February 28, 2011 

MEMORANDUM 

To: Toxics Rule Docket

From: 

Subject: Review of Proximity Analysis

1.0 INTRODUCTION 

This memorandum transmits the methods and results of the proximity-based demographic 

assessment conducted in support of Section J. of the Toxics Rule which regulates coal- and oilfried electrical generation units (EGUs) greater than 25 megawatts (MWs). The objective of the 

assessment is to determine the demographics of populations located in the vicinity of these 

EGUs. These populations have the potential to be significantly affected by the emissions of this 

source category. However, these EGUs usually have very tall emission stacks that tend to 

disperse pollutants fairly far from the source. In addition, several of the pollutants emitted by 

this source category (e.g., mercury (Hg) and sulfur dioxide (SO2)) are known to travel long 

distances and can cause harm to both the environment and human health as far as hundreds or 

even thousands of miles from where they were first emitted into the atmosphere. Therefore, 

while this analysis gives some indication of populations that may be exposed to levels of 

pollution that can cause concern, it does NOT identify the demographic characteristics of the 

people or communities most affected by the pollutants from this source category or the emissions 

reductions that would result from this proposed rule. 

2.0 ANALYTICAL APPOACH

The following analysis uses the “proximity-to-a-source”approach as a surrogate for 

potential exposure to identify those populations living in designated “study areas” around 

affected sources. The site location data for the sources subject to the Toxics Rule were taken 

from the Office of Air and Radiation’s Clean Air Markets Divison’s (CAMD) 

NEEDSv410_NODA database. 

Data collected by EPA from potentially affected sources in 2010 under the Information 

Collection Rule (ICR) indicated that numerous EGUs identified as oil-fired listed in the CAMD 

database don't fire oil unless they absolutely have to (e.g., when natural gas service is interrupted 

due to cold weather, hurricanes, etc.). The ICR data also indicated that, given the price 

differential between natural gas and oil, some “oiled-fired” EGUs had not fired oil in years. In 

addition, the responses to the ICR revealed that the data on oil-fired EGUs contain multiple 

miscoding, for example several "oil-fired boilers" were really "combustion turbines" with heat 

recovery steam generators. As a result of the uncertainities in data on the oil-fired EGUs, we 
decided to focus the proximity analysis only on coal-fired EGUs greater than 25 MWs. 

Therefore the resulting analysis represents approximately 97% of likely affected sources (i.e., 

coal- and oil-fired EGUs) contained in the CAMD data base. 

The demographic data for this analysis are based on census block level data extracted 

from the 2000 US census which was provided to the EPA by the US Census Bureau. The sociodemographic parameters used in the analysis include the following categories: Racial (White, 

African American, Native American, Other or Multiracial, All Other Races); Ethnicity 

(Hispanic, non-Hispanic); and Other (Number of people below the poverty line). 

This proximity analysis determined the demographic composition for two study areas 

around each emissions source. The first study area consisted of those census blocks whose 

centriods lay within a 5 km (approximately 3 miles) radius of the affected sources. This radius 

has been used in previous demographic analyses focused on areas around industrial sources1,2. 

While a 5 km radius has been used in previous rulemakings, in the case of EGUs it is potentially 

a poor estimate of the affected area. As noted above, power plants have very tall emissions 

stacks that are known to disperse pollutants broadly across a large area. 

Therefore, this proximity analysis also included a second study area that consisted of 

those census blocks whose centriods lay within a 50 km (approximately 31.1 miles) radius of the 

affected sources. The radius of 50 km was used to approximate the distance from a source where 

elevated levels of non-mercury hazardous air pollutants such as arsenic and cadimum are most 

likely to occur. This radius also coincides with the maximum dispersion modeling radius used in 

the 16 case studies contained in Section III of the preamble for this rule. It should be noted that 

neither of these study areas (i.e., the 5 km or the 50 km radii circles) necessarily describe the 

individuals or communities with the greatest risk of exposure to mercury from the consumption 

of fish contaminated by the emissions from EGUs. 

During the course of the analysis, we determined that the centroids of some census blocks 

were within 5 km or 50 km of more than one coal-fired power plant. This meant that the analysis 

ran the risk of “double counting.” This was particularly problematic as the study areas expanded 

from 5 km to 50 km. We resolved this issue by counting each census block only once when 

aggregating the summary data for each study radius. The summary data presented in Table 3.1 

are based on "absolute" numbers that eliminate any double counting. 

3.0 SUMMARY OF THE DEMOGRAPHIC ANALYSES 

The proximity analyses indicate that All Other Races’share of the population living 

within a 5 km of coal-fired EGUs is higher than the national average. For these same areas, the 

percent of African Americans and the percent of the population below the poverty line are also 

 

1 Mohai P, Saha R. “Reassessing Racial and Socio-economic Disparities in Environmental Justice Research.” 

Demography. 2006; 43(2): 383–399.

2 Mohai P, Saha R. “Reassessing Racial and Socio-economic Disparities in Environmental Justice Research.” 

Demography. 2006; 43(2): 383–399. 
slightly higher than their respective national averages. The specific values for these 

demographic variables compared to their respective national averages are as follows: All Other 

Races - 29.2% v 24.9% respectively; African American - 15.8% v. 12.3% respectively; Hispanic 

- 15.5% v. 13.7%, respectively, and number below poverty level - 15.6% v. 13.1% respectively. 

The demographic characteristics of the population living within 50 km of coal-fired 

EGUs is different. Although the percent of All Other Races remains slightly above the national 

average (i.e., 25.5% v. 24.9%, respectively) as does percent African Americans (i.e., 15.2% v. 

12.3% respectively), the percent of Other and Multiraicial (including Native Americans), and the 

percent of the population living below the poverty line decrease below their respective national 

averages. In addition, it should be noted that the analysis indicates that 75% of Americans live 

within 50 km of at least one coal-fired EGU. (See Table 3.1 below.)

Table 3.1 COMPARATIVE SUMMARY OF THE DEMOGRAPHICS WITHIN 5 KM (3 MILES) 

AND 50 KM (31 MILES) RADII OF THE AFFECTED SOURCES 

Nationwide 

5 Km (3 mile) 

 Study Area 

50 Km (31.1 mile) 

 Study Area 

Total population 285,339,125 6,558,259 214,108,152 

Race by percent 

White 75.1 70.8 74.5 

All Other Races 24.9 29.2 25.5 

Race by percent 

White 75.1 70.8 74.5 

African American 12.3 15.8 15.2 

Native American 0.9 0.7 0.5 

Other and 

Multiracial 11.7 12.7 9.7 

Ethnicity by percent 

Hispanic 13.7 15.5 9.9 

Non-Hispanic 86.3 84.5 90.1 

Below Poverty Line by percent 

Below poverty level 13.1 15.6 11.6 

Above poverty level 86.9 84.4 88.4