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

Revising the Air Quality Index and Se tting a Significant Harm Level for PM 2.5 February 12, 2007

Introduction In October 2006, EPA revised the nationa l ambient air quality standards for PM 2.5 by strengthening the 24-hour standard to prot ect public health agains t effects associated with short-term exposure to fine particles (PM 2.5 ). 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 cardiov ascular diseases; increased respiratory symptoms such as coughing, wheezing and shor tness 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 pr ovide our initial t houghts about possible revisions to the Air Quality Index (AQI) sub-index for PM 2.5 , reflecting this strengthening of the 24-hour st andard. We are also providi ng our initial thoughts about setting a Significant Harm Level (SHL) for PM 2.5 , 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 part ners that use the AQI on a daily basis to communicate with the public about daily air qua lity and any associated health risks. We expect these discussions to occur over the next few months, starti ng 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 PM 2.5 with a final rule in the fall of
2007. To facilitate these discussions we ha ve identified a few key questions which can be found in this paper. The responses to th ese 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 unifo rm air quality index used by States for daily reporting to the general pub lic in accordance with section 319 of the Clean Air Act.

1OAQPS Issue Paper for Discussion at Nati onal Air Quality Conference, February 2007

The changes included, among others, the addition of a new sub-index for PM 2.5 . The new
sub-index for PM 2.5 was developed using an approach that was conceptually consistent with past practice for selecting the air qual ity concentrations associated with the AQI breakpoints. The Agency’s hist orical approach to selecti ng 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, whic h include a short-term standard designed to protect against the h ealth 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 s hort-term standard in effect defines a level of health protection provided against short-term risks and t hus can be a useful benchmark against which to compare daily air quality concentrations. In the case of the 1997 PM 2.5 standards, we took a different approach to protecting against the health risk s associated with short-term e xposures. 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 stan dards working in concert. In fact, the
annual standard was intended to serve as th e principal vehicle for protecting against
short-term PM 2.5 exposures by lowering the entire day-by-day distribution of PM 2.5 concentrations in an area. Because the 24- hour standard served to provide additional protection against very high s hort-term concentrations, loca lized “hotspots” or risks arising from seasonal emissions that would not be well-c ontrolled by a national annual standard, we consequently conc luded that it would be appropr iate to caution members of sensitive groups exposed to c oncentrations below the level of the 24-hour standard. We also concluded that it would be inappropriate to compare daily air qua lity 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 re flect the general level of health protection against short-term risks offered by the annua l 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 ra nge between the annual
and the 24-hour PM 2.5 standards (i.e., 40 μg/m 3 ) to reflect the dual ro le of the 24-hour and

2OAQPS Issue Paper for Discussion at Nati onal Air Quality Conference, February 2007

the annual PM 2.5 standards in protecting against short- term risks. Given that decision, we also concluded that it was appr opriate 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 PM 2.5 standard (see Table 1). In 2006, EPA revised its national ambient air quality standards for PM 2.5 by strengthening the 24-hour standard to 35 μg/m 3 . We used a significantly expanded and stronger body of evidence from short-term exposure PM 2.5 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 PM 2.5 . Numerous U.S. and Canadian studies were available that provided evidence of associations between short-term exposure to
PM 2.5 and serious health effects in areas with air quality at and above the level of the
current annual PM 2.5 standard (15 μg/m 3 ). The standard level we established generally will require improvements in air quality in ar eas in which the distribution of daily shortterm exposure to PM 2.5 can reasonably be expected to be associated with serious health effects. In setting th e level of the 24-hour PM 2.5 standard at 35 μg/m 3 , we noted that this standard would protect public health with an adequate marg in of safety from serious health effects, including premature mortality and hospital admissions for cardiorespiratory causes that are asso ciated with short-term exposure to PM 2.5 . With the promulgation of the 2006 24-hour PM 2.5 standard, the short-term standard defines a level of health protection provided against short- term risks that again serves as an appropriate benchmark agai nst which to compare daily air quality concentrations. As a result, it is appropria te to consider setting an AQI value of 100 equal to the level of the 24-hour PM 2.5 standard (35 μg/m 3 ). We believe it is appropriate to consider reducing the AQI value of 150 (now 65 μg/m 3 ) in proportion to the reduction in the AQI value of 100 (to 55 μg/m 3 – rounded to the nearest 5 μg/m 3 ). This conclusion is based on the more extensive and stronger body of evidence linking 24-hour PM 2.5 concentrations with serious morbidity and mortality effects, includi ng: 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 cardiov ascular diseases; increased respiratory symptoms such as coughing, wheezing and shor tness of breath; lung function changes,

3OAQPS Issue Paper for Discussion at Nati onal Air Quality Conference, February 2007

especially in children and people with lung diseases such as asthma; changes in heart rate variability; and irregular heartb eat. We have also conclude d that it is appropriate to
retain the level of the annual standa rd for an AQI value of 50 (15 μg/m 3 ).

Possible revisions to the AQ I value of 500 and the upper end of the range (including setting an SHL for PM 2.5 ) 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 leve l of the annual standard, if there is one, or
at one-half the level of the short-term sta ndard if there is not. The upper bound index value of 500 corresponds to the Significant Ha rm 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 im minent and substantial endangerment to public health. We have not yet promulgated an SHL for PM 2.5 . In 1999, we set an index value of 500 equal to 500 μg/m 3 PM 2.5 , 24-hour average, and indicated that if we promulgated
an SHL for PM 2.5 and it was different, we would revise the PM 2.5 sub-index accordingly (63 FR 67829). This level was established on the basis of incr eased mortality found during historical wi ntertime pollution episodes in London, where PM concentrations measured as British Smoke were in the 500 to 1000 μg/m 3 range. British Smoke provided an approximate measurement of fine pa rticles, since it is considered to measure PM with a cut-point of approximately 4.5 mi crons. These measurements and health
rationale are also the basis for the current SHL for PM 10 (600 μg/m 3 ). At that time, we considered using the proportion of the PM 10 SHL (600 μg/m 3 ) that comprises PM 2.5 to set the 500 level of the AQI , but we did not have enough PM 2.5 monitoring data to establish a ratio (PM 2.5 to PM 10 ) for that purpose. Since then there has been a significant increase in PM 2.5 monitoring, and we now have sufficient PM 2.5 monitoring data to be able to estimate that in the United States, on average, 60 percent of
PM 10 consists of PM 2.5 . This allows us to scale the SHL for PM 2.5 to the SHL for PM 10 , rather than basing the PM 2.5 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 PM 10 SHL. Using this approach results in a PM 2.5 SHL of 350 μg/m 3 (rounded to the nearest 50 μg/m 3 ).

4OAQPS Issue Paper for Discussion at Nati onal Air Quality Conference, February 2007

For the intermediate breakpoints in the AQI between the values of 100 and 500,
the PM 2.5 concentrations under consideration generally reflect a linear relationship between increasing index values and increasing PM 2.5 values (see Table 1). The available scientific evidence of health effects related to population exposures to PM 2.5 concentrations between the level of the 24-hour standard and the le vel of the SHL under consideration suggest a continuum of eff ects in this range, with increasing PM 2.5 concentrations being associated with increa singly large numbers of people likely to experience such effects. The generally lin ear relationship between AQI values and PM 2.5 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 wint ertime pollution episodes in London because
there is little available info rmation about similar, more recent, air pollution episodes and associated impacts on community health upon wh ich to base a decision about an SHL for
PM 2.5 . 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 PM 2.5 levels. We have compared the level of the
SHL for PM 2.5 under consideration with air quality data (October 2003 to October 2006) in the AQS database. We looked at all approp riate data for this an alysis. AIRNow data collected using non-reference method technolog ies 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. Du ring these three years, only one monitor
exceeded the level of the PM 2.5 SHL (350 μg/m 3 ) 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 shorte r averaging period (i.e., 1-hour PM 2.5 levels) for use during PM 2.5 episodes (such as wildfires), when th ere can be extremely high short-term peaks of PM 2.5 that aren't well communicated by a dvisories based on 24-hour average
PM 2.5 concentrations. We intend to issue gui dance that air agencies can use during PM 2.5 episodes. There is a growing body of studies that provide additional evidence of effects associated with exposure periods shorte r than 24 hours (e.g., one to several hours)

5OAQPS Issue Paper for Discussion at Nati onal Air Quality Conference, February 2007

(OAQPS PM Staff Paper, December 2005, sec tion 3.5.5.1). While the PM Staff Paper
concluded that this information remains too li mited to serve as a basis for establishing a
shorter-than-24-hour PM 2.5 primary standard, this information will be useful in helping develop guidance that can be used during PM 2.5 episodes. Some agencies already issue
advisories based on 1-hour PM 2.5 levels. We are requesting that agencies using such advisories send us information about the approach used and incl uding associated strengths and weaknesses.

Possible Implications for Air Quality Forecasting For many areas around the country, loweri ng 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 curr ently do not reach USG levels may do so once the threshold is lowered. Therefore, a lo wer 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 th ese 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 great er 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 2 nd quarter of 2007 with answers to the above questions along with c onclusions and recommendations.

6OAQPS Issue Paper for Discussion at Nati onal Air Quality Conference, February 2007

Table 1. Current breakpoints and possible revisions to PM 2.5 sub-index

AQI Category Index Values PM 2.5 24-hour (μg/m 3 )
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 Weinstoc k (weinstock.lewis@epa.gov)

Supplemental Information Air Quality Index Reporting Propos ed Rule (December 9, 1998): http://epa.gov/airnow/h ealth/aqi_proposal_1998.pdf

Air Quality Index Reporting Final Rule (August 4, 1999): http://www.epa.gov/ttn/oar pg/t1/fr_notices/airqual.pdf

National Ambient Air Quality Standards for Particulate Matter Proposed Rule (January 17, 2006):

7OAQPS Issue Paper for Discussion at Nati onal 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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