emissions from the primary sources of 

pollution in Imperial County, which are various forms of dust and motor 

vehicles. However, if the plants were located in Imperial County, they 

would be required, among other things, to offset their emissions by 

reducing emissions from other pollution sources in the region. 

Emissions from the 

Mexicali Plants Are 

Comparable to New 

Plants in California, 

but Offset 

Requirements Would 

Apply in Imperial 

County 
Page 13 GAO-05-823 Mexicali Power Plants 

Power plants in Mexico are not required to report to federal agencies in 

the United States on actual emissions of key pollutants generated during 

plant operations. Therefore, we believe that the best data available to 

estimate emissions from the Sempra and Intergen power plants comes 

from emission performance tests conducted by independent third-party 

contractors hired by the power plants. The average emissions from the 

Sempra and Intergen plants based on the results of the third-party testing 

are presented in table 2. 

Table 2: Average Emissions from the Sempra and Intergen Power Plants Based on 

Third-party Testing 

 Sempra resultsa

 Intergen resultsb 

 Unit 1 Unit 1 Unit 2 

Pollutant

Turbine

 1

Turbine

 2

Turbine

 1 

Turbine 

2 

Turbine 

3 

Turbine 

1 

NOX 

(ppm) 2.33 2.08 15.33c13.37c 2.41 3.14

PM10

(lbs/hr) 12.80 11.86 7.73 3.18 3.09 7.10

CO (ppm) 0.00 0.00 0.71 1.24 0.86 0.73

NH3

(ppm) 0.45 0.41 d d 1.24 1.73

VOC 

(lbs/hr) e e 0.07 0.11 0.83 0.86

Legend: NOX = nitrogen oxides; ppm = parts per million; PM10 = particulate matter; lbs/hr = pounds per 

hour; CO = carbon monoxide; NH3 = ammonia; VOC = volatile organic compounds. 

Sources: Sempra and Intergen. 

a

These tests were conducted on June 4-6, 2003, and July 8-9, 2003. 

b

These tests were conducted between September 20 and September 28, 2004. 

c

Selective catalytic reduction systems were installed on these turbines in March and April 2005, after 

these tests had been completed. These turbines have been emitting NOX at a level below 2.5 ppm 

since the installation of the new equipment. 

d

NH3 is a by-product of selective catalytic reduction control technology and was not emitted by these 

turbines at the time the tests were conducted because the control technology had not yet been 

installed. 

e

VOC emissions were undetectable at the plant during these tests. 

We were not able to compare emissions data from the Sempra and 

Intergen plants with emissions data from an individual plant in Imperial 

County to determine whether the plants would likely meet emissions 

requirements because no similar natural gas-fired power plant has recently 

been permitted for construction in the county. Therefore, we evaluated the 

Estimated Emissions from 

the Sempra and Intergen 

Plants Are Comparable to 

Emissions from New 

California Plants 
Page 14 GAO-05-823 Mexicali Power Plants 

Sempra and Intergen data against a range of allowable emission limits 

from the 23 natural gas-fired power plants of similar size and 

specifications that were given permits to operate elsewhere in California 

by the California Energy Commission between 2000 and 2004. These 23 

plants are among the cleanest fuel-fired plants in the United States. We 

found that the levels of emissions for major pollutants (NOX, PM10, CO, 

NH3

, and VOC) from the Sempra and Intergen plants are generally 

comparable to the range of emissions limits for the recently permitted 

California plants (see table 3). 

Table 3: Comparison of Estimated Sempra and Intergen Plant Emissions and the 

Emission Limits of Recently Permitted Power Plants in California 

Legend: NOX = nitrogen oxides; ppm = parts per million; PM10 = particulate matter; lbs/hr = pounds per 

hour; CO = carbon monoxide; NH3 = ammonia; VOC = volatile organic compounds. 

Sources: GAO analysis of data from the California Energy Commission, Sempra, and Intergen. 

a

The range of emission limits is based on best available control technology requirements for 23 

similar plants permitted in CA between 2000 and 2004. Although these limits are based primarily on 

the use of modern emissions control equipment, best available control technology is determined on a 

case-by-case basis and may take into consideration factors such as potential economic impacts, as 

well as design or operational standards. 

b

Sempra and Intergen emissions data are based on the average of emissions from the individual 

turbines recorded during testing. 

c

ppm for the California plants are based on a 1 or 3-hour average, depending on the testing method 

required by the local air pollution control agency. NH3 levels are computed based on a 1, 3, or 24-

hour average. 

d

NH3, or ammonia, is a by-product of selective catalytic reduction control technology and was not 

being emitted at this unit at the time of the tests because the control technology had not yet been 

installed. 

e

VOC emissions were undetectable during tests conducted at the plant. 

As shown in table 3, the average NOX emissions from the Intergen power 

plant were the only emissions that exceeded the range of emissions from 

recently permitted plants in California. This was the case, in part, because 

the plant was not originally designed to meet California requirements. 

However, as of April 7, 2005, all combustion turbines at the Intergen plant 

Pollutant 

Range of CA 

emission 

limitsa

Sempra 

averageb

Intergen averageb 

(US export)

Intergen average 

(Mexico)

NOX (ppm)c 2.0 – 2.5 2.2 2.8 14.4

PM10 (lbs/hr) 3.0 – 18.5 12.3 5.1 5.5

CO (ppm) 2.0 – 10.0 0.0 0.8 1.0

NH3 (ppm) 5.0 – 10.0 0.4 1.5 d 

VOC (lbs/hr) 1.6 – 6.6 e 0.8 0.1
Page 15 GAO-05-823 Mexicali Power Plants 

had been equipped with the selective catalytic reduction control 

technology for nitrogen oxide that is common in the newer California 

plants. With the exception of one turbine, which will continue to operate 

at a maximum NOX limit of 3.5 ppm, all other turbines are expected to emit 

NOX at a level below 2.5 ppm. According to Intergen plant officials, the last 

two turbines to be equipped with selective catalytic reduction systems 

have been meeting these levels since the systems became operational in 

March and April 2005, respectively. Data provided by plant officials, based 

on continuous monitoring of all emissions from these turbines over a 1 

week period, also indicate that both turbines are achieving the expected 

NOX reductions. 

One way to assess the environmental impact of emissions from power 

plants is to examine the tons of pollutants they emit on an annual basis. 

The third-party performance tests discussed above provide the best 

available data to estimate annual emissions likely to occur during actual 

operations at the Sempra and Intergen plants because the data are based 

on observations of the actual equipment in operation. Other options for 

estimating annual emissions from these plants include using (1) the 

maximum allowable emissions levels for similar plants in California and 

(2) the emissions estimates that DOE developed during its environmental 

impact assessment of the Sempra and Intergen plants. Table 4 presents 

annual emissions estimates based on each of these three alternative 

operating assumptions. 

Table 4: Summary of Annual Emission Estimates for the Sempra and Intergen 

Power Plants Using Three Alternative Operating Assumptions (Tons per Year) 

Source: GAO analysis of data from Sempra, Intergen, the California Energy Commission, and DOE. 

Under the first scenario, annual emissions levels were estimated using the 

values determined by third-party contractors during turbine performance 

tests at the Sempra and Intergen plants. For the two Intergen turbines that 

Estimated Annual 

Emissions from the 

Sempra and Intergen 

Plants Are Low Relative to 

Emissions from Sources in 

Imperial County 

Pollutant

Third-party 

testing

Maximum allowable 

emissions levels in 

California 

DOE’s environmental 

impact statement

NOX 374 610 610

PM10 200 352 1,210

CO 919 1,897 3,089

NH3 86 518 646

VOC 57 105 1,026
Page 16 GAO-05-823 Mexicali Power Plants 

did not have selective catalytic reduction systems installed when the 

testing was conducted, we estimated annual NOX emissions using the 

testing values recorded for the similar turbine that was operating with 

such equipment. We did so because these two turbines are now equipped 

with selective catalytic reduction systems and their future emissions are 

likely to be similar to those from the turbine that was using this 

technology during the tests. These estimates do not take into account 

start-up and shutdown operations of the plant, which may contribute to 

increased plant emissions for approximately 1 to 2 hours. However, the 

total annual estimate is based on the conservative assumption that the 

plants are operating at maximum emission levels, 24 hours a day, 365 days 

a year. The actual operation of the plants, and the resulting emissions, 

would be less than this because of scheduled maintenance, forced 

outages, and varying electrical demand in California. 

The second estimates of annual emissions were based on maximum 

allowable emissions determined during the permitting process for similar 

California plants. These maximum allowable emissions are higher than the 

estimates based on third-party testing data. California grants permits to 

construct power plants on a case-by-case basis. As a condition of receiving 

a permit, the state places limits on emissions of individual pollutants. 

These limits are based on the use of best available control technology and 

take into consideration energy, environmental, and economic impacts. 

Under this estimating scenario, the annual estimates also account for short 

term variations in emissions levels that may occur during start-up and 

shutdown operations and are based on the conservative assumption that 

the plants are operating at maximum emissions levels, 24 hours a day, and 

365 days a year. 

The final, and highest, emissions estimates are based on the values DOE 

used in its environmental impact statement. DOE’s estimates are based on 

either the maximum emissions allowable by permit from the Mexican 

government or the vendor guarantee limits, which are the maximum 

emissions levels specified by the manufacturer that a piece of equipment is 

likely to produce. These values tend to be much higher than the levels that 

typically occur during normal power plant operations. For example, the 

vendor guarantee limit of PM10 for turbines at the Intergen plant is 52.3 

pounds per hour. However, the actual emissions of PM10 at plants using 

similar equipment are typically below 10 pounds per hour. In addition, 

DOE’s estimates also assume that the Sempra and Intergen plants are 

operating at these levels 100 percent of the time, 365 days per year. 

Averaged on an annual basis, these estimates are likely to be significantly 

higher than the actual emissions resulting from operations at these plants. 
Page 17 GAO-05-823 Mexicali Power Plants 

According to DOE’s environmental impact statement, emissions from the 

Sempra and Intergen power plant would result in increases of ambient 

concentrations of NOX, PM10, and CO in Imperial County. However, it is 

difficult to determine the actual percentage of plant emissions that will 

reach Imperial County annually. For most of the year, the winds in the 

vicinity of the Sempra and Intergen plants travel predominantly from the 

United States to Mexico. However, during the months of June, July, and 

August, this trend reverses and the winds travel predominantly from 

Mexico to the United States. Even assuming the plants operate at the 

maximum emissions levels allowed in California, and that all of those 

emissions reach Imperial County, annual emissions from the plants are 

low compared with various forms of dust and emissions from motor 

vehicles—the primary sources contributing to nonattainment of the 

standards for PM10 and ozone in Imperial County (see fig. 3). 
Page 18 GAO-05-823 Mexicali Power Plants 

Figure 3: Relative Emissions Contributions by Source Assuming the Sempra and 

Intergen Plants Operated at Maximum Allowable Emissions and All Emissions 

Reached Imperial County 

Note: 

Emissions contributions of less than 0.5 percent were not included in this figure. In the PM10 category, 

mobile sources (0.5 percent), the Sempra and Intergen power plants (0.4 percent), and fuel 

combustion (0.2 percent) emissions were omitted. 

a

The “mobile sources” category includes emissions from sources such as on and off-road motor 

vehicles, airplanes, trains, boats, and farm equipment. 

b

The “fuel combustion” category includes other stationary sources, such as electric utilities, 

manufacturing, food and agricultural processing, and service and commercial operations. 

c

The “other sources” category includes all subcategories identified by the California Air Resources 

Board, such as farming, fires, waste burning, and mineral processing that contribute emissions not 

included in the other categories listed. 

According to 2004 California emissions inventory estimates, road and 

windblown dust constituted almost 89 percent of total PM10 emissions 

within Imperial County. Mobile sources, which include both personal and 

commercial vehicles, accounted for 79 percent of total NOX emissions in 

the county. Even if they were located in Imperial County and operated at 

maximum allowable California emissions levels 24 hours per day, the 

Source: GAO analysis of data from Sempra, Intergen, and the California Air Resources Board.

0

20

40

60

80

100

PM CO 10 NOx

Percentage

Mobile sourcesa

Dust sources

Fuel combustionb

Other sourcesc

Sempra and Intergen power plants
Page 19 GAO-05-823 Mexicali Power Plants 

plants would emit 352 tons per year of PM10, compared with nearly 77,000 

tons per year from road and windblown dust, and 610 tons per year of 

NOX, compared with almost 10,000 tons per year from mobile sources (see 

table 5). 

Table 5: Estimated Annual Average Emissions in 2004 for Imperial County, 

California (Tons per Year) 

Emissions sources NOX PM10 CO

Stationary sources 

Fuel combustiona 2,537 150 325

Other stationary sources 11 1,011 22

Subtotal 2,548 1,161 347

Area-wide sources

Paved and unpaved road dust 0 13,647 0

Fugitive windblown dust 0 63,068 0

Waste burning and disposal 106 799 4,395

Other miscellaneous 37 7,194 252

Subtotal 142 84,709 4,647

Mobile sources 

On-road motor vehicles 5,143 139 25,831

Other mobile sources 4,847 252 8,629

Subtotal 9,990 391 34,460

Total 12,680 86,260 39,454

Legend: NOX = nitrogen oxides; PM10 = particulate matter; CO = carbon monoxide. 

Source: GAO analysis of California Air Resources Board data. 

a

The Fuel Combustion subcategory includes stationary sources such as existing electric utilities, 

manufacturing, food and agricultural processing, and service and commercial operations. 

Another way to examine the environmental impact of a power plant is to 

evaluate the amount of pollution emitted per unit of electricity produced. 

This calculation has been used within the energy industry to measure how 

efficiently power plants produce electricity. As illustrated in table 6, the 

Sempra and Intergen plants produce much lower emissions of NOX for 

each megawatt of energy generated than do other power plants operating 

in Imperial County and the border region of Baja California, Mexico. For 

example, Sempra’s estimated emission rate for NOX of .04 pounds per 
Page 20 GAO-05-823 Mexicali Power Plants 

megawatt of electricity is over 35 times lower than that rate at El Centro, 

the only major fuel-fired plant operating in Imperial County in 2002.11

Table 6: Comparison of NOX Emission Rates from the Sempra and Intergen Plants 

and Power Plants in Imperial County and Baja California 

Power plants 

NOX

emissions rate 

(lbs/MW)

Annual NOX

emissions (lbs) 

Net annual 

energy 

generation (MW)

Mexicali plants (2004) 

Sempra 0.04 89,668 2,389,549

Intergen 0.30a1,309,422 4,306,690

Imperial County (2002)

El Centro 1.45 610,674 421,736

Baja California (2002) 

C.C.C. Presidente 

Juarez (Rosarito) 2.38 4,942,713 2,077,250

C.TG. Presidente 

Juarez (Tijuana) 4.15 2,694,021 648,420

Legend: NOX = nitrogen oxides; lbs/MW = pounds per megawatt. 

Sources: GAO analysis of data from Sempra, Intergen, and the Commission for Environmental Cooperation. 

a

Intergen’s values represent emissions produced in 2004 before the installation of selective catalytic 

reduction technology on two of the four units. Future NOX emissions rates will likely be comparable to 

those at the Sempra plant. 

If the Sempra and Intergen plants were located in Imperial County, to help 

improve air quality, California regulations would require, among other 

things, offsets for all emissions from the plants that contribute to 

nonattainment of the PM10 and ozone standards in the county. Under the 

specific offsetting rules established by the Imperial County Air Pollution 

Control District, the operators of each plant would be required to reduce 

emissions from other pollution sources in Imperial County by at least 1.2 

tons for every ton of emissions the plants released.12 In addition to 

offsetting emissions of PM10 and NOX generated by the plant, Sempra and 

 11North American Power Plant Air Emissions, Commission for Environmental 

Cooperation of North America, 2004. 

12Imperial County Air Pollution Control District, Rule 207, Section C.3 (Revised Sept. 14, 

1999). 

If the Plants Were Located 

in Imperial County, 

California, among Other 

Things, Standards Would 

Require Them to Offset 

Their Emissions 
Page 21 GAO-05-823 Mexicali Power Plants 

Intergen would also be required to offset all emissions of VOC, which, in 

combination with NOX, contribute to the formation of ozone.13

As shown in table 7, potential offsets identified by the Imperial County Air 

Pollution Control District in DOE’s environmental impact statement 

include (1) paving roads, (2) retrofitting emission controls on existing 

power plants in Imperial County, (3) funding projects designed to increase 

the use of natural gas in motor vehicles, (4) controlling Imperial County 

airport dust, and (5) retrofitting diesel engines for off-road heavy duty 

vehicles. According to the Air Pollution Control District, repaving 

approximately 23 miles of roads could reduce PM10 emissions in Imperial 

County by about 650 tons per year—more than the estimated annual PM10

emissions from both plants based on the maximum allowable emissions 

levels in California. The District estimated the paving project would cost 

approximately $430,000 per mile for a two-lane road, bringing the total 

cost to about $9.9 million. 

 13The border city of Calexico, California, is also in violation of the state ambient air quality 

standard for CO. Therefore, if the plants were located in Imperial County close enough to 

Calexico to impact the city’s air quality, the plants would also be required to offset their 

emissions of CO. 
Page 22 GAO-05-823 Mexicali Power Plants 

Table 7: Potential Offsets in Imperial County, California, Identified by the Imperial County Air Pollution Control District 

Potential offset Description of project Estimated cost 

Estimated emission 

reduction 

Paving roads • Pave 50 road segments in Imperial County, totaling 23 

miles. 

$9,890,000 650 tons per year 

(PM10) 

Enhancing use of natural 

gas in motor vehicles 

• Fund maintenance of El Centro natural gas facility. 

• Fund natural gas facility to be constructed at the Calexico 

Unified School District. 

• Acquire land in Brawley, California, for construction of a 

new natural gas facility. 

• Replace/update fleet of fifteen Imperial Valley buses. 

$150,000 

$250,000 

$250,000 to 

$500,000 

$4 million to $5 

million 

0.1 tons per yeara

(PM10) 

Controlling Imperial County 

Airport dust 

• Begin treatment of bare desert soil with chemical dust 

retardants or cover soil with crushed rock in the most 

sensitive areas. 

$150,000 15 tons per year 

(PM10) 

Retrofitting diesel engines • Update the diesel engines of off-road vehicles used in 

agriculture, earthmoving, or construction. 

$250,000 3.3 tons per year 

(PM10) 

Retrofitting emission 

controls on existing power 

plants 

• Install selective catalytic reduction technology on one main 

unit of the existing steam plant at the Imperial Irrigation 

District, as well as all of the smaller units used only during 

periods of peak electricity demand. 

Not Estimated The main unit is 

already scheduled to 

be retrofitted in 2007-

2008 

(NOX) 

Legend: NOX = nitrogen oxides; PM10 = particulate matter. 

Source: DOE. 

a

Estimated emission reduction applies only to the bus replacement project. Emission reductions from 

the other projects were not quantified. 

In addition to the potential offsets identified above for Imperial County, 

according to DOE, mitigation measures may be even more abundant and 

cost-effective if applied on the Mexican side of the border. Some potential 

projects include paving roads in Mexicali, Mexico; replacing older 

automobiles and buses with newer, less polluting ones; and converting 

brick kilns to run on natural gas. However, according to DOE, it does not 

have the authority to impose or enforce offsets in Mexico. 

Finally, if the power plants were located in California, the Intergen plant 

would likely be required to make additional equipment modifications to be 

consistent with other plants recently constructed in California. These 

modifications would include installing additional carbon monoxide control 

equipment and achieving a small reduction in NOX emissions in one of the 

plant’s four combustion turbines. Although emissions testing data indicate 

that Intergen’s carbon monoxide levels are generally comparable to those 

of California plants without this equipment, nearly all of the plants 
Page 23 GAO-05-823 Mexicali Power Plants 

recently permitted in California have installed oxidizing catalyst systems 

to control carbon monoxide emissions. In addition, the Intergen plant 

would likely be required to lower the maximum NOX emissions in one 

turbine by 1.0 ppm—from 3.5 ppm to 2.5 ppm. Although this turbine is 

currently equipped with a selective catalytic reduction system to control 

NOX emissions, Intergen has stated that certain technical aspects of the 

design of the turbine prevent it from attaining emissions levels of 2.5 ppm. 

Emissions from the Sempra and Intergen power plants may contribute to 

adverse health impacts in Imperial County, but the extent of those impacts 

is unknown for several reasons. First, in its environmental impact 

statement, DOE did not calculate the total health impacts in the county 

because it did not analyze all the likely asthma-related or other health 

impacts from the increased pollution caused by the Sempra and Intergen 

plants. Second, DOE did not analyze the health impacts from increased 

power plant emissions on particularly susceptible populations, such as 

asthmatic children and low-income populations. Finally, because of 

uncertainty in DOE’s modeling of ozone increases due to emissions from 

the power plants, the health impacts related to ozone may be larger than 

DOE estimated. 

In its December 2004 final environmental impact statement, DOE 

estimated that emissions from the Sempra and Intergen power plants 

would result in increased concentrations of NOX, PM10, and CO in Imperial 

County. DOE used EPA’s “significant impact levels” to help assess the 

impact of these emissions increases on the residents of Imperial County. 

Generally, significant impact levels are thresholds below which the 

environmental and health impacts of air pollution are not viewed as 

significant; however, EPA designed them to be used only in areas that 

meet air quality standards.14 Although the plants will add more pollution to 

an area already violating the national standards for PM10 and ozone, DOE 

reported that because all pollution increases would be below EPA’s 

significant impact levels, emissions from the plants would not produce any 

significant air quality impacts in Imperial County. Specifically, DOE 

 14EPA has established significant impact levels for NO2

, SO2, CO, and PM10 in the context of 

permitting a major source or major modification to an existing pollution source in the 

United States. 40 C.F.R. § 51.165(b)(2).

Emissions Generated 

by the Sempra and 

Intergen Power Plants 

May Contribute to 

Various Adverse 

Health Impacts in 

Imperial County, but 

the Extent of Such 

Impacts Is Unknown 

DOE Did Not Analyze All 

the Likely Asthma-Related 

and Other Health Impacts 

of Increased Pollution 

from the Sempra and 

Intergen Power Plants 
Page 24 GAO-05-823 Mexicali Power Plants 

calculated that emissions from the power plants would be expected to 

increase asthma hospitalizations in the county by less than one case per 

year. However, DOE’s analysis did not quantify all of the health impacts 

from the increase in PM10 emissions. 

Health experts told us that the potential impact on asthmatics would be 

broader than the minimal increase in hospitalizations described in the 

environmental impact statement because hospitalization occurs only in the 

most acute asthma cases. According to these experts, an increase in PM10

pollution could exacerbate the underlying condition of anyone suffering 

from asthma. According to data from the 2003 California Health Interview 

Survey, 21,000 Imperial County residents, or 14 percent of the county’s 

population, have been diagnosed with asthma.15 Approximately 13,000 of 

these asthmatics experienced asthma symptoms during the previous year. 

The health experts we spoke with agreed that hospitalizations and other 

adverse health effects are part of a pyramid of potential adverse health 

effects. While the number of hospitalizations is represented at the top of 

the pyramid, other adverse health impacts such as emergency room visits, 

physician visits, asthma medication use, and increased asthma symptoms 

are layered vertically downward, with the number of people increasing in 

each subsequent group as you move to the bottom of the pyramid (see fig. 

4). 

 15The California Health Interview Survey—maintained at the UCLA Center for Health Policy 

Research in Los Angeles, California—is the state’s largest health survey. The telephone 

survey of adults, adolescents, and children is a collaborative project of the UCLA Center 

for Health Policy Research, the California Department of Health Services, and the Public 

Health Institute, and is conducted every 2 years. 
Page 25 GAO-05-823 Mexicali Power Plants 

Figure 4: Pyramid of Potential Health Impacts for Asthmatics 

In addition, the DOE study did not address the extent to which increased 

emissions of particulate matter would cause other adverse health impacts, 

such as other respiratory or cardiovascular problems. These impacts could 

include chronic obstructive pulmonary disease, pneumonia, 

cardiovascular disease, as well as increased symptoms of upper and lower 

respiratory disease, decreased lung function, or premature death. 

According to the project manager of DOE’s analysis, the expected 

incidence of other adverse health effects resulting from PM10 exposure has 

not been quantified because of a lack of data. 

Degree of severity becomes

greater toward the top

Number of people affected becomes

greater toward the bottom

Source: GAO.
Page 26 GAO-05-823 Mexicali Power Plants 

Studies funded by EPA, the Health Effects Institute,16 and others have 

concluded that certain groups are likely to be more susceptible to 

particulate matter than others, and therefore experience more adverse 

health effects. For example, these studies identified asthmatics, especially 

children, as a potentially susceptible subpopulation. According to data 

from the 2003 California Health Interview Survey, approximately 19 

percent of Imperial County children ages 1 through 17 have been 

diagnosed with asthma, or about 9,000 children. In addition, the 

relationship between socioeconomic factors and asthma exacerbation has 

been documented in various studies. Imperial County is ranked as one of 

the poorest counties in California, with some of the highest poverty and 

unemployment rates in the state. An estimated 22 percent of the overall 

population lives below the national poverty level, in comparison with 13 

percent statewide. Results from a 2001 California asthma report indicate 

that asthmatic adults with family incomes below the national poverty level 

are nearly twice as likely to experience symptoms every day or every week 

as those with incomes three times the poverty level, in part, because they 

have less access to health care.17

Finally, residents of Imperial County are currently exposed to airborne 

particulate pollution exceeding the Clean Air Act’s health-based National 

Ambient Air Quality Standard for PM10. For instance, in 2003, Imperial 

County residents were exposed to an annual average concentration of 

PM10 that was 30 micrograms, or 63 percent higher, than the national 

standard of 50 micrograms per cubic meter of air, as shown in table 8. As a 

result, Imperial County residents can be expected to have higher incidence 

of adverse health effects caused by airborne particulate pollution than 

residents living in areas with less of that contaminant. 

 16The Health Effects Institute (HEI) is an independent, nonprofit corporation chartered in 

1980 to provide impartial research on the health effects of air pollution. Supported jointly 

by the U.S. Environmental Protection Agency and industry, HEI has funded over 170 

studies and published over 100 research reports. HEI supported research has produced 

findings on the health effects of a variety of pollutants, such as carbon monoxide, nitrogen 

oxides, ozone, and most recently, particulate air pollution. 

17UCLA Center for Health Policy Research, Asthma in California: Findings from the 2001 

California Interview Health Survey, November 2003. 

DOE Did Not Analyze the 

Health Impacts on 

Particularly Susceptible 

Populations in Imperial 

County 
Page 27 GAO-05-823 Mexicali Power Plants 

Table 8: Imperial County Air Quality Compared with National Standard for PM10

(1997-2003) 

Micrograms per cubic meter of air 

 Imperial County 

PM10

National 

standard 1997 1998 1999 2000 2001 2002 2003

Maximum 24 hour 

concentration 150 532 176 227 268 647 373 840

Calculated days 

over national 

standard 12 12 32 38 18 21 25

Annual average 50 77.7 66.1 77.8 95.2 86.2 81.3 80

Legend: PM10 = particulate matter. 

Source: DOE. 

In its environmental impact statement, DOE acknowledges that there are 

preexisting conditions of concern in Imperial County, such as asthmatic 

children and low-income populations, both of whom are particularly 

susceptible to health problems from pollution, and the fact that the county 

is a Clean Air Act nonattainment area for PM10. However, DOE did not fully 

explore these conditions to determine their potential health impacts. DOE 

believes that because the increases in emissions from the plants are below 

EPA’s significant impact levels any health impacts will be negligible. 

However, some health studies have found that even the smallest 

incremental increase in particulate matter air pollution increases the 

incidence of adverse health effects. 

DOE conducted air dispersion ozone modeling for the Imperial ValleyMexicali air basin to determine what impact emissions from the Sempra 

and Intergen plants would have on the formation of ozone. DOE 

concluded, based on its modeling, that there would be no meaningful 

change in ozone levels as a result of the operation of the Sempra and 

Intergen power plants. Consequently, DOE concluded that the health 

impacts from ozone formation as a result of plant emissions would be 

minimal. However, if the modeling is not accurate, then the health impacts 

could be larger than DOE estimated. 

EPA officials have raised concerns about the accuracy of DOE’s modeling 

of estimated ozone increases. In its comments on DOE’s draft 

environmental impact statement, EPA stated that it is difficult to quantify 

the impact of a small number of facilities (i.e., the two power plants) on 

Health Impacts from 

Ozone Formation May Be 

Larger Than Estimated by 

DOE 
Page 28 GAO-05-823 Mexicali Power Plants 

the maximum ozone concentration in an air basin. The lack of areaspecific information, such as temperature, relative humidity, and levels of 

volatile organic compounds (an ozone precursor), in the Imperial 

County-Mexicali air basin makes modeling ozone formation particularly 

difficult. Because these data were not available, DOE used surrogate 

values from Phoenix, Arizona.18 Furthermore, DOE’s analysis relied on air 

monitoring data and the EPA ozone model to determine the potential 

influence of NO2

 emissions—the primary pollutant emitted from the 

Sempra and Intergen power plants—on ozone concentrations in Imperial 

Valley. DOE concluded that increased NOX emissions from the plants could 

produce a decrease in ozone concentrations. In its comments on the draft 

environmental impact statement, EPA stated that peak ozone 

concentrations generally occur in areas away from sources of high NOX

emissions, not at the monitor where high NO2

 concentrations are 

measured. They emphasized that if modeled ozone projections are not 

correct, impacts to air quality from the plants’ emissions could be 

significant. Recent research funded by EPA and others has found that 

increases in ozone pollution raise the risk of premature death. Finally, 

EPA recommended that DOE require Sempra and Intergen to implement 

mitigation measures to ensure that increased concentrations of ozone do 

not occur in the air basin. 

In response to EPA’s comments, in its December 2004 final environmental 

impact statement, DOE presented a sensitivity analysis that indicated that 

power plant emissions could result in either increases or decreases in 

peak ozone concentrations depending on model input assumptions. 

However, DOE again concluded that its modeling of ozone formation did 

not indicate any meaningful change in ozone levels as a result of the 

operation of the Sempra and Intergen power plants and therefore chose 

not to require any mitigation requirements in its record of decision. 

According to DOE officials, although the record of decision states that 

mitigation of emissions is the preferred option, the sum total of emissions 

from the plants is so minimal that it is not cost-effective to require 

mitigation measures in the United States. However, DOE has not 

conducted any analysis to support its claim that mitigation measures are 

not cost-effective. Furthermore, DOE said that it does not have the 

resources needed to conduct a comprehensive monitoring program to 

 18DOE used data from Phoenix, Arizona because it is one of the 10 cities that was already 

built into the EPA ozone model (OZIPR) database, and they believe Phoenix, Arizona is the 

most representative proximate city in terms of climate, latitude, and physiography. 
Page 29 GAO-05-823 Mexicali Power Plants 

ensure that mitigation projects are completed satisfactorily. Finally, DOE 

acknowledged in the environmental impact statement that mitigation 

measures may be more abundant and cost-effective in Mexico. However, 

DOE told us that while it has the authority to require the plants to take 

mitigation measures in the United States, it does not have the authority to 

require or enforce such measures in Mexico. 

Because the Sempra and Intergen power plants are not subject to either 

the federal Clean Air Act or the California Clean Air Act, they are not 

required to provide offsets for their emissions. In addition, relevant 

agreements among the United States, Canada, and Mexico may not provide 

adequate mechanisms to address adverse health impacts resulting from 

emissions from these plants. As a result, policymakers have limited 

options to ensure that emissions from these plants do not adversely affect 

the health of residents in Imperial County. 

Existing U.S. law provides few options to ensure that emissions from the 

Intergen and Sempra plants do not adversely affect the health of residents 

in Imperial County. Because the Intergen and Sempra plants are not 

located in the United States, federal and California environmental agencies 

do not have authority over the plants. The federal Clean Air Act contains 

no language extending the statute’s coverage to pollution sources that are 

located outside of the United States. Similarly, the text of the California 

Clean Air Act limits its application to pollution sources that are located in 

California. Because neither of these laws applies to the Sempra or Intergen 

plants, U.S. environmental agencies have no authority under existing law 

to require the plants to implement pollution control measures. 

Similarly, existing international agreements provide few options to ensure 

that emissions from the Sempra and Intergen plants do not adversely 

affect the health of residents of Imperial County. The governments of the 

United States and Mexico have ratified two agreements that are of 

particular importance to environmental conditions in the border region. 

The first was signed at La Paz, Mexico, in 1983. The La Paz Agreement 

creates a framework for promoting cooperation between the United States 

and Mexico on issues of environmental protection in the border region. 

Policymakers Have 

Limited Options to 

Ensure That 

Emissions from the 

Sempra and Intergen 

Power Plants Do Not 

Adversely Affect the 

Health of Residents in 

Imperial County 

Existing Laws and 

Agreements May Not 

Provide Effective 

Mechanisms to Address 

Adverse Health Impacts 
Page 30 GAO-05-823 Mexicali Power Plants 

For example, the agreement states that the United States and Mexico will 

“cooperate in the solution of the environmental problems of mutual 

concern in the border area,” and that high officials from the two countries 

will meet annually to review the agreement’s implementation. The 

agreement does not require either government to implement specific 

pollution control requirements or provide a course of action for either 

country to pursue if a particular project in the border region harms the 

health of border region inhabitants. 

The other environmental agreement, the North American Agreement on 

Environmental Cooperation (NAAEC), also provides few options to ensure 

that emissions from the Sempra and Intergen plants do not adversely 

affect the health of residents of Imperial County. The United States, 

Canada, and Mexico signed the NAAEC in 1993 to supplement the 

provisions of the North American Free Trade Agreement. The NAAEC 

provides a dispute resolution procedure under which the United States, 

Mexico, or Canada may request consultation with another party to the 

agreement regarding whether there has been a persistent pattern of failure 

by that other party to effectively enforce its environmental law. The 

parties must make every attempt to resolve the matter through the 

consultative process. However, if consultation fails to lead to a 

satisfactory resolution, then either party may take a series of steps that 

may culminate in the meeting of an impartial, five-member arbitration 

panel. This panel can determine whether the party complained against has 

persistently failed to enforce its environmental law. If the panel issues a 

decision finding such a persistent failure, it may formulate an action plan 

to remedy the enforcement failure and may ultimately impose monetary 

penalties if the enforcement failure persists. Thus, the NAAEC dispute 

resolution procedure provides an option for U.S. policymakers, but only if 

the Mexican government persistently fails to enforce the Mexican 

environmental laws that apply to the two plants. The NAAEC dispute 

resolution procedure does not provide a useful option for U.S. 

policymakers if the Intergen and Sempra plants comply with Mexican law, 

even if the plants adversely affect the health of the residents of Imperial 

County. 

There are some actions policymakers could take to protect Imperial 

County from increased emissions from the Sempra and Intergen power 

plants. For example, the Congress could enact legislation restricting the 

importation of electricity generated by power plants whose electrical 

output is dedicated exclusively to the United States if they do not meet 

certain U.S. emission and offset requirements. While this action would 

Policymakers Have 

Limited Options to Protect 

Imperial County Residents 

from Adverse Health 

Impacts 
Page 31 GAO-05-823 Mexicali Power Plants 

have benefits to air quality and health, it would also have costs, such as 

possibly reducing energy supplies available to southern California. In 2003 

a bill was introduced in the Senate and House that would have prohibited 

the exportation of natural gas from the United States to Mexico for use in 

power plants near the U.S. border if the plants do not provide air quality 

protection that is at least equivalent to the protection provided by air 

quality requirements applicable in the United States. Each chamber 

referred the bill to committee; neither the Senate nor the House committee 

reported the bill to the full chamber for consideration. 

Similarly, DOE could modify its regulations that apply to applicants for 

presidential permit seeking to build new international transmission lines 

to import electricity into the United States from Mexico. The modified 

regulations could require that the lines connect to plants that employ 

specified emissions controls and obtain offsets in the United States. 

However, limiting the import of electricity from Mexico into California 

could jeopardize some electricity supplies for parts of southern California, 

which could be problematic especially during peak consumption periods. 

According to the California Independent System Operator, the demand for 

energy in California is growing at nearly 4 percent annually. During the 

summer of 2004, the peak demand record set in 1999 was broken seven 

times, and the California Independent System Operator believes that the 

record will likely fall again during the summer of 2005. 

Moreover, both of the above options would need to be assessed to 

determine if they are compliant with the North American Free Trade 

Agreement (NAFTA). The agreement allows either the U.S. or Mexico to 

restrict energy imports for a range of reasons, including protection of 

human life or health. However, such import restrictions must meet a 

variety of conditions. For example, though NAFTA recognizes a country’s 

right to license imports and exports of energy, any such licensing system 

must be consistent with NAFTA and not frustrate its overall objectives of 

eliminating trade barriers, promoting fair competition, and increasing 

investment opportunities. NAFTA also requires energy regulatory agencies 

to minimize disruptions to contractual relationships in applying their 

regulations. 

A third option would be for the United States and Mexico to expand 

cooperation under the existing binational initiative to address 

transboundary air pollution in the U.S.-Mexico border region by providing 

economic incentives, such as emissions trading, to reduce pollution. Such 

programs have proven successful in the United States in reducing 

emissions that contribute to acid rain. At the international level, the United 
Page 32 GAO-05-823 Mexicali Power Plants 

States and Canada have developed an air pollution agreement that could 

possibly serve as a model for a similar agreement between the United 

States and Mexico. However, based on the Canadian example, developing 

the legal and regulatory framework needed to create a binational 

emissions trading program with Mexico is likely to take a significant 

amount of time. The United States and Canada initiated cooperative 

efforts in 1980 through a memorandum of understanding, and 11 years 

later the 1991 U.S.-Canada Air Quality Agreement identified market-based 

mechanisms, including emissions trading, as areas for further discussion. 

In April 1997, the United States and Canada agreed on a joint plan of 

action for addressing transboundary air pollution, expanding the initial 

focus on acid rain to also examine ground-level ozone and particulate 

matter. In 2000, the Air Quality Agreement was formally expanded to 

address transboundary ground-level ozone issues. In 2004, the United 

States and Canada initiated a joint study to examine the feasibility of 

establishing a binational emissions trading program. Issues being 

addressed include the legal authority and the air pollution monitoring, 

assessment, and reporting system that would be needed to implement 

such a program. 

At the state level, Texas passed legislation in 2001 that authorized the 

state’s environmental agency to accept reductions in emissions from brick 

kilns in Ciudad Juarez, Mexico, to satisfy new state emission control 

requirements passed by the Texas Legislature in 1999. In return for air 

emission allowances under Texas law, the local utility, El Paso Electric, 

arranged the destruction of older, high-polluting, open-top kilns and 

replaced them with less polluting closed-top kilns. This emission control 

project serves the Paso Del Norte air basin, which is officially recognized 

in the La Paz Agreement, and includes El Paso, Texas, and Ciudad Juarez, 

Mexico. 

Finally, another potential option is the development of a binational clean 

air trust fund that could provide grants and loans to support projects that 

would improve the air quality of U.S. and Mexican cities that share air 

basins in the border region. Implementing such a program could help 

offset emissions generated by a variety of sources, including power plants 

in Mexico that are not required to offset their emissions. Funds from a 

variety of sources, such as appropriations from both nations’ legislatures, 

fast-lane fees for cars and trucks at ports of entry, and fees from airports 

and railroads operating along the border, could be held in a joint 
Page 33 GAO-05-823 Mexicali Power Plants 

U.S.-Mexican trust fund for distribution to states, counties, cities, or local 

air pollution control districts along the shared border.19 The binational 

clean air trust fund could also potentially obtain funds from power plants 

located in the U.S.-Mexico border region that are looking for opportunities 

to offset their emissions, although they are not required to do so by law. 

Both Intergen and Sempra have shown an interest in supporting projects 

aimed at improving the air quality in the border region. For example, 

Intergen supports an applied research grant program to improve air quality 

in the California–Mexico border region,20 and Sempra is developing a fund 

to support the implementation of environmental projects, such as road 

paving, in the border city of Mexicali, Mexico, that it expects to implement 

before the end of 2005. 

The Sempra and Intergen plants near Mexicali, Mexico, are modern power 

plants that use advanced air pollution control technologies. As a result, the 

pollution they emit is comparable to that emitted by similar plants that 

have recently received permits to operate in California and is low relative 

to dust and emissions from vehicles, the primary sources of pollution in 

Imperial County. Nevertheless, the plants emit some pollutants into an air 

basin that already does not meet some air quality standards and is home to 

many asthmatic children and a low-income population that may be 

particularly susceptible to adverse health consequences from any level of 

pollution increase. DOE concluded in its environmental impact statement 

that pollution from the plants would not result in significant health 

impacts in Imperial County and therefore did not require the plants to 

offset their emissions. However, the DOE analysis did not fully examine 

several issues that could have led to an assessment of a larger adverse 

health impact in Imperial County. In addition, if the plants were located 3 

miles north in Imperial County, California, they would be required to fund 

projects to reduce pollution from other sources to offset their emissions 

regardless of whether there was a documented adverse health impact. 

However, now that DOE has determined that no offsets are required, 

options available to U.S. policymakers in the short term to directly address 

 19Richard Ryan, “Financing Clean Air on the Border: Establishing a Binational Clean Air 

Trust Fund (BiCAT)” (paper presented at the Border Institute VII, Transboundary Air 

Quality Management conference, in Rio Rico, Ariz., April 2005). 

20The program, known as the Border Ozone Reduction and Air Quality Improvement 

Program, is administered by a Harvard University-affiliated nonprofit organization, 

LASPAU: Academic and Professional Programs for the Americas. 

Conclusions 
Page 34 GAO-05-823 Mexicali Power Plants 

the existing health concerns are limited. In the long term, the United States 

and Mexico could implement an emissions trading program or a clean air 

trust fund to address pollution in the border area, but such programs are 

likely to take years, and require significant binational effort to develop. 

We provided draft copies of this report to the Department of Energy 

(DOE) and to the Environmental Protection Agency (EPA) for their review 

and comment. We received a written response from DOE’s Director, Office 

of Electricity Delivery and Energy Reliability. EPA provided technical 

comments which we incorporated in the report. 

DOE disagreed with our assertion that it did not analyze all of the likely 

asthma-related and other health impacts of increased pollution from the 

Sempra and Intergen power plants. Specifically, DOE stated that the 

environmental impact statement for the two plants (1) notes the full range 

of respiratory effects associated with exposure to airborne particulate 

matter (PM10) and (2) uses the number of potential additional asthma 

hospitalizations in Imperial County as a representative estimate of the 

number of potential health effects cases associated with power plant 

emissions of PM10. While DOE’s environmental impact statement 

acknowledges that increases in PM10 concentrations could have adverse 

health impacts such as increased asthma symptoms and chronic 

bronchitis, up to hospitalization and death, DOE did not quantify or report 

quantified estimates for any adverse health effects other than asthma 

hospitalizations. For example, DOE did not quantify the extent to which 

asthma sufferers would have increased doctor visits or medication use 

related to increased pollution. Furthermore, while asthma hospitalizations 

are one measure of potential adverse health impacts from increased 

emissions of particulate matter, there are many other adverse health 

effects that have been documented, such as chronic lung disease, chronic 

bronchitis, pneumonia, and cardiovascular disease that DOE did not 

quantify. For these reasons, asthma hospitalizations do not represent the 

“full range” of potential adverse health impacts in Imperial County. 

DOE also disagreed with our assertion that it did not analyze the potential 

health impacts of pollution from the Sempra and Intergen power plants on 

susceptible populations in Imperial County. According to DOE, because 

the environmental impact statement’s estimate of increased asthma 

hospitalizations is based on data that include children ages 14 and under, it 

accounts for health impacts on susceptible populations. However, 

asthmatic children are not the only susceptible population mentioned in 

our report, and asthma hospitalization is not the only potential health 

Agency Comments 

and Our Evaluation 
Page 35 GAO-05-823 Mexicali Power Plants 

impact. Moreover, DOE’s analysis does not differentiate among different 

population subgroups in terms of their susceptibility to the effects of air 

pollution but instead characterizes potential adverse health effects for the 

population as a whole. Consequently, we continue to believe that DOE’s 

environmental impact statement did not address the full range of potential 

health impacts on susceptible populations in Imperial County. 

Finally, DOE does not agree that the health impacts from ozone formation 

may be larger than it estimated in the environmental impact statement. 

DOE said that it addressed EPA’s concerns regarding the uncertainty in 

the ozone modeling in the final environmental impact statement. However, 

while EPA acknowledged in its comments on the final environmental 

impact statement that the document clarified the limitations of the ozone 

modeling analysis, it also reiterated its support for off-site mitigation 

efforts to address these limitations to ensure that there is no net increase 

in air pollution in Imperial County. As a result, we continue to believe that 

ozone formation may have larger health impacts than estimated in the final 

environmental impact statement. DOE’s specific comments and our 

detailed responses are presented in appendix II of this report. 

We are sending copies of this report to the Secretary of Energy, and the 

Administrator of the Environmental Protection Agency, and appropriate 

congressional committees. We will also provide copies to others upon 

request. In addition, the report will be available at no charge on the GAO 

Web site at http://www.gao.gov. 
Page 36 GAO-05-823 Mexicali Power Plants 

If you or your staffs have any questions about this report, please contact 

me at (202) 512-3841 or stephensonj@gao.gov. Contact points for our 

Offices of Congressional Relations and Public Affairs may be found on the 

last page of this report. GAO staff who made major contributions to this 

report are listed in appendix III. 

John B. Stephenson 

Director, Natural Resources 

 and Environment 
Appendix I: Objectives, Scope, and 

Methodology 

Page 37 GAO-05-823 Mexicali Power Plants 

The objectives of this report were to determine (1) how emissions from 

the Sempra and Intergen power plants compare to emissions from recently 

permitted plants in California and emissions from sources in Imperial 

County, and what emissions standards the plants would be subject to if 

they were located in Imperial County; (2) the health impacts of emissions 

from the power plants on Imperial County residents; and (3) what options 

exist for U.S. policymakers to ensure that emissions from these power 

plants do not adversely affect the health of Imperial County residents. To 

address all three of these objectives we visited the Sempra and Intergen 

plants in Mexicali, Mexico; interviewed plant representatives, various U.S. 

federal, state, and local air quality officials, and other stakeholders; and 

reviewed relevant documents and studies. 

To determine emissions from the Sempra and Intergen plants we obtained 

data from emissions performance tests conducted at the plants by third 

party contractors (GE Mostardi Platt and Air Hygiene). These tests were 

designed to document the average emissions of selected pollutants 

(nitrogen oxide, particulate matter, carbon monoxide, volatile organic 

compounds, and ammonia) from the combustion turbines at each of these 

plants. The results of these tests were reported in standard units of 

measurement, namely parts per million or pounds per hour. According to 

the contractors, they completed the tests according to Environmental 

Protection Agency and California-approved methods and conducted 

quality assurance activities related to their test results. We assessed the 

reliability of the data by (1) reviewing documentation of test objectives 

and quality control procedures provided by the third party contractors, (2) 

conducting interviews with plant officials to determine the scope and 

generalizability of the tests, and (3) reviewing reports of actual NOX

emissions submitted to the Mexican government to ensure consistency 

with the test results. Based on this assessment, we determined that the 

data were sufficiently reliable for the purposes of this report. 

To determine annual emissions estimates from these plants, we used the 

results from the emissions tests to calculate the annual tonnage that these 

plants would be likely to emit. We computed these values based on the 

conservative assumption that these plants would be operating 24 hours a 

day, 365 days a year. In addition to the estimates obtained from the testing 

results, we also used the maximum allowable emission limits of 

comparable plants in California to develop a more conservative estimate 

of annual emissions from these plants. For Sempra, we utilized the Elk 

Hills power plant as the primary basis for developing comparative 

estimates. This natural gas-fired power plant is partially owned by Sempra 

Energy and utilizes very similar equipment and pollution control 

Appendix I: Objectives, Scope, and 

Methodology 
Appendix I: Objectives, Scope, and 

Methodology 

Page 38 GAO-05-823 Mexicali Power Plants 

technology as the Mexicali plant. For the Intergen plant, we used a 

combination of comparable estimates because no similar, Intergen owned 

facilities were recently constructed in California. To estimate nitrogen 

oxide (NOX) and ammonia (NH3

), we used vendor guarantee limits, which 

are the maximum emissions levels specified by the manufacturer that a 

specific piece of equipment is likely to produce (e.g. selective catalytic 

reduction systems). Particulate matter (PM10) was estimated using the 

average allowable emissions limit from all comparable plants in California 

permitted between 2000 and 2004. Because the Intergen plant is not 

equipped with an oxidation catalyst, carbon monoxide (CO) was estimated 

using a specific plant in California, permitted in 2000, that was the only 

one licensed without such control equipment. Finally, because some 

California permits establish volatile organic compounds (VOC) limits in 

parts per million and others do so in pounds per hour, we were not able to 

develop an average for all recently permitted plants in California. For this 

reason, we used emissions limits from the Elk Hills power plant to 

estimate annual emission levels of VOC at the Intergen plant. 

To determine how estimated emissions from the Sempra and Intergen 

plants compare to recently permitted plants in California, we developed a 

range of maximum allowable emission limits for all natural gas-fired 

power plants in California with similar specifications, licensed between 

2000 and 2004. This time frame was chosen because it corresponded to the 

dates that the Sempra and Intergen plants in Mexicali were designed, 

permitted, and began commercial operations. Because all California power 

plants are permitted on a case-by-case basis, emissions limits may vary 

with each project. Therefore, we used the entire range of emission limits 

for the 23 plants that were identified during our selection process. We then 

compared the range of emission limits from the 23 plants that we 

identified with the third party testing results we obtained from the Sempra 

and Intergen plants. 

To determine how the emissions from these plants compare to emissions 

from sources in Imperial County we utilized the 2004 estimated annual 

average emissions inventory for Imperial County developed by the 

California Air Resources Board. We also met with officials from the 

California Air Resources Board and reviewed emissions reports for 

stationary sources obtained from the Imperial County Air Pollution 

Control District. To determine the levels of nitrogen oxide emissions from 

the Sempra and Intergen plants in relation to existing plants in Imperial 

County and Baja California, Mexico, we obtained reports developed for 

the Mexican government that included annual emissions of nitrogen 

oxides based on data from the continuous emissions monitoring system on 
Appendix I: Objectives, Scope, and 

Methodology 

Page 39 GAO-05-823 Mexicali Power Plants 

each turbine. Comparable data for the El Centro plant in Imperial County 

and the two Baja California plants were obtained from a report produced 

by the Commission for Environmental Cooperation of North America.1

 To 

assess the reliability of these data sources we (1) spoke with officials at 

the California Air Resources Board and reviewed documentation related to 

data collection and quality control procedures used to develop the annual 

emissions inventory, and (2) corroborated the emissions data related to 

the El Centro plant with the EPA Clean Air Markets database. Based on 

these assessments, we determined that the data were sufficiently reliable 

for the purposes of this report. 

To determine what emissions standards the plants would be subject to if 

located in Imperial County we reviewed the principal federal regulations 

applicable to new power plants located in the United States and the 

emission limits of similar plants recently permitted in California. The 

primary federal regulations we reviewed were those established under 

EPA’s New Source Review program for new or modified major pollution 

sources. We reviewed selected state and local air pollution regulations 

because state and local agencies have responsibility for implementing 

specific permitting activities as part of the federal program. The state and 

local regulations we reviewed included the permitting conditions of 

several power plants licensed by the California Energy Commission to 

determine the standard permitting criteria and the air quality rules 

established by the Imperial County Air Pollution Control District for 

sources located in Imperial County. 

To identify the potential health impacts from emissions generated by the 

Sempra and Intergen power plants, we reviewed the health assessment in 

DOE’s environmental impact statement. We met with the project manager 

of DOE’s health assessment to gather additional information about the 

assessment methodology. We reviewed EPA’s comments on the 

environmental impact statement, and interviewed EPA officials and health 

experts regarding DOE’s health assessment methodology. In addition, we 

reviewed relevant EPA reports, and other health studies regarding the 

impacts of particulate matter and ozone on human health. Finally, we 

reviewed a recent California health survey to obtain current information 

 1

The Commission for Environmental Cooperation of North America was established by the 

North American Agreement on Environmental Cooperation—one of the two side 

agreements to the North American Free Trade Agreement. 
Appendix I: Objectives, Scope, and 

Methodology 

Page 40 GAO-05-823 Mexicali Power Plants 

on asthmatic populations in Imperial County and other California 

counties. 

To determine the policy options available to ensure that emissions from 

the Sempra and Intergen plants do not adversely affect the health of 

Imperial County residents, we reviewed the federal Clean Air Act, the 

California Clean Air Act, key provisions of the North American Free Trade 

Agreement, as well as environmental agreements between the United 

States and Mexico, such as the La Paz agreement, and a trilateral 

agreement between the United States, Mexico, and Canada—the North 

American Agreement on Environmental Cooperation; and academic 

research. We also participated in a transboundary air quality management 

conference where officials from various federal, state, and local agencies 

in the United States and Mexico met to discuss strategies to address 

binational air pollution. 

We conducted our work between September 2004 and August 2005 in 

accordance with generally accepted government auditing standards. 
Appendix II: Comments from the Department 

of Energy 

Page 41 GAO-05-823 Mexicali Power Plants 

Appendix II: Comments from the Department 

of Energy 

Note: GAO comments 

supplementing those in 

the report text appear at 

the end of this appendix. 

See comment 4. 

See comment 3. 

See comment 2. 

See comment 1. 

Now on p. 23. 
Appendix II: Comments from the Department 

of Energy 

Page 42 GAO-05-823 Mexicali Power Plants 

See comment 7. 

Now on p. 27. 

See comment 6. 

See comment 5. 

Now on p. 26. 
Appendix II: Comments from the Department 

of Energy 

Page 43 GAO-05-823 Mexicali Power Plants 

See comment 8. 
Appendix II: Comments from the Department 

of Energy 

Page 44 GAO-05-823 Mexicali Power Plants 

The following are GAO’s comments on DOE’s written comments provided 

in their letter dated July 29, 2004. 

1. While DOE’s environmental impact statement acknowledges that 

increases in PM10 concentrations could have adverse health impacts 

such as increased asthma symptoms and chronic bronchitis, up to 

hospitalization and death, DOE did not quantify any potential adverse 

health effects other than asthma hospitalizations. Instead, the EIS 

relies solely on comparisons to significant impact levels to gauge the 

magnitude of potential adverse impacts on human health. In doing so, 

DOE determined that emissions from the two plants were below 

significant impact levels, and therefore, these emissions would not 

produce any significant air quality or adverse health impacts in 

Imperial County. As we stated in our report, significant impacts levels 

were designed to be used only in areas that meet air quality standards; 

Imperial County is currently designated as a nonattainment area for 

PM10 and ozone because it does not meet air quality standards for these 

two pollutants. 

2. Asthma hospitalizations are just one measure of potential adverse 

health impacts from increased emissions of particulate matter. While 

asthma hospitalizations are more severe and likely to occur less often 

than doctor visits or increased medication use for asthma, they cannot 

be considered representative of the “full range” of potential adverse 

health impacts associated with asthma in Imperial County. In addition 

to asthma-related adverse health effects, numerous studies have linked 

increased exposure to particulate matter to other non-asthma-related 

adverse health effects, such as chronic bronchitis, chronic lung 

disease, pneumonia, and cardiovascular disease. 

3. We disagree with DOE that hospitalizations are the best parameter for 

representing impacts on asthma. While asthma hospitalizations in 

Imperial County may be well documented, the 2003 California Health 

Survey provides information on other asthma-related health impacts in 

Imperial County. For example, the survey contains information on the 

number of Imperial County residents who take medication to control 

asthma. In addition, the survey presents information on the number of 

Imperial County residents who had asthma symptoms within a 

specified time frame and who visited an emergency room or urgent 

care facility for asthma-related health problems during that time frame. 

Such information could have been used, along with information on 

hospitalizations, to create a more complete estimate of the potential 

asthma-related health effects from increases in pollution from the 

power plants. 

GAO Comments 
Appendix II: Comments from the Department 

of Energy 

Page 45 GAO-05-823 Mexicali Power Plants 

4. The report does not use the health effects pyramid, or suggest it should 

be used, to compute instances of potential health effects from air 

pollution. However, we believe that the health effects pyramid is useful 

for understanding the variety of ways in which increased pollution can 

aggravate asthma suffering in Imperial County. In so doing, it also 

highlights the full range of potentially quantifiable effects related to 

asthma. 

5. During our review of the health effects literature, we identified a 

number of studies that support a linear relationship between increases 

in particulate matter pollution and increased incidence of 

cardiovascular diseases. 

6. Asthmatic children are not the only susceptible population mentioned 

in our report, and asthma hospitalization is not the only potential 

health impact. Consequently, we continue to believe that DOE’s 

environmental impact statement did not address the full range of 

potential health impacts on susceptible populations in Imperial 

County. DOE’s quantification of just one adverse health impact for the 

entire population of Imperial County masks the differential effects that 

can beset more susceptible subpopulations in the County. 

7. In commenting on the final environmental impact statement, EPA 

acknowledged that DOE had clarified the limitations and uncertainties 

of the ozone modeling analysis. However, in its comments EPA said it 

continues to support and encourage off-site mitigation efforts to 

address the limitations in the ozone modeling to ensure that there is no 

net increase of air pollution in Imperial County. 

8. We believe that EPA’s comment regarding peak ozone concentrations 

is relevant because it is presented in the context of EPA’s comments 

on the draft environmental impact statement. We also note in the 

report that DOE took action in response to EPA’s comment. 
Appendix III: GAO Contact and Staff 

Acknowledgments 

Page 46 GAO-05-823 Mexicali Power Plants 

John B. Stephenson (202) 512-3841 

In addition to the contact named above, Leo G. Acosta, Charles Bausell, 

Nancy Crothers, Brandon Haller, Ryan Lambert, Omari Norman, Kim 

Raheb, and Stephen Secrist made key contributions to this report. 

Appendix III: GAO Contact and Staff 

Acknowledgments 

GAO Contact 

Acknowledgments 

(360516) 
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