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From: Lydia Wegman/RTP/USEPA/US

To: nfrey@omb.eop.gov

Cc: martin.karen@epa.gov, Beth Hassett-Sipple/RTP/USEPA/US@EPA, Joseph-J 

Dougherty/DC/USEPA/US@EPA, hannon.john@epa.gov, Steven Silverman/DC/USEPA/US@EPA, Richard 

Wayland/RTP/USEPA/US@EPA, Pat Dolwick/RTP/USEPA/US@EPA, James 

Hemby/RTP/USEPA/US@EPA

Date: 06/12/2012 05:22 PM

Subject: Further revisions to AQI section of draft PM NAAQS preamble

Nathan, 

We have given further consideration to the comments from the FLMs and others on the Air Quality Index. Based 

on our further thinking, we have made revisions that we believe fully accommodate the concerns that the 

interagency reviewers have expressed. The attached section from the preamble reflects these changes 

compared to the interagency review draft. We'd appreciate it if you would distribute this to the interagency review 

group as soon as possible. Please let me know if you have any questions. Thanks. 

Lydia
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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. 
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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. 
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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 annual PM2.5 standards

summarized in section III.F above, the EPA proposes a conforming changes to the AQI to be consistent 

with the proposed changes to the annual 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, in which the current AQI value of 50 is set at 15 μg/m3, consistent with the level of the current 

primary annual PM2.5 standard. and t The EPA sees no basis for deviating from this approach in this 

review. 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 final 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, which is the basis for advisories to individuals in sensitive 

groups, 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 discussednoted above, is 

to set an AQI value of 100 at the same level as the short-term standard. The EPA recognizes that some 

 

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).
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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 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. This 

was the approach used in the 1999 rulemaking to set the AQI value of 100 at a level of 40 μg/m3 when 

the 24-hour standard level was 65 μg/m3.

4

 This alternative approach was used in the case of the PM2.5

sub-index because the annual and 24-hour PM2.5 standards set in 1997 were designed to work together, 

and the intended degree of health protection against short-term risks was not defined by the 24-hour

standard alone, but by the combination of the two standards working in concert. Indeed, at that time, the 

24-hour standard was set to provide supplemental protection relative to the principal protection provided 

by the annual standard. The EPA is consideringsoliciting comment on this alternative approach in 

recognition that, as proposed, the 24-hour PM2.5 standard is intended to continue to 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 in such a 

case, 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 

 

4 Currently, we are cautioning members of sensitive groups at the AQI value of 100 at 35 μg/m3

, 24-hour 

average, consistent with more recent guidance from EPA with regard to the development of State 

emergency episode contingency plans (Harnett, 2009, Attachment B).

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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 

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. The AQI value 

of 150 is currently set at a level of 65 μg/m3. 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, 5is appropriate to use in conjunction with an AQI value of 100 

 

5 We note that this level is consistent with the level recommended in the more recent EPA guidance

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

With respect to an AQI value of 500, a 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 
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this value for PM10, weEPA 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 set in the 1999 rulemaking 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 awere 

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

Unhealthy for 

Sensitive Groups

101 -

150

35.5 – 55.4

Unhealthy

151 -

200

55.5 - 140.4

Very 201 - 140.5 - 210.4

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Unhealthy 300

Hazardous

301 -

400

210.5 - 280.4

401 -

500

280.5 - 350.4

In considering whether it is appropriate to retain or revisesions to an the AQI value of 500 for 

PM2.5, the EPA has taken notes that the proposed and final actions in 1998 and 1999 rulemaking wasere

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), such that it would be reasonable 

to expect that a level of 500 μg/m3based on the British Smoke method would generally be equivalent to 

a somewhat lower level based on the current PM2.5 method. However, the EPA recognizes that 

regardless of measurement method, extremely high PM concentrations that would most likely be 

associated with combustion sources (e.g., coal burning in historic the London event, wildfires in 

contemporary U.S. environments) are typically dominated by fine particles, such that there may be very 

little difference between these measurement methods. To further explore this issue, Because of the 

dramatic increase in PM2.5 monitoring data since that rulemaking, the EPA concludes that considers it 

appropriate to consider the extent to which more recent air quality data can inform consideration of the 

appropriate level 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. 

In considering more recent air quality data, EPA notes that oOn average, about 60 percent of 

ambient PM10 measured in the northeastern, southeastern, and industrial midwest portions of the U.S. 

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consists of PM2.5.

6 Using the 60 percent factor to scale the AQI value of 500 for PM10 (600 μg/m3

) 

would results in an AQI value of 500 for PM2.5 of about 350 μg/m3.

7 On the other hand, the EPA 

recognizes that the ratio between PM10 and PM2.5 based on such regional average data is not likely to be 

representative of the ratio at the atypically high concentrations that may occur in some areas of the 

country, such as those associated with wildfires and biomass burning, that would be relevant to consider 

in conjunction with the range of AQI values between 300 and 500 that defines the “hazardous” category.

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

Further, in considering the body of more recent health effects evidence available in this review, 

tThe EPA concludes that there is little information about more recent air pollution episodes similar to 

the wintertime pollution episodes in London and associated impacts on community health upon which to 

base a decision. Thus, the EPA concludes that it remains appropriate to has used the historical 

wintertime pollution episodes in London as the basis for to propose ssetting an AQI value of 500 for 

PM2.5 as described above because it is still the best available directly relevant information --- there is

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

community health upon which to base a decision. HoweverNonetheless, the EPA takes note of thea

limited number of studies cited in the Integrated Science Assessment that evaluated wood smoke health 

impacts which 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 

 

6 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). 

7 We note that this level is consistent with the level recommended in the more recent EPA guidance

(Harnett, 2009, Attachment B).

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al., 2009; Schreuder et al., 2006), albeit at much lower levels (US EPA, 2009a, sections 6.2 and 6.6). 

The EPA believes that tThese recent health studies may provide some additional qualitative support for

considering 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.

Based on the above considerations, the EPA provisionally concludes that it is appropriate to 

propose both to retain the current level of 500 μg/m3for the AQI value of 150 and, in the alternative, to 

propose to revise the level to within a range from below 500 μg/m3down to a level of 350 μg/m3. A 

decision to retain the current level would reflect placing significant weight on the uncertainties 

associated with characterizing an appropriate scaling factor to translate the historic London air quality 

data into corresponding PM2.5 levels, and on the uncertainties in translating the more recent health 

effects studies, conducted at relatively low PM2.5 levels, into a basis for selecting a level for an AQI 

value of 500. A decision to lower the level for the AQI value of 500 to within the proposed range would 

reflect placing more weight on the overall body of recent health effects information and on studies 

available in this review that specifically examine health impacts related to exposure to wood smoke. 

The EPA solicits comment on whether it is appropriate to retain or revise the current level of 500 μg/m3

for the AQI value of 500. To the extent that support is expressed for revising the current level to a level 

within the proposed range, the EPA solicits comment on the recommended level and on the rationale 

that would support such a level. The EPA also solicits any additional information that may be useful to 

inform a final decision on the appropriate level to set the AQI value of 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 

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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.

Based on the proposed levels discussed above, Table 2 below summarizes the proposed 

breakpoint for the PM2.5 subindex. Table 2 shows the proposed level of 500 μg/m3for the AQI value of 

500, which is consistent with retaining the current AQI value, and the associated intermediate 

breakpoints between AQI values of 150 and 500. Intermediate breakpoints that would be associated with 

an AQI value of 500 set a different level within the range proposed in the alternative would be 

calculated based on a linear relationship between the selected levels for AQI values of 150 and 500.

Table 2. Proposed Breakpoints 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

Unhealthy for 

Sensitive Groups 101 - 150 35.5 – 55.4

Unhealthy 151 - 200 55.5 - 150.4

Very Unhealthy 201 - 300 150.5 - 250.4

Hazardous 301 - 400 250.5 - 350.4

401 - 500 350.5 - 500.4

In proposing to retain or 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 

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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 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; August 4, 1999), 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.