EJ Indexes

The EJ index is a combination of environmental and demographic information. There are eleven EJ Indexes in EJSCREEN reflecting the 11 environmental indicators. The 11 EJ Index names are:

1. National Scale Air Toxics Assessment Air Toxics Cancer Risk
2. National Scale Air Toxics Assessment Respiratory Hazard Index
3. National Scale Air Toxics Assessment Diesel PM (DPM)
4. Particulate Matter (PM2.5)
5. Ozone
6. Lead Paint Indicator
7. Traffic Proximity and Volume
8. Proximity to Risk Management Plan Sites
9. Proximity to Treatment Storage and Disposal Facilities
10. Proximity to National Priorities List Sites
11. Wastewater Discharge Indicator

What the Environmental Justice Index Means

EJSCREEN provides reports and maps that can show each environmental indicator and each demographic indicator, one at a time. To summarize how an environmental indicator and demographics come together in the same location, EJSCREEN uses an EJ Index.

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 minority population

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 minority residents with a higher environmental indicator value.

For each environmental factor, the EJ Index finds the block groups that contribute the most toward the nationwide disparity in that environmental factor. By "disparity" in this case we mean the difference between the environmental indicator's average value among these demographic groups and the average in the US population.

How the EJ Index Works

• Demographics in the EJ Index
• Excess Risk

To calculate a single EJ Index, EJSCREEN uses a formula to combine a single environmental factor with the demographic indicator. It considers how much the local demographics are above the national average. It does this by looking at the difference between the demographic composition of the block group, as measured by the Demographic Index, and the national average (which is approximately 35%). It also considers the population size of the block group, although most block groups are similar in population size.

EJSCREEN calculates the EJ Index by multiplying together three items:

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Demographics in the EJ Index

The demographic portions of the EJ Index can be thought of as the additional number of susceptible individuals in the block group, beyond what you would expect for a block group with this size total population. The terms "susceptible" or "potentially susceptible individuals" are used informally in these examples, as a way to think of the Demographic Index times the population count in a block group. This is essentially the average of the count of minorities and count of low-income individuals¹. It is easiest to think of the average of these counts as "the susceptible individuals" in these examples.

The number of potentially susceptible individuals (Demog. Index times population count) of course is typically less than the actual number who are minority, low-income or both. The demographic breakdown is not reported by block group –the ACS does not provide that level of resolution on the overlaps².

• View Examples

  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.

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Excess Risk

The EJ Index uses the concept of "excess risk" by looking at how far above the national average the block group's demographics are. For example, assume a block group with 1000 people in it. In that block group, one would expect 350 potentially susceptible individuals (1000 people here x US average of 35%). However, if the Demographic Index for that block group is 75%, well above the US average, then there are the equivalent of 750 potentially susceptible people in that block group, or 400 more than expected for a block group with a population of 1000.

This formula for the EJ Index is useful because for each environmental factor it finds the block groups that contribute the most toward the national disparity in that environmental factor. It can highlight which locations are driving the overall net disparity. By "disparity" in this case we mean the difference between the environmental indicator’s average value among certain demographic groups and the average in the rest of the US population.

Minority and low-income individuals live in older housing more often than the rest of the US population, for example. The EJ Index for lead paint (pre-1960 housing) tells us how much each block group contributes toward this "excess population risk" or "excess number" of people in older housing, for potentially susceptible individuals. "Excess" here simply means the number of potentially susceptible individuals in older housing is above what it would be if they were in older housing at the same rate as the rest of the U.S. population.