Page 1 of 10

From: Lydia Wegman/RTP/USEPA/US

To: nfrey@omb.eop.gov

Cc: martin.karen@epa.gov, Scott Jenkins/RTP/USEPA/US@EPA, Joseph-J Dougherty/DC/USEPA/US@EPA, 

Beth Hassett-Sipple/RTP/USEPA/US@EPA, Susan Stone/RTP/USEPA/US@EPA, Alison 

Davis/RTP/USEPA/US@EPA, hannon.john@epa.gov, Steven Silverman/DC/USEPA/US@EPA

Date: 06/08/2012 05:02 PM

Subject: Revisions to the AQI section of PM NAAQS preamble

Nathan, 

Please find attached revisions to the AQI section of the PM NAAQS preamble. These revisions address the 

interagency comments we received both in meetings this week and in writing on June 6. We can discuss these 

edits during our Monday meeting on the secondary standard if any of the reviewers have questions about these 

revisions. Thanks and have a good weekend. 

Lydia
Page 2 of 10

I. Communication of Public Health Information

Sections 319(a)(1) and (3) of the CAA require the EPA to establish a uniform air quality index 

for reporting of air quality. These sections specifically direct the Administrator to ‘‘promulgate 

regulations establishing an air quality monitoring system throughout the United States which utilizes 

uniform air quality monitoring criteria and methodology and measures such air quality according to a 

uniform air quality index’’ and ‘‘provides for daily analysis and reporting of air quality based upon such 

uniform air quality index...’’ In 1979, the EPA established requirements for index reporting (44 FR 

27598, May 10, 1979). The requirement for State and local agencies to report the AQI appears in 40 

CFR 58.50 and the specific requirements (e.g., what to report, how to report, reporting frequency, 

calculations) are in appendix G to 40 CFR part 58.

Information on the public health implications of ambient concentrations of criteria pollutants is 

currently made available primarily through EPA’s AQI program. The current AQI has been in use since 

its inception in 1999.1It provides accurate, timely, and easily understandable information about daily 

levels of pollution (40 CFR 58.50). The AQI establishes a nationally uniform system of indexing 

pollution levels for ozone, carbon monoxide, nitrogen dioxide, PM and sulfur dioxide. The AQI is also 

recognized internationally as a proven tool to effectively communicate air quality information to the 

public. In fact, many countries have created similar indices based on the AQI. 

The AQI converts pollutant concentrations in a community’s air to a number on a scale from 0 to 

500. Reported AQI values enable the public to know whether air pollution levels in a particular location 

are characterized as good (0–50), moderate (51–100), unhealthy for sensitive groups (101– 150), 

unhealthy (151–200), very unhealthy (201–300), or hazardous (300–500). The AQI index value of 100 

typically corresponds to the level of the short-term (e.g., daily or hourly standard) NAAQS for each 

 

1

In 1976, the EPA established a nationally uniform air quality index, then called the Pollutant Standard 

Index (PSI), for use by State and local agencies on a voluntary basis (41 FR 37660, September 7, 1976). 

In August 1999, the EPA adopted revisions to this air quality index (64 FR 42530, August 4, 1999) and 

renamed the index the AQI. 
Page 3 of 10

pollutant. Below an index value of 100, an intermediate value of 50 was defined either as the level of the 

annual standard if an annual standard has been established (e.g., PM2.5, nitrogen dioxide), or as a 

concentration equal to one-half the value of the short-term standard used to define an index value of 100 

(e.g., carbon monoxide). An AQI value greater than 100 means that a pollutant is in one of the unhealthy 

categories (i.e., unhealthy for sensitive groups, unhealthy, very unhealthy, or hazardous) on a given day. 

An AQI value at or below 100 means that a pollutant concentration is in one of the satisfactory 

categories (i.e., moderate or good). Decisions about the pollutant concentrations at which to set the 

various AQI breakpoints that delineate the various AQI categories draw directly from the underlying 

health information that supports the NAAQS review.

Historically, state and local agencies have primarily used the AQI to provide general information 

to the public about air quality and its relationship to public health. For more than a decade, many states 

and local agencies, as well as the EPA and other Federal agencies, have been developing new and

innovative programs and initiatives to provide more information to the public, in a more timely way. 

These initiatives, including air quality forecasting, real-time data reporting through the AIRNow 

website2, and air quality action day programs, can serve to provide useful, up-to-date, and timely 

information to the public about air pollution and its effects. Such information will help individuals take 

actions to avoid or to reduce exposures to ambient pollution at levels of concern to them and can 

encourage the public to take actions that will reduce air pollution on days when levels are projected to be 

at levels of concern to local communities. Thus, these programs have significantly broadened the ways 

in which state and local agencies can meet the nationally uniform AQI reporting requirements, and are 

contributing to state and local efforts to provide community health protection and to attain or maintain 

compliance with the NAAQS. The EPA and state and local agencies recognize that these programs are 

interrelated with AQI reporting and with the information on the effects of air pollution on public health 

 

2

See http://www.airnow.gov/index.cfm?action=airnow.showlocal&cityid=204. 
Page 4 of 10

that is generated through the periodic review, and revision when appropriate, of the NAAQS.

In recognition of the proposed changes to the suite of primary PM2.5 standards summarized in 

section III.F above, the EPA proposes conforming changes to the AQI to be consistent with the proposed 

changes to the standards. The health effects information that supports the proposed decisions on the 

PM2.5 standards as discussed in section III.B above is also the basis for the proposed changes in the AQI 

discussed below in this section. The EPA intends to finalize conforming changes to the AQI in 

conjunction with the Agency’s final decisions on the primary annual and 24-hour PM2.5 standards, if 

revisions to such standards are promulgated. Note that, currently, we are cautioning members of 

sensitive groups at the AQI value of 100 at 35 μg/m3, 24-hour average, because this is the level that was 

provided in guidance (Harnett, 2009, Attachment B) which, in essence, superseded the AQI value of 100 

established in 1999 (64 FR 42530, August 4, 1999).3

With respect to an AQI value of 50, as discussed above, the historical approach is to set it at the 

same level of the annual standard, if there is one. This is consistent with the current AQI sub-index for 

PM2.5 and the EPA sees no basis for deviating from this approach. Thus, the EPA proposes to set an AQI 

value of 50 atwithin a range of 12 to 13 μg/m3, 24-hour average, the same level asconsistent with the 

proposed annual PM2.5standard. If the annual5 standard is set at a different level, an (section III.F). The

AQI value of 50 will be set at the level of the annual PM2.5 standard that is promulgated. 

With respect to an AQI value of 100, there are two general approaches that could be used to 

select the associated PM2.5 level. By far the most common approach, which has been used with the other 

sub-indices as discussed above, is to set an AQI value of 100 at the same level as the short-term 

standard. The EPA recognizes that some state and local air quality agencies have expressed a strong 

preference that the Agency set an AQI value of 100 equal to any short-term standard. These agencies 

typically express the view that this linkage is useful for the purpose of communicating with the public 

 

3

In addition, this guidance document recommended an AQI value of 150 at 55 μg/m3, 24-hour average, 

which is in use by many State and local agencies (Harnett, 2009, Attachment B).
Page 5 of 10

about the standard, as well as the health impacts associated with daily air quality. The EPA proposes to 

use this approach to set the AQI value of 100 at 35 μg/m3, 24-hour average, consistent with the proposal 

to retain the current 24-hour PM2.5 standard (section III.F).

The secondAn alternative approach is to directly evaluate the health effects evidence to select the

level for an AQI value of 100 which is the basis for advisories to individuals in sensitive groups. The 

EPA is consideringsoliciting comment on this alternative approach in recognition that, as proposed, the 

24-hour PM2.5 standard provides supplemental protection against effects associated with short-term 

exposures of PM2.5 by working in conjunction with the annual standard to reduce 24-hour exposures to 

PM2.5. The EPA recognizes that some state and local air quality agencies have expressed support for this 

alternative approach, when the annual standard provides some protection from effects associated with 

short-term exposures. This alternative approach of evaluating the health evidence directly resultswould

result in the consideration of a range of 35 to 30 μg/m3, 24-hour average, based on the discussion of the 

health information pertaining to the level of the 24-hour standard in section III.E.4 above. Setting an 

AQI value of 100 at the upper end of the range (35 μg/m3, 24-hour average)), which would be 

essentially equivalent to the proposed approach, would place more weight on the multi-city, short-term 

exposure studies that generally support retaining the current 24-hour standard level at 35 μg/m3in 

conjunction with lowering the level of the annual standard to within a range of 12 to 13 μg/m3. Setting 

an AQI value of 100 at the lower end of the range (30 μg/m3, 24-hour average) would place more weight 

on the limited evidence from one multi-city study (Bell et al., 2008) that reported positive and 

statistically significant effects with an overall 98th percentile value below 35 μg/m3, 24-hour average, in 

conjunction with an overall long-term mean concentration slightly less than 13 μg/m3(US EPA, 2011a, 

p. 2-84). 

Both of these approaches result in the consideration of setting an AQI value of 100 for the PM2.5

sub-index within the range of 35 to 30 μg/m3, 24-hour average. In making a final decision on the air 

quality level for an AQI value of 100, the EPA will first decide on the approach that is appropriate, and 
Page 6 of 10

then the level that is consistent with that approach. If the first approach is selected, the Agency will set 

the AQI value of 100 at the level of the 24-hour PM2.5 that is promulgated. If the second approach is 

selected, the Agency will base the decision to set the PM2.5 level for an AQI value of 100 by evaluating 

the health effects evidence. The EPA stronglyThe EPA encourages state and local air quality agencies 

that use the AQI to comment on both the approach and the level at which to set an AQI value of 100. 

With respect to an AQI value of 150, this level is based upon the same health effects information 

that informs the selection of the level of the 24-hour standard and the AQI value of 100. As noted above, 

in guidance the EPA indicated that a 55 μg/m3, 24-hour average, PM2.5 level should be used for an AQI 

value of 150, in conjunction with an AQI value of 100 at 35 μg/m3, 24-hour average (Harnett, 2009.,

Attachment B). In considering what level to propose for an AQI value of 150, we still believe that the 

health effects evidence indicates that the level of 55 μg/m3, 24-hour average, is appropriate to use in 

conjunction with an AQI value of 100 set at the proposed level of 35 μg/m3. Thus, if the EPA sets an 

AQI value of 100 at the PM2.5 level of 35 μg/m3, 24-hour average, the Agency proposes to set an AQI 

value of 150 at the PM2.5 level of 55 μg/m3, 24-hour average. If, however, the EPA decides to set an 

AQI value of 100 at 30 μg/m3, using one ofbased on the two approachesalternative approach discussed 

above, then the EPA would adjust an AQI value of 150 proportionally to the PM2.5 level of 50 μg/m3, 

24-hour average. The Agency’s approach to selecting the levels at which to set the AQI values of 100 

and 150 inherently recognizes that the epidemiological evidence upon which these decisions are based 

provides no evidence of discernible thresholds, below which effects do not occur in either sensitive

groups or in the general population, at which to set these two breakpoints. Therefore, EPA concludes the 

use of a proportional adjustment is appropriate. Thus, if the EPA sets an AQI value of 100 at the PM2.5

level of 35 μg/m3, 24-hour average, the Agency proposes to set an AQI value of 150 at the PM2.5 level 

of 55 μg/m3, 24-hour average. However, if the EPA sets an AQI value of 100 at the PM2.5 level of 30 

μg/m3, 24-hour average, the Agency proposes to set an AQI value of 150 at the PM2.5 level of 50 μg/m3, 

24-hour average. would be appropriate.
Page 7 of 10

The EPA also proposes to set an AQI value of 500 at 350 μg/m3, as discussed below, and 

proposes proportional revisions to the AQI breakpoints between the 150 and 500 levels. The proposed 

breakpoints are shown in Table 2 below. A review of the history of the AQI value of 500 for PM10 and 

of the AQI value of 500 for PM2.5 is useful background for explaining the basis for proposing to set an 

AQI value of 500 at 350 μg/m3, 24-hour average, for PM2.5. The current AQI value of 500 for PM10 was 

set in 1987 at the level of 600 μg/m3, 24-hour average, on the basis of increased mortality associated 

with historical wintertime pollution episodes in London (52 FR 24687 to 24688, July 1, 1987). Particle

concentrations during these episodes, measured by the British Smoke method, were in the range of 500 

to 1000 μg/m3. In the 1987 rulemaking that established the upper bound index value for PM10, the EPA

cited a generally held opinion that the British Smoke method measures PM with a cutpoint of 

approximately 4.5 microns (52 FR 24688, July 1, 1987). In establishing this value for PM10, we assumed 

that concentrations of PM10, which includes both coarse and fine particles, during episodes of concern, 

would be about 100 μg/m3higher than the PM concentration measured in terms of British Smoke (52 FR

24688, July 1, 1987). The upper bound index value of 600 μg/m3 was developed by selecting the lower 

end of the range of harmful concentrations during the historical wintertime pollution episodes in London 

(500 μg/m3) and adding a margin of 100 μg/m3to account for this measurement difference. The current 

PM2.5 concentration corresponding to an AQI value of 500 is 500 μg/m3, 24-hour average. This was 

proposed in 1998 and finalized in 1999 (63 FR 67829, December 9, 1998, and 64 FR 42530, August 4, 

1999, respectively). Because there were few PM2.5 monitoring data available at that time, the 

proposaldecision was based on the stated assumption that PM concentrations measured by the British 

Smoke method are approximately equivalent to PM2.5 concentrations. 

Table 2. Proposed Revisions toBreakpoints for PM2.5 Sub-Index

AQI Category Index Values Proposed Breakpoints

(μg/m3, 24-hour average)

Good 0 - 50 0.0 – (12.0 -13.0)

Moderate 51 - 100 (12.1 – 13.1) – 35.4
Page 8 of 10

Unhealthy for 

Sensitive Groups 101 - 150 35.5 – 55.4

Unhealthy 151 - 200 55.5 - 140.4

Very Unhealthy 201 - 300 140.5 - 210.4

Hazardous 301 - 400 210.5 - 280.4

401 - 500 280.5 - 350.4

In considering revisions to an AQI value of 500 for PM2.5, the EPA has taken note that the

proposed and final actions in 1998 and 1999 were based on an assumption of approximate equivalence 

between the British Smoke method and the current PM2.5 method. This assumption is not entirely

consistent with the view cited in 1987 that the British Smoke method has a cutoff of 4.5 microns (52 FR 

24688, July 1, 1987). Because of the dramatic increase in PM2.5 monitoring data since that rulemaking, 

the EPA considers it appropriate to set the AQI value of 500 for PM2.5 at a level below 500 μg/m3so that 

it is more consistent with the findings from the London episodes and with the AQI value of 500 for 

PM10. 

On average, about 60 percent of ambient PM10 measured in the northeastern, southeastern, and 

industrial midwest portions of the U.S. consists of PM2.5.

4 Using the 60 percent factor to scale the AQI 

value of 500 for PM10 (600 μg/m3) results in an AQI value of 500 for PM2.5 of about 350 μg/m3. The 

Agency believes it is appropriate that this scaling approach results in a PM2.5 value for the upper bound 

index value that is below the 500 μg/m3 concentration of PM4.5 by the British Smoke method. The EPA 

therefore proposes this level for the upper bound level of the PM2.5 sub-index of the AQI. The EPA 

solicits comment on this scaling approach for translating the AQI value of 500 for PM10 into an 

appropriate corresponding value for PM2.5

The EPA has used the historical wintertime pollution episodes in London to propose setting an 

AQI value of 500 for PM2.5 as described above because it is still the best available information --- there 

is little information about similar, more recent, air pollution episodes and associated impacts on 

 

4 More specifically, in 2001 to 2003, the median of the ratio of 24-hour PM2.5 and PM10 among 

monitoring sites in the northeastern, southeastern, and industrial midwest portions of the U.S to the 

annual average was between 0.60 and 0.65 (US EPA, 2005, Figure 2-19). 
Page 9 of 10

community health upon which to base a decision. However, the limited number of studies cited in the 

Integrated Science Assessment that evaluated wood smoke health impacts found effects such as 

cardiovascular morbidity and mortality (e.g., Delfino et al., 2009; Mar et al., 2000; Sarnat et al., 2008) as 

well as respiratory effects (e.g., Barregard et al., 2008; Delfino et al., 2009; Schreuder et al., 2006) at 

much lower levels (US EPA, 2009a, sections 6.2 and 6.6). These recent health studies provide additional 

qualitative support for lowering the air quality level associated with an AQI value of 500 to 350below 

the current value of 500 μg/m3and, therefore, also lowering the other levels in the range of AQI values 

between 150 and 500. 

For the intermediate breakpoints in the AQI between the values of 150 and 500, the EPA 

proposes PM2.5 concentrations that generally reflect a linear relationship between increasing index 

values and increasing PM2.5 values (Table 2 above). The available scientific evidence of health effects 

related to population exposures to PM2.5 concentrations between the level of the 24-hour standard and an 

AQI value of 500 suggest a continuum of effects in this range, with increasing PM2.5 concentrations 

being associated with increasingly larger numbers of people likely to experience such effects. The 

generally linear relationship between AQI values and PM2.5 concentrations in this range is consistent 

with the health evidence. This also is consistent with the Agency’s practice of setting breakpoints in 

symmetrical fashion where health effects information does not suggest particular levels.

In proposing to revise the 500 level for the AQI as described above, we note that the EPA is not 

proposing to establish a Significant Harm Level (SHL) for PM2.5. The SHL is an important part of air 

pollution Emergency Episode Plans, which are required for certain areas by CAA section 110(a)(2)(G) 

and associated regulations at 40 CFR 51.150, under the Prevention of Air Pollution Emergency Episodes 

program. The Agency believes that air quality responses established through an Emergency Episode 

Plan should be developed through a collaborative process working with State and Tribal air quality, 

forestry and agricultural agencies, Federal land management agencies, private land managers and the 

public. Therefore, if in future rulemaking EPA proposes revisions to the Prevention of Air Pollution 
Page 10 of 10

Emergency Episodes program, the proposal will include a SHL for PM2.5 that is developed in 

collaboration with these organizations. As discussed in the Air Quality Index Reporting Rule (64 FR 

42530), if such a rulemaking process results in a SHL that is different from the 500 level of the AQI for 

PM2.5, the AQI will be revised accordingly.