OAQPS Issue Paper for Discussion at National Air Quality Conference, February 2007 

Revising the Air Quality Index and Setting a Significant Harm Level for PM2.5 

February 12, 2007 

Introduction 

In October 2006, EPA revised the national ambient air quality standards for PM2.5 

by strengthening the 24-hour standard to protect public health against effects associated 

with short-term exposure to fine particles (PM2.5). These effects include: premature death 

in people with heart and lung disease; non-fatal heart attacks; increased hospital 

admissions, emergency room visits and doctor’s visits for respiratory diseases; increased 

hospital admission and ER visits for cardiovascular diseases; increased respiratory 

symptoms such as coughing, wheezing and shortness of breath; lung function changes, 

especially in children and people with lung diseases such as asthma; changes in heart rate 

variability; and irregular heartbeat. 

The purpose of this issue paper is to provide our initial thoughts about possible 

revisions to the Air Quality Index (AQI) sub-index for PM2.5, reflecting this 

strengthening of the 24-hour standard. We are also providing our initial thoughts about 

setting a Significant Harm Level (SHL) for PM2.5, which historically is set at the same 

level as an AQI value of 500. We intend to encourage discussion and comment about 

these changes among the many agencies and partners that use the AQI on a daily basis to 

communicate with the public about daily air quality and any associated health risks. We 

expect these discussions to occur over the next few months, starting at the National Air 

Quality Conference in February, and to lead to the publication of a summer 2007 

proposed rule to revise the AQI and to set an SHL for PM2.5 with a final rule in the fall of 

2007. To facilitate these discussions we have identified a few key questions which can 

be found in this paper. The responses to these questions will provide information useful 

to the rulemaking. At the end of this issue paper there is a list of key EPA contacts and 

links to supplemental information. 

Possible revisions to the AQI value of 100 and the lower end of the range 

In 1999, EPA adopted revisions to the uniform air quality index used by States for 

daily reporting to the general public in accordance with section 319 of the Clean Air Act. 

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OAQPS Issue Paper for Discussion at National Air Quality Conference, February 2007 

The changes included, among others, the addition of a new sub-index for PM2.5. The new 

sub-index for PM2.5 was developed using an approach that was conceptually consistent 

with past practice for selecting the air quality concentrations associated with the AQI 

breakpoints. The Agency’s historical approach to selecting index breakpoints had been 

to simply set the AQI value of 100 at the level of the short-term standard for a pollutant 

and the AQI value of 50 at the level of the annual standard, if there is one, or at one-half 

the level of the short-term standard if there is not. This method of structuring the index 

may be appropriate for a typical suite of air quality standards, which include a short-term 

standard designed to protect against the health effects associated with short-term 

exposures and an annual standard designed to protect against health effects associated 

with long-term exposures. In such cases the short-term standard in effect defines a level 

of health protection provided against short-term risks and thus can be a useful benchmark 

against which to compare daily air quality concentrations. 

In the case of the 1997 PM2.5 standards, we took a different approach to protecting 

against the health risks associated with short-term exposures. The intended level of 

protection against short-term risk was not defined by the 24-hour standard but by the 

combination of the 24-hour and the annual standards working in concert. In fact, the 

annual standard was intended to serve as the principal vehicle for protecting against 

short-term PM2.5 exposures by lowering the entire day-by-day distribution of PM2.5 

concentrations in an area. Because the 24-hour standard served to provide additional 

protection against very high short-term concentrations, localized “hotspots” or risks 

arising from seasonal emissions that would not be well-controlled by a national annual 

standard, we consequently concluded that it would be appropriate to caution members of 

sensitive groups exposed to concentrations below the level of the 24-hour standard. We 

also concluded that it would be inappropriate to compare daily air quality concentrations 

directly with the level of the annual standard by setting an AQI value of 100 at that level. 

We wanted to set the AQI value of 100 to reflect the general level of health protection 

against short-term risks offered by the annual and 24-hour standards combined, consistent 

with the underlying logic of the historical approach. We concluded that it was 

appropriate to set the AQI value of 100 at the midpoint of the range between the annual 

and the 24-hour PM2.5 standards (i.e., 40 μg/m3

) to reflect the dual role of the 24-hour and 

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OAQPS Issue Paper for Discussion at National Air Quality Conference, February 2007 

the annual PM2.5 standards in protecting against short-term risks. Given that decision, we 

also concluded that it was appropriate to retain the approach of using the level of the 

annual standard for an AQI value of 50 and of setting the AQI value of 150 at the level of 

the 24-hour PM2.5 standard (see Table 1). 

In 2006, EPA revised its national ambient air quality standards for PM2.5 by 

strengthening the 24-hour standard to 35 μg/m3

. We used a significantly expanded and 

stronger body of evidence from short-term exposure PM2.5 studies as the principal basis 

for selecting the level of this standard, which is aimed at protecting against health effects 

associated with short-term exposures to PM2.5. Numerous U.S. and Canadian studies 

were available that provided evidence of associations between short-term exposure to 

PM2.5 and serious health effects in areas with air quality at and above the level of the 

current annual PM2.5 standard (15 μg/m3

). The standard level we established generally 

will require improvements in air quality in areas in which the distribution of daily shortterm exposure to PM2.5 can reasonably be expected to be associated with serious health 

effects. In setting the level of the 24-hour PM2.5 standard at 35 μg/m3

, we noted that this 

standard would protect public health with an adequate margin of safety from serious 

health effects, including premature mortality and hospital admissions for 

cardiorespiratory causes that are associated with short-term exposure to PM2.5. 

With the promulgation of the 2006 24-hour PM2.5 standard, the short-term 

standard defines a level of health protection provided against short-term risks that again 

serves as an appropriate benchmark against which to compare daily air quality 

concentrations. As a result, it is appropriate to consider setting an AQI value of 100 

equal to the level of the 24-hour PM2.5 standard (35 μg/m3

). We believe it is appropriate 

to consider reducing the AQI value of 150 (now 65 μg/m3

) in proportion to the reduction 

in the AQI value of 100 (to 55 μg/m3

 – rounded to the nearest 5 μg/m3). This conclusion 

is based on the more extensive and stronger body of evidence linking 24-hour PM2.5 

concentrations with serious morbidity and mortality effects, including: premature death 

in people with heart and lung disease; non-fatal heart attacks; increased hospital 

admissions, emergency room visits and doctor’s visits for respiratory diseases; increased 

hospital admission and ER visits for cardiovascular diseases; increased respiratory 

symptoms such as coughing, wheezing and shortness of breath; lung function changes, 

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OAQPS Issue Paper for Discussion at National Air Quality Conference, February 2007 

especially in children and people with lung diseases such as asthma; changes in heart rate 

variability; and irregular heartbeat. We have also concluded that it is appropriate to 

retain the level of the annual standard for an AQI value of 50 (15 μg/m3

). 

Possible revisions to the AQI value of 500 and the upper end of the range (including 

setting an SHL for PM2.5) 

Historically, the AQI value of 100 is set at the level of the short-term standard for 

a pollutant and the AQI value of 50 at the level of the annual standard, if there is one, or 

at one-half the level of the short-term standard if there is not. The upper bound index 

value of 500 corresponds to the Significant Harm Level (SHL), established in section 

51.16 of the CFR under the Prevention of Air Pollution Emergency Episodes program. 

The SHL is set at a level that represents imminent and substantial endangerment to public 

health. We have not yet promulgated an SHL for PM2.5. In 1999, we set an index value 

of 500 equal to 500 μg/m3PM2.5, 24-hour average, and indicated that if we promulgated 

an SHL for PM2.5 and it was different, we would revise the PM2.5 sub-index accordingly 

(63 FR 67829). This level was established on the basis of increased mortality found 

during historical wintertime pollution episodes in London, where PM concentrations 

measured as British Smoke were in the 500 to 1000 μg/m3 range. British Smoke 

provided an approximate measurement of fine particles, since it is considered to measure 

PM with a cut-point of approximately 4.5 microns. These measurements and health 

rationale are also the basis for the current SHL for PM10 (600 μg/m3

). 

At that time, we considered using the proportion of the PM10 SHL (600 μg/m3) 

that comprises PM2.5 to set the 500 level of the AQI, but we did not have enough PM2.5 

monitoring data to establish a ratio (PM2.5 to PM10) for that purpose. Since then there has 

been a significant increase in PM2.5 monitoring, and we now have sufficient PM2.5 

monitoring data to be able to estimate that in the United States, on average, 60 percent of 

PM10 consists of PM2.5. This allows us to scale the SHL for PM2.5 to the SHL for PM10, 

rather than basing the PM2.5 SHL on measurements of British Smoke, and to maintain the 

health-based rationale that we used for setting the 500 level of the AQI and the PM10 

SHL. Using this approach results in a PM2.5 SHL of 350 μg/m3

 (rounded to the nearest 

50 μg/m3). 

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OAQPS Issue Paper for Discussion at National Air Quality Conference, February 2007 

For the intermediate breakpoints in the AQI between the values of 100 and 500, 

the PM2.5 concentrations under consideration generally reflect a linear relationship 

between increasing index values and increasing PM2.5 values (see Table 1). The available 

scientific evidence of health effects related to population exposures to PM2.5 

concentrations between the level of the 24-hour standard and the level of the SHL under 

consideration suggest a continuum of effects in this range, with increasing PM2.5 

concentrations being associated with increasingly large 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 our practice of setting breakpoints in symmetrical fashion where health effects 

information does not suggest particular levels. 

We have used the historical wintertime pollution episodes in London because 

there is little available information about similar, more recent, air pollution episodes and 

associated impacts on community health upon which to base a decision about an SHL for 

PM2.5. In light of this, we are encouraging state and local air agencies to send us 

information about air quality measurements and associated public health impacts, if 

available, related to episodes of high PM2.5 levels. We have compared the level of the 

SHL for PM2.5 under consideration with air quality data (October 2003 to October 2006) 

in the AQS database. We looked at all appropriate data for this analysis. AIRNow data 

collected using non-reference method technologies were used to fill in for missing FRM 

data as necessary. Individual site analysis required 75 percent data completeness resulting 

in 845 observations out of 1137 possible. During these three years, only one monitor 

exceeded the level of the PM2.5 SHL (350 μg/m3

) now under consideration, and that 

episode was related to wildfires. 

As a related matter, we recognize that it would be quite useful to have a public 

advisory system based on a shorter averaging period (i.e., 1-hour PM2.5 levels) for use 

during PM2.5 episodes (such as wildfires), when there can be extremely high short-term 

peaks of PM2.5 that aren't well communicated by advisories based on 24-hour average 

PM2.5 concentrations. We intend to issue guidance that air agencies can use during PM2.5 

episodes. There is a growing body of studies that provide additional evidence of effects 

associated with exposure periods shorter than 24 hours (e.g., one to several hours) 

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OAQPS Issue Paper for Discussion at National Air Quality Conference, February 2007 

(OAQPS PM Staff Paper, December 2005, section 3.5.5.1). While the PM Staff Paper 

concluded that this information remains too limited to serve as a basis for establishing a 

shorter-than-24-hour PM2.5 primary standard, this information will be useful in helping 

develop guidance that can be used during PM2.5 episodes. Some agencies already issue 

advisories based on 1-hour PM2.5 levels. We are requesting that agencies using such 

advisories send us information about the approach used and including associated 

strengths and weaknesses. 

Possible Implications for Air Quality Forecasting 

For many areas around the country, lowering the AQI thresholds will result in 

more days reaching an AQI equal to 100 which is considered Unhealthy for Sensitive 

Groups (USG). In addition, areas that currently do not reach USG levels may do so once 

the threshold is lowered. Therefore, a lower threshold could impact existing forecasting 

programs and may necessitate that additional cities develop forecasting programs. It is 

EPA’s goal to conduct analyses to quantify these impacts. Most, if not all, of the 

following questions will be answered by EPA to serve as information for state and local 

forecast programs as well as a basis for discussion: 

$	 How will the frequency distribution of AQI levels change by season with the 

lowering of thresholds? 

$	 Which cities may want to consider implementing a forecasting program? For 

existing forecast programs, will PM2.5 forecasts need to be extended into 

additional seasons and/or made at a greater frequency (e.g. daily versus weekly). 

$	 How much time will be required to forecast with additional action days and 

additional "close-call" days? 

A report will be available during the 2nd quarter of 2007 with answers to the 

above questions along with conclusions and recommendations. 

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OAQPS Issue Paper for Discussion at National Air Quality Conference, February 2007 

Table 1. Current breakpoints and possible revisions to PM2.5 sub-index 

AQI Category Index Values 

PM2.5 24-hour (μg/m3

) 

Current Under Consideration 

Good 0 - 50 0.0 - 15.4 0.0 - 15.4 

Moderate 51 - 100 15.5 - 40.4 15.5 - 35.4 

Unhealthy for 

Sensitive 

Groups 

101 - 150 40.5 - 65.4 35.5 - 55.4 

Unhealthy 151 - 200 65.5 - 150.4 55.5 - 140.4 

Very 

Unhealthy 

201 - 300 150.5 - 250.4 140.5 - 210.4 

Hazardous 301 - 400 250.5 - 350.4 210.5 - 280.4 

401 - 500 350.5 - 500 280.5 - 350.4 

Contacts
 

AIRNow: John White (white.johne@epa.gov) 

Air Quality: Mike Rizzo (rizzo.michael@epa.gov) 

Communications: Alison Davis (davis.alison@epa.gov) 

Forecasting: Scott Jackson (jackson.scott@epa.gov) 

Health: Susan Lyon Stone (stone.susan@epa.gov) 

Monitoring: Lewis Weinstock (weinstock.lewis@epa.gov) 

Supplemental Information
 

Air Quality Index Reporting Proposed Rule (December 9, 1998): 

http://epa.gov/airnow/health/aqi_proposal_1998.pdf 

Air Quality Index Reporting Final Rule (August 4, 1999): 

http://www.epa.gov/ttn/oarpg/t1/fr_notices/airqual.pdf 

National Ambient Air Quality Standards for Particulate Matter Proposed Rule (January 

17, 2006): 

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OAQPS Issue Paper for Discussion at National Air Quality Conference, February 2007 

http://a257.g.akamaitech.net/7/257/2422/01jan20061800/edocket.access.gpo.gov/2006/pd 

f/06-177.pdf 

National Ambient Air Quality Standards for Particulate Matter Final Rule (October 17, 

2006): 

http://a257.g.akamaitech.net/7/257/2422/01jan20061800/edocket.access.gpo.gov/2006/pd 

f/06-8477.pdf 

Review of the National Ambient Air Quality Standards for Particulate Matter: OAQPS 

Staff Paper (December 2005): 

http://www.epa.gov/ttn/naaqs/standards/pm/data/pmstaffpaper_20051221.pdf 

Particle Pollution and Your Health: http://www.epa.gov/airnow//particle/pm-color.pdf 

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