EJ and Supplemental Indexes in EJScreen

EJ & Supplemental Indexes
What the EJ Index Means
How the EJ Index Works
What the Supplemental Index Means
How the Supplemental Index Works

EJ & Supplemental Indexes

The EJ and supplemental indexes are a combination of environmental and socioeconomic information. There are twelve EJ indexes and supplemental indexes in EJScreen reflecting the 12 environmental indicators. The 12 EJ index and supplemental index names are:

Particulate Matter 2.5
Ozone
Diesel Particulate Matter
Air Toxics Cancer Risk
Air Toxics Respiratory Hazard Index
Traffic Proximity
Lead Paint
RMP Facility Proximity
Hazardous Waste Proximity
Superfund Proximity
Underground Storage Tanks
Wastewater Discharge

What the Environmental Justice Index Means

An EJ index combines demographic factors with a single environmental factor. For example, the EJ index for traffic is a combination of the following populations residing in the Census block group:

The traffic indicator
The low-income population
The people of color populations

Note that an EJ index does not combine various environmental factors into a cumulative score -- each environmental indicator has its own EJ index.

The EJ index is higher in block groups with large numbers of mainly low-income and/or people of color residents with a higher environmental indicator value.

How the EJ Index Works

To calculate a specific EJ index, EJScreen uses a formula to combine a single environmental factor with the demographic index (which averages low income and people of color populations).

EJScreen calculates the EJ index by multiplying together three items:

EJ index =

(The Environmental Indicator Percentile for Block Group)
X (Demographic Index for Block Group)

What the Supplemental Index Means

The supplemental indexes offer a different perspective on community-level vulnerability. The supplemental indexes use the same EJScreen methodology but incorporate a five-factor supplemental demographic index (as opposed to the two factor demographic index which averages low income and people of color populations). The supplemental demographic index averages:

% Low Income
% Unemployed
% Limited English Speaking
% Less than High School Education
Low Life Expectancy

The supplemental demographic index is then combined with a single environmental indicator, to display areas with the highest intersection between these socioeconomic factors and the environmental indicator.

The supplemental indexes provide flexibility in the ways the data can be considered within EJScreen. They also increase EJScreen’s functionality and may be more relevant for use in certain situations, such as awarding grants.

How the Supplemental Index Works

To calculate a single supplemental index, EJScreen uses a formula to combine a single environmental factor with the supplemental demographic indicator (which averages five socioeconomic factors: low income, unemployment, limited English, less than high school education, and low life expectancy).

EJScreen calculates the supplemental index by multiplying together two items:

Supplemental Index =

(The Environmental Indicator Percentile for Block Group)
X (Supplemental Demographic Index for Block Group)

Example 1:
Suppose that in a certain block group of 1000 people, 350 (35%) are minority and 350 (35%) low-income. There might be 200 (20%) who are low-income but not minority, and 200 (20%) who are minority but not low-income. In that case, there would be 150 (15%) who are both, and 450 (45%) who are neither. Therefore, there actually would be 550 (55%) who were either minority, low-income or both.
The Demographic Index would use 35% in this case, which falls between the 15% who were both minority and low-income, and the 55% who were in at least one of these groups. These detailed numbers cannot be obtained from the ACS by block group. Therefore, to represent both groups in a simple way, the average is used.

Example 2 (extreme example):
Suppose a block group has 1000 people but is 0% minority and 100% low-income. The demographic index would be 50%, or the equivalent of 500 "potentially susceptible individuals" in this case. The same would be true in a block group that was 100% minority but 0% low-income – it would treated as having the equivalent of 50% (500) "potentially susceptible" for the sake of these examples.

¹ To be precise, the percent low-income times population is not always exactly the same as the count of low-income residents. The percent low-income is calculated as a fraction of those for whom poverty status could be determined, which is less than the full population in some block groups. As an approximation, these examples gloss over that detail.

² The closest available data would be Census table B17001 and related tables in the ACS, which provide tract resolution cross-tabulations of race/ethnic groups by poverty status, but this is not available for block groups and does not provide the income to poverty ratio data needed to calculated "low-income" as defined in EJScreen.