Table 1 PSI values and health descriptors
Pollutant concentrations Index value
Health effects descriptor
d
Air quality level
NO
2
1 h O
3
1 h PM
10
24 h SO
2
24 h CO 8 h
ppm ppm
mgm
3
mgm
3
ppm 2.0
Hazardous 500
Significant 0.6
50 600
2620 harm
1.6 400
Emergency 500
2100 40
0.5 1.2
300 Warning
420 1600
30 0.4
Very unhealthy 0.6
200 Unhealthy
Alert 0.2
350 800
15 0.12
a
Moderate 100
NAAQS
b
150 365
9 Good
a
50 50 of
4.5 0.06
50
c
80
c
NAAQS
b a
a
No index values reported at concentrations below ‘Alert’ levels. The applicable short term NAAQS for NO
2
is 0.053 ppm EPA 1998, table 2.1, p. 7.
b
National Ambient Air Quality Standards.
c
Annual primary NAAQS.
d
Refers to human health only. For more details, see EPA 1994. Source: EPA 1998, p. 62.
of pollution on all populations and materials. The limited current focus helps keep the empirical
analysis tractable. The paper is organized as follows. Following
Section 1, Section 2 critically reviews the method- ology underlying the PSI. Section 3 presents an
improved index of environmental pollution. Sec- tion 4 illustrates the index with data on air quality
indicators for 135 counties and Metropolitan Statistical Areas in the United States and com-
pares the results with the PSI. Section 5 concludes with suggestions for further research.
2. The PSI
The PSI measures daily air pollution levels due to the five gases for which the EPA has estab-
lished National Ambient Air Quality Standards NAAQS. These are large particulate matter
PM
10
, sulphur dioxide SO
2
, carbon monoxide CO, ground level ozone O
3
, and nitrogen diox- ide NO
2
. The index combines the NAAQS with an epidemiological function to determine a de-
scriptor of human health effects due to short-term exposure 24 h or less to each pollutant EPA,
1994, 1998. It translates the short-term concen- trations of the five gases into a number between 0
and 500 that indicates the source of the highest level of pollution experienced on each day. In
other words, the daily index value is determined by the gas with highest concentration relative to
the society-wide dose response function. PSI val- ues and the associated health descriptors are
shown in Table 1.
1
The EPA tracks air quality by the number of days per year for which PSI values
exceed 100 in urban areas with populations greater than 200 000.
The PSI provides a good rule for determining whether a region is polluted or not. If the ambient
concentration of any one of the five pollutants exceeds its NAAQS, the PSI value exceeds 100
and the region may be considered polluted. How- ever, the EPA also uses the PSI to determine how
polluted a region is vis-a`-vis other regions, and how the air quality of regions changes over time.
For these purposes, the PSI is problematic.
1
There is no PSI value for lead since the EPA does not have a short-term NAAQS, a Federal Episode Criteria, or a Signifi-
cant Harm level for it EPA, 1998, p.62.
Table 2 1997 ambient concentrations and Pollutant Standards Index PSI values selected counties
SO
2
24 h O
3
1 h, 2nd Max PM
10
24 h, 99th p’ile
d
State County
CO 8 h NO
2
1 h, AM
c
ppm ppm
mgm
3
ppm mgm
3
Ambient concentrations in
1997
a
2.2 0.0193
0.131 79
PA 65.5
Montgomery 10.48
129 0.097
0.0178 4.4
AZ Pima
107.42 0.113
99 IL
Cook 5.3
0.0336 PSI 6alues for each gas
b
100–200 50–100
PA Montgomery
50–100 50–100
AZ Pima
50–100 50–100
50–100 IL
Cook 50–100
50–100 PSI 6alue for county
b
100–200 PA
Montgomery AZ
Pima 50–100
IL Cook
50–100
a
Source: EPA 1998, Table A-12, pp. 122–139. The data for SO
2
are reported in ppm. They were converted to mgm
3
using the EPA’s conversion factor of 1 ppm = 2620 mgm
3
D. Mintz, personal communication, March 29, 1999.
b
Own calculations based on Table 1.
c
Arithmetic mean.
d
99th percentile.
Consider the three counties shown in Table 2. The data in this table represent the composite
average of 1997 readings for these counties at various monitoring sites. They are interpreted to
represent the air quality on a typical day in each of these counties.
2
PSI values are calculated on the basis of Table 1. According to this index, Cook and
Pima Counties are equally polluted whereas Mont- gomery County has worse air quality. However,
note that the PSI is in the 50 – 100 range for all five gases in the case of Cook County, but only for two
gases in case of Pima. Furthermore, the ambient concentrations of all gases except O
3
are lower in Montgomery County as compared to the other two
counties. Yet, according to the PSI, Montgomery is the most polluted of the three counties since
the index value is determined entirely by its O
3
concentration, which puts it in the 100 – 200 range regardless of the ambient concentration of the other
gases. The aim of the paper is to develop an index that
is based on the level of each pollutant, their individual physical impacts, and the consequent
welfare losses while building upon the framework established under the PSI. The welfare losses
provide the common metric in terms of which of the ambient concentrations of different environ-
mental indicators may be aggregated into an over- all pollution index.
3. An improved pollution index
