Interim Exceptional Events Rule 

Frequently Asked Questions

United States Environmental Protection Agency

May 2013

Note:  This May 14, 2013, document replaces the original document posted on May 13, 2013, to

include the inadvertent omission of footnote 16 on page 28. There are no additional changes and

document pagination is the same in both versions of this document.  
Interim Exceptional Events Rule Frequently Asked Questions 

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ATTACHMENT 1 

Interim Exceptional Events Rule Frequently Asked Questions 

The Exceptional Events Rule of 20071 superseded the EPA’s previous Exceptional Events 

guidance and policy documents and created a regulatory process codified at 40 CFR parts 50 

and 51 (50.1, 50.14 and 51.930). The Exceptional Events Rule (EER) recognizes that each 

potentially eligible event can have different or unique characteristics, and thus, necessitates a 

case-by-case demonstration and evaluation. Therefore, the EER adopts a “weight-ofevidence” approach for reviewing each demonstration to justify excluding data affected by an 

exceptional event. The EPA acknowledges that extreme2

 exceptional events may justify more 

limited demonstration packages. 

Air agencies and other stakeholders have raised technical questions and issues related to 

implementation since the EPA promulgated the EER. This Question and Answer (Q&A) 

document is intended to respond to some of these frequently asked questions and to provide 

guidance and clarification to air agencies3

 implementing the EER. The EPA recognizes the 

limited resources of the air agencies that prepare and submit exceptional event demonstration 

packages and of the EPA regional offices that review these demonstration packages. One of 

the EPA’s goals in developing exceptional event implementation guidance is to establish 

clear expectations to enable affected air agencies to better manage resources as they prepare 

the documentation required under the EER. Submitters should prepare and submit the 

appropriate level of supporting documentation, which will vary on a case-by-case basis under 

the weight-of-evidence approach. The EPA anticipates that the resources needed to prepare 

(and review) packages will decrease as we continue to identify ways to streamline the 

process and continue to build our database of example demonstrations and analyses. In 

addition, as noted above, the EPA acknowledges that extreme exceptional events may justify 

more limited demonstration packages. 

For organizational ease, this document has been divided into the following topical sections: 

A. Historical Fluctuations 

B. “But For” Test 

C. Exceptional Event Data Flagging Schedules 

D. General AQS Procedures 

E. General Exceptional Events Rule Applicability and Implementation Issues 

 

1

 “Treatment of Data Influenced by Exceptional Events; Final Rule,” 72 FR 13563, March 22, 2007. 

2

 Extreme exceptional events may justify a more limited demonstration package. Whether a particular event 

should be considered “extreme” for this purpose depends on the type and severity of the event, pollutant 

concentration, spatial extent, temporal extent, and proximity of the event to the violating monitor. Several 

meteorological phenomena that could be considered extreme events include hurricanes, tornadoes, haboobs, and 

catastrophic volcanic eruptions. The EPA addresses “extreme” high wind dust events in Question 17a in this 

document. 

3

 References to “air agencies” are meant to include state, local, and tribal air agencies responsible for 

implementing the EER. 
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F. Exceptional Event Data Flagging for Air Quality Concentrations that Could 

Contribute to an Exceedance or Violation of the National Ambient Air Quality 

Standards 

Each section contains related questions. Readers of this document can find additional 

information at the EPA’s Exceptional Events website located at 

http://www.epa.gov/ttn/analysis/exevents.htm. The EPA’s interim guidance documents and 

the exceptional events website present examples to illustrate specific points. The example 

analyses and level of rigor are not necessarily needed for all demonstrations. 

Disclaimer 

The Exceptional Events Rule is the source of the regulatory requirements for exceptional 

events and exceptional event demonstrations. This interim Q&A document provides 

guidance and interpretation of the Exceptional Events Rule rather than imposing any new 

requirements and shall not be considered binding on any party. Note: If and when the EPA 

takes a regulatory action that hinges on a decision to exclude data under the Exceptional 

Events Rule, the EPA will consider and appropriately respond to any public comments on 

any aspect of a supporting exceptional events demonstration submittal. 
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A. Historical Fluctuations 

40 CFR 50.14(c)(3)(iv): “The demonstration to justify data exclusion shall provide 

evidence that: 

* * * 

(C) The event is associated with a measured concentration in excess of normal 

historical fluctuations, including background; 

1. Question: Is the Exceptional Events Rule demonstration requirement to provide 

evidence to support “a measured concentration in excess of normal historical fluctuations, 

including background” a test that can be “passed” or “failed” based on the outcome of the 

statistical comparison? For example, must the concentration affected by an event exceed 

a specific percentile rank in the historical data? 

Answer: The “historical fluctuations” criterion is a test, but there is no specific 

percentile rank that the EPA will use to determine whether the test has been passed. The 

EPA will use a weight-of-evidence approach to review each demonstration on a case-bycase basis. The air agency’s role in satisfying this element is to provide appropriate 

analyses and statistics and conclude that the provided data show that the event was in 

excess of normal historical fluctuations. The EPA will review the information provided 

by the air agency. “Normal historical fluctuations” will generally be defined by those 

days without events for the previous years. The EPA acknowledges that natural events 

can recur and still be eligible for exclusion under the EER; therefore, events do not 

necessarily have to be rare to satisfy this element. 

 

The submittal of data showing how the event concentration compared with historical 

concentrations will help the EPA determine whether the air agency has satisfied the 

“clear causal relationship,” “but for,” and “affects air quality” criteria. Air agencies need 

to satisfy these EER criteria, as well as “not reasonably controllable or preventable,” for 

the EPA to concur on an exceptional event claim. The EPA anticipates that less 

conclusive historical fluctuation comparisons will likely indicate less conclusive “clear 

causal relationship” and/or “but for” relationships. However, a demonstration without a 

historical fluctuations comparison would prevent the EPA from being able to approve 

exclusion of the data in question. 

The EPA recommends that each “historical fluctuation” demonstration submittal contain 

a minimum set of statistical analyses described in more detail in Questions 2 and 3. The 

EPA generally will consider submission of the identified statistical analyses to have met 

the requirement to “provide evidence.” 

It is important to note, however, that there is no outcome of the “historical fluctuation” 

statistical comparison that, by itself, can guarantee successful demonstration of the clear 

causal relationship and “but for” elements. The EPA will consider in its weight-ofevidence approach the comparison of the concentrations during event(s) in question with 

historical concentration data. For example, a uniquely high concentration in an area (and 

season) with no previous exceedances, with a clear causal connection, and with no 
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evidence of any other plausible explanation would be a case in which the weight-ofevidence would generally indicate that the “but for” criterion has been demonstrated. In 

contrast, if the event-affected concentration does not stand out much from normally 

occurring exceedance concentrations for the same place and season, the statistical 

comparison generally will not by itself provide much support for “but for” in the weightof-evidence consideration. 

2. Question: What evidence does the EPA want included in the demonstration as part of a 

comparison of a measured concentration with normal historical fluctuations, including 

background?

Answer: The EPA would prefer an analysis showing how the observed concentration 

compares to the distribution of historical concentrations. To aid the EPA’s review, reduce 

requests for additional information, and facilitate the EPA’s understanding of the air 

agency’s position, a submitting air agency can consider providing some of the following 

types of statistics, graphics, and explanatory text: 

 Comparison of concentrations on the claimed event day with past historical data (see 

Question 3 for additional detail). The historical comparisons can be made on an annual 

and/or seasonal basis, depending on which is more appropriate. For example, if PM or 

ozone data at the location show clear seasonality (i.e., exceedances are nonexistent or 

extremely rare in some seasons but not others, or concentrations vary according to season 

due to meteorological conditions), discussing that information in the demonstration is 

likely appropriate. In contrast, if exceedances are likely throughout the year, analysis of 

annual data may be more appropriate. For seasonal comparisons, the EPA recommends 

using all available seasonal data from 3-5 years (or more, if available). The analysis 

should discuss the seasonal nature of pollution for the location being evaluated. 

Depending on the quantity of data, it may be appropriate to present monthly maximums; 

however, generally it is not appropriate to present monthly-averaged daily data or any 

other average of the daily data as this masks high values. Regardless of whether seasonal 

or annual data are presented, data are most helpful when provided in the form relevant to 

the standard that is being considered for data exclusion (see Question 30). Specific 

examples of analyses of annual and seasonal data, as well as analyses of historical 

speciated PM2.5 fluctuations and spatial distribution fluctuations are included in the 

presentation located at 

http://www.epa.gov/ttn/analysis/docs/IdeasforShowingEEEvidence.ppt. Examples of 

graphics are also included in the response to Question 3. 

Additionally, it may be useful for the comparison of concentrations on the claimed event 

day with past historical data to label appropriate data points as being associated with 

concurred exceptional events, suspected exceptional events, or other unusual occurrences. 

As additional evidence to use in interpreting the data, it may also be useful to include 

comparisons omitting such points. The intent of these comparisons is to present a time 

series of concentration data for the event area, thereby fully and accurately portraying the 

historical context for the claimed event day. 
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 Comparison of concentrations on the claimed event day with a narrower set of similar 

days: Similar days could include neighboring days (e.g., a time series of two weeks) 

and/or other days with similar meteorological conditions (possibly from other years). 

This type of comparison could demonstrate that the event caused higher concentrations 

than would be expected for given meteorological and/or local emissions conditions. 

 Percentile rank of concentration relative to annual data. The percentile rank of the 

event-day concentration should be provided for the event day relative to all measurement 

days over the previous 3-5 years. To ensure statistical robustness, the EPA generally 

recommends that submitting agencies include a minimum of 300 data points in this 

calculation. The daily statistic (e.g., 24-hour average, maximum 8-hour average, or 

maximum 1-hour) should be appropriate for the form of the standard being considered for 

data exclusion (see Question 30). 

 Percentile rank of concentration relative to seasonal data. The percentile rank of the 

event-day concentration should be provided for the event day relative to all measurement 

days for the season (or appropriate alternative 3-month period) of the event over the 

previous 3-5 years. It is generally appropriate to use the same time horizon as used for the 

percentile rank calculated relative to annual data. 

(Note: The use of percentile ranks is illustrative and should not be seen as a bright line 

to be passed or failed when comparing observed concentrations with historical values.)

3. Question: How will the EPA consider the submitted “historical fluctuations” evidence 

when assessing whether the “but for” and “clear causal relationship” criteria are met? 

Answer: The EPA will review the submitted analyses showing how the observed 

concentration compares to the distribution of historical concentrations to determine 

whether the event is associated with a measured concentration in excess of normal 

historical fluctuations and will assess the other criteria, in part, based on this historical 

fluctuations comparison. When the observed concentration is higher than all or nearly all 

normal historical concentrations (i.e., concentrations when there was not an event), the 

EPA may need less additional evidence to demonstrate the “but for” finding. When the 

concentration is similar to or lower than a large number of normal historical values, the 

EPA may want additional evidence (e.g., PM or VOC speciation data) to support the “but 

for” and “clear causal relationship” demonstration requirements. The additional evidence 

will help differentiate the concentration increment caused by the event in question from 

other, non-event causes. 

Stated another way, the EPA’s intended use of the data is to review the historical 

fluctuations prong, which may influence how much information of other types is needed 

to successfully meet the other demonstration criteria (i.e., “but for” and “clear causal 

relationship”) of 40 CFR § 50.14 based, in part, on the degree to which the measured 

concentration is in excess of normal historical fluctuations. 
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Submitting agencies are encouraged to discuss available historical fluctuation evidence 

with the appropriate EPA regional office prior to submitting the event demonstration 

package to determine if specific information might assist in the review process. 

Additional Examples and Explanation Concerning “Historical Fluctuations” Evidence

(Note: The discussion and graphics that follow illustrate the type of analyses and 

discussion that are described in this question and in Question 2 and that an air agency 

might include in a submittal showing that an event is associated with a measurement “in 

excess of normal historical fluctuations.”) 

The evidence comparing the event-affected concentration with historical concentrations 

is most helpful to an air agency’s demonstration if it shows that the event-affected 

concentration is high compared to all, or nearly all, historical concentrations generated by 

normal emissions and ambient conditions. This scenario makes it more plausible that the 

event caused the observed excess concentration rather than that some other causal event 

occurred on the same day as the known event. If similar events have been very rare in the 

past, it may be possible to make this point by labeling appropriate data points as being 

associated with concurred exceptional events, suspected exceptional events, or other 

unusual occurrences. To facilitate the EPA’s understanding of the influence of these 

events, air agencies may also include comparisons omitting such points. 

The following figures demonstrate the concept of seasonal emissions fluctuations. The 

first figure shows an exceedance level PM2.5 value in late spring that is outside the range 

of the 3 to 5-year historical data set for non-wintertime PM2.5, while the second figure 

shows a similar data value for a different part of the country where similar exceedance 

concentrations occur throughout the year, suggesting that some non-event process(es) can 

cause high concentrations all during the year. In the first case, a seasonal assessment of 

historical fluctuations generally would be appropriate, while annualized data analysis 

might be more appropriate for the second case to provide the most robust yet also 

representative historical data set. 
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4. Question: The Preamble to the EER states that less documentation or evidence may be 

needed to demonstrate that an event affected air quality for flagged data > 95th percentile 

than for values > 75th percentile. For ozone, PM10 and 24-hour PM2.5, in areas near the 

standard, exceedances are often near or above the 95th percentile of historical data. In 

these cases, will the EPA accept less documentation to demonstrate that an event affected 

air quality simply because an event-affected concentration is above the 95th percentile of 

the historical concentrations? 

Answer: The preamble statement paraphrased in the question above was intended to 

address National Ambient Air Quality Standards (NAAQS) that are based on averaging 

Historical Seasonal Fluctuations in PM2.5, Seasonal Data, 2005-2009

0

10

20

30

40

50

60

70

1 92 183 274 365

Day of Year

24-hour PM2.5, ug/m3

1st Quarter 2nd Quarter 3rd Quarter 4th Quarter

Flagged Value Potentially Outside 

Normal Seasonal Historical Fluctuations

Historical Seasonal Fluctuations in PM2.5, Non-Seasonal Data, 2005-2009

0

10

20

30

40

50

60

70

1 92 183 274 365

Day of Year

24-hour PM2.5, ug/m3

1st Quarter 2nd Quarter 3rd Quarter 4th Quarter

Flagged Value Not Potentially Outside 

Normal Seasonal Historical Fluctuations
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periods of many days, such as annual, quarterly and/or 3-month rolling average NAAQS. 

NAAQS with 1-hour, 8-hour or 24-hour averaging periods only allow a small percentage 

of days to have concentrations above the level of the NAAQS. Flagging and excluding 

data falling at around the 75th percentile point of the historical concentrations are 

extremely unlikely to influence an area’s attainment status with respect to such a shortterm NAAQS. Data around the 75th percentile point can, however, affect compliance with 

NAAQS having a quarterly average, 3-month average, or annual average standard. For 

the annual PM2.5 NAAQS, it is true that showing that the Exceptional Events Rule criteria 

are met will be more difficult for values near the 75th percentile point than for values near 

the 95th percentile point because it is more likely that values near the 75th percentile point 

are related to non-event causes. 

Other questions and answers in this Q&A document address situations involving NAAQS 

with short averaging periods. 

5. Question: Some pollutant demonstrations do not (or poorly) characterize the historical 

fluctuations of the observed concentrations at the monitor affected by the event. How can 

one judge whether the demonstration is adequate in this regard? 

Answer: As previously stated in the response to the historical fluctuations question, the 

EPA will review the submitted analyses showing how the observed concentration 

compares to the distribution of historical concentrations to assess whether the event is 

associated with a measured concentration in excess of normal historical fluctuations, and 

when assessing the exceptional event demonstration criteria of “affects air quality,” 

“clear causal relationship,” and “but for” causation. Because the “historical fluctuations” 

showing is not a statistical demonstration with any defined bright line, air agencies 

should consider submitting (with appropriate descriptions and discussion) the type of 

statistical analyses described in the responses to Questions 2 and 3. The EPA will review 

these analyses and look at both the relationship between the claimed concentration and 

historical concentrations and the strength of the data set to help inform the evidence 

needed to demonstrate the clear causal relationship and “but for” criteria. 

In the response to Question 2, we identified that air agencies completing historical 

fluctuation analyses should consider using 3 to 5 years of data to ensure a representative 

dataset. We recognize, however, that these data may not be available for all monitors 

and/or all pollutants. If data are not available, please consult with the reviewing EPA 

regional office. 

B. “But For” Test 

 

Section 319 of the Clean Air Act requires that “a clear causal relationship must exist 

between the measured exceedances of a national ambient air quality standard and the 

exceptional event to demonstrate that the exceptional event caused a specific air 

pollution concentration at a particular air quality monitoring location...” and that 

[States] can petition [EPA] to “[E]xclude data that is directly due to exceptional 

events from use in determinations...with respect to exceedances or violations.”
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The implementing language in the EER at 40 CFR 50.14(c)(3)(iv) states: “The 

demonstration to justify data exclusion shall provide evidence that: 

* * * 

(D) There would have been no exceedance or violation but for the event. 

6. Question: What types of evidence can air agencies include in a demonstration that ozone 

exceedances would not have occurred but for the effect of a fire event? 

Answer: Air Agencies may include any evidence that they consider relevant to the “but 

for” requirement recognizing that the effects of a fire on ozone are complex. Fire can 

generate ozone precursors, but it can also reduce solar radiation needed to drive ozone 

formation. Also, fire plumes containing ozone and ozone precursors can pass over a 

monitoring site without mixing down to ground level and affecting the monitored 

concentration. Additionally, wildfires often occur during the same seasons that exhibit 

high ozone caused by anthropogenic precursor emissions making it difficult to separate 

the wildfire contribution from a high ozone event that would have occurred without the 

fire. 

Examples of relevant evidence follow. The EPA recognizes that the following example 

analyses have limitations and may not conclusively or quantitatively demonstrate the “but 

for” criterion. For this reason, the EPA considers “but for” evidence using a weight-ofevidence approach on a case-by-case event basis. 

 Statistical evidence that shows that for the place, time of year, and prevailing 

weather conditions at the time of the event, past ozone data show no history of 

exceedances on days that were not affected by a fire event, or shows that 

exceedances were so infrequent as to make the fire at issue the more likely cause 

of the observed exceedance. 

 Unusual diurnal patterns of hourly or minute-by-minute ozone concentrations, 

such as a spike or peak other than at the normal time of day. This could be 

demonstrated by comparing the event pattern to the range of diurnal patterns 

exhibited on typical high ozone days. 

 Evidence that the normally good correlation between the affected monitor and a 

monitor clearly outside the area of influence of the fire was disrupted on the day 

of the fire event in a manner not seen on non-fire days. 

 Evidence that there were no known unusual emission releases from non-fire 

sources at the time of the fire event, such as from traffic due to a sports or 

entertainment event or source non-compliance. 

 Evidence that the plume from the fire passed over the location of the monitoring 

site and mixed down to ground level. This can include satellite images, wind data 

including HYSPLIT trajectories, visual smoke observations, and chemical 

analysis of PM filters showing elements and compounds that are markers for 

biomass burning. 
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 Altered pollutant amounts, ratios, or patterns that indicate the influence of the 

event rather than non-event sources. This information could include the level, 

timing and patterns of CO and PM; PM size distribution or composition; 

indicators of precursor composition and “age,” such as oxygenated VOCs, 

radicals, sulfates, and timing and pattern of NO2 and NO; and pollutant ratios, 

such as CO/NOx, CO/PM10, Elemental Carbon (EC)/Organic Carbon (OC), 

O3/NOy and O3/CO. 

 A prediction that the “normal” ozone concentration would have been below the 

level of the NAAQS. “Normal” ozone concentrations can be predicted using 

statistical methods based on previous-day ozone and same-day weather variables 

(like methods used for air quality advisories in some areas) or using air quality 

models. The EPA asks that demonstration packages using these predictive 

techniques also include an easily understandable narrative describing the 

application of the technique and information on the uncertainty of the prediction 

methods (i.e., information on its past success in predicting normal ozone levels). 

 A prediction based on air quality/photochemical modeling of the incremental 

ozone concentration due to the emissions from the fire, from comparing modeling 

results with and without the emissions from the fire. A demonstration that 

includes such evidence should address the uncertainties in the emission estimates 

for the fire including the speciation of the VOC and NOx emissions, and the 

uncertainties due to other aspects of the modeling platform such as grid cell size, 

etc. 

The EPA is currently developing a separate guidance document for preparing a 

demonstration for wildfire events that are believed to have affected ozone concentrations. 

In addition, the EPA will post on its exceptional events website example demonstration 

packages that illustrate the type and scope of analyses that constitute complete submittals 

for ozone-related exceptional events.4 

C. Exceptional Event Data Flagging Schedules 

Note: “Flag” is the common terminology for a data qualifier code in the EPA’s AQS (Air 

Quality System). Unless explicitly noted, the process of “flagging” data refers to adding 

Request Exclusion (“R”) data qualifier codes to selected data in AQS. “R” flags are the 

only AQS flags that satisfy the EER requirement for initial data flagging. The EPA can 

act/concur only on an “R” flag. 

7. Question: When the EPA revises the National Ambient Air Quality Standards, how will 

it notify air agencies of the schedules and deadlines for flagging and documenting 

exceptional event data for designations purposes? 

Answer: When the EPA promulgated 40 CFR § 50.14, “Treatment of Air Quality 

Monitoring Data Influenced by Exceptional Events,” in March 2007, the EPA was 

mindful that designations would be occurring under the then-recently revised PM2.5

NAAQS. Exceptions to the generic deadline of July 1 of the calendar year following the 

 

4

 http://www.epa.gov/ttn/analysis/exevents.htm 
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datum year (see 40 CFR § 50.14(c)(2)(iii)) were included for PM2.5 in the rule. The EPA 

was also mindful that similar issues would arise for subsequent new or revised NAAQS. 

The Exceptional Events Rule at section 50.14(c)(2)(vi) indicates “when EPA sets a 

NAAQS for a new pollutant, or revises the NAAQS for an existing pollutant, it may 

revise or set a new schedule for flagging data for initial designation of areas for those 

NAAQS.” See as examples, the data flagging schedule identified in the 2012 SO2

NAAQS final rule at 75 FR 35592, the data flagging schedule identified in the 2010 NO2

NAAQS final rule at 75 FR 6531, or the data flagging schedule identified in the 2012 

PM2.5 NAAQS final rule at 78 FR 3086. 

D. General AQS Procedures 

8. Question: What is the difference between the “R” series flags and the “I” series flags, 

and how should they be used? 

Answer: Within AQS, monitoring agencies can use two types of data validation, or data 

qualifier, codes: the Request Exclusion flags (“R”) and the Informational Only flags (“I”). 

Agencies should use the “I” series flags when identifying informational data and the “R” 

series flags to identify data points for which the agency intends to request an exceptional 

event exclusion and the EPA’s concurrence. As an example, air agencies may use an “I” 

series flag to initially identify values they believe were affected by an event. Once the air 

agency collects additional supporting data, it may change the flag to an “R” series flag 

and submit an initial event description. Or, the air agency may find that additional 

information does not support flagging the data as an exceptional event, and the air agency 

may, therefore, delete the flag or retain the “I” series flag. Air agencies may also use the 

“I” series flags simply to note activities or conditions occurring on the data collection day 

that are unrelated to exceptional events. 

The EPA does not intend to review or concur on the “I” series flags. Air agencies must 

submit “R” flags by July 1 of the calendar year following the year in which the flagged 

measurement occurred or by the other deadlines identified with individual NAAQS 

revisions (see Question 7). Air agencies intending to change “I” flagged data to “R” 

flagged data should be aware of the EER flagging and initial event description deadline 

of July 1 of the year following the sample measurement. Air agencies should change the 

flag status from “I” to “R” BEFORE the July 1 deadline. Normally, air agencies should 

not modify the flag status after this date and, therefore, if they went beyond July 1, they 

may not be able to meet the EER initial flagging and event description deadlines.

9. Question: May an air agency flag any data in AQS? 

 

Answer: Yes, but the EPA asks air agencies to use the “R” flags to identify data that 

might have a regulatory consequence and for which an air agency intends to request 

exclusion and submit an approvable demonstration. Air agencies should use the “I” series 

flags to identify values for informational purposes (see Question 8). AQS only allows the 

EPA to place concurrence flags on data identified with an “R” flag. “I” flags never affect 

regulatory summary statistics (e.g., design values, number of exceedances, 98th percentile 
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values) generated by AQS for NAAQS determinations purposes. “R” flags will not affect 

the regulatory summary statistics unless or until they are concurred by the EPA. 

Further, while the EER does not prohibit air agencies from flagging individual 

concentration values below the level of the NAAQS, in general, air agencies can only 

request exclusion for data that contribute to a violation or an exceedance of the NAAQS. 

See Questions 29-31 for more information, including clarifications and examples, 

particularly for PM2.5 and PM10, in which flagging individual concentration values below 

the level of the NAAQS is acceptable. 

10. Question: The EPA requires air agencies to provide an initial description for data 

flagged with an “R” data qualifier code. Is it possible for an initial description to be 

inadequate (for example, "fires in surrounding states")? 

Answer: Although the EPA is not specifying pass/fail criteria for the initial description 

associated with “R” flagged data, it is possible for an air agency to enter inadequate 

initial descriptions in AQS. The preamble to the Exceptional Events Rule explains: "At 

the time the [request exclusion] flag is inserted into the AQS database, the State must also 

provide an initial description of the event in the AQS comment field. This initial 

description should include such information as the direction and distance from the event 

to the air quality monitor in question, as well as the direction of the wind on the day in 

question." 72 FR 13568 (emphasis added). AQS maintains event definitions, including 

their initial descriptions, in fields separate from the raw data flagging fields. As a result, 

air agencies can enter more detailed event descriptions either before or after the raw data 

measurements are flagged. Regardless of precise timing, the intent of this initial 

description is to initially explain why the flagged data warrant consideration as 

exceptional events. Although the initial description is not likely to provide enough 

information to assist the EPA with exceptional event planning and prioritization, the act 

of providing the initial description encourages air agencies to review and identify data 

having regulatory consequence and for which they are likely to submit an approvable 

demonstration. To facilitate the EPA’s review of the initial event description, the EPA 

suggests that air agencies notify the appropriate regional office after the air agency 

creates the event description. This allows the air agency and the EPA to discuss and, if 

necessary, develop a mutually agreed-upon description. This initial discussion and the 

optional letter of intent (see Question 27) can assist the EPA and air agencies with 

exceptional event review and prioritization. 

11. Question: The “j” flag was "Construction/Demolition." The new “IE/RE” flag is 

demolition; can it also be used for construction? 

Answer: The “j” flag is obsolete and can no longer be used. The “IE/RE” flag should not

be used for construction. 

Generally, construction activity is not considered to be exceptional. Reasonable and 

appropriate controls capable of preventing localized NAAQS exceedances should be 

available during most construction events. In some cases, however, construction activities 
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may involve very high-energy, emissions-generating physical processes, such as 

explosive excavation. Dust control measures may not be adequate to prevent exceedances 

/ violations in the vicinity of this type of activity. 

If an agency wishes to “flag” data related to exceedances caused by some construction 

activity, the agency should use the Other (“IL/RL”) exceptional events flag. Air agencies 

should use the “IE/RE” flag only when an exceptional demolition event occurred and the 

air agency wishes to flag the data for exclusion as an exceptional event. Air agencies 

using either the “IE/RE” flag or the “IL/RL” flag to identify an exceptional event should 

show in a demonstration submittal that all reasonable and appropriate controls were in 

place during the construction / demolition activity, and that those controls proved 

inadequate to prevent NAAQS exceedances. The demonstration would also need to meet 

all other requirements of the Exceptional Events Rule. 

11a.Question: What flags does AQS use to describe fires? 

Answer: Land Management Agencies modified their fire-related definitions after the 

EPA promulgated the Exceptional Events Rule. The EPA has incorporated the firerelated terminology in the exceptional events guidance documents to ensure consistency 

(see also Question 20a). These definitional changes result in corresponding changes to 

fire-related flags in AQS. The EPA eliminated from AQS the Wildland Fire Use Fire – 

United States (“IU”) and (“RU”) flags and the Forest Fire (“E”) flag. The EPA continues 

to use the following flags to describe fires: 

 IF – Fire – Canadian (Informational Only) 

 IG – Fire – Mexico/Central America (Informational Only) 

 IM – Prescribed Fire (Informational Only) 

 IP – Structural Fire (Informational Only) 

 IT – Wildfire – US (Informational Only) 

 RF – Fire – Canadian (Request Exclusion) 

 RG – Fire – Mexico/Central America (Request Exclusion) 

 RM – Prescribed Fire (Request Exclusion) 

 RP – Structural Fire (Request Exclusion) 

 RT – Wildfire – US (Request Exclusion) 

The EPA believes it is appropriate to retain the Fire – Canadian (“IF/RF”) and Fire – 

Mexico/Central America (“IG/RG”) flags because these flags indicate the jurisdictional 

origin of the fire (i.e., outside of the submitting state/outside of the United States). 

Emissions from fires originating outside of the United States that affect air quality 

concentrations in the United States may qualify for regulatory treatment under the 

international transport provisions of 40 CFR part 179(b) of the Clean Air Act. 

12. Question: The National Park Service operates ozone monitors in some locations that 

meet all requirements of 40 CFR part 58. Can an air agency request exclusion of data 

from such monitors under the EER, and exclusion of other data not collected by the air 

agency itself that may lead to a nonattainment finding? 
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Answer: Yes. However, air agencies should take special steps with regard to data 

handling within AQS. To maintain data integrity, AQS is generally designed so that only 

the agency updating a monitoring site may enter or alter data for that site. Under normal 

circumstances, an air agency will not have access rights to apply event flags to data from 

monitors operated by other entities, such as the National Park Service or other state, 

local, or tribal agencies. When an air agency believes that an exceptional event affected 

the concentration recorded by monitors operated by other agencies, the air agency should 

contact the agency operating the monitor and request that the operating agency flag the 

identified data range for exclusion. The affected air agency should also develop and 

forward to the operating agency an initial event description that the operating agency can 

enter in AQS as it enters the appropriate “R” series flags (see Question 10). If an air 

agency is unsuccessful in requesting that another agency apply the appropriate “R” series 

flags and initial event description, the air agency should contact the EPA regional office. 

If the EPA regional office is aware of the request, and if the request was prior to July 1st

of the year following the datum year, the EPA will generally still consider the affected air 

agency’s request. Air agencies should notify the EPA regional office of such an instance 

as soon as possible. 

Regardless of whether the monitor operator flags the data in question or the air agency 

notifies the regional office that a flag is needed, it is the air agency’s responsibility to 

develop an initial event description, prepare the demonstration, and submit it to the EPA 

under the applicable schedule. The agency operating the monitor may choose to assist in 

this process. 

13. Question: Events can make an air concentration significantly higher than it would have 

been in the absence of the event contribution, and elevate the 3-year design value for a 

NAAQS pollutant. Depending on the magnitude of the effect and how the “normal” 

concentration compares to the NAAQS, the “but for” test may not be satisfied in that 

there may have been a violation with or without the event. Thus, it appears that data 

associated with the event cannot be handled as an exceptional event. However, retaining 

such data in the calculation of a design value for a nonattainment area can make it seem 

that the area needs more emissions reduction to attain the NAAQS than is actually the 

case. How will the EPA deal with such a situation when reviewing an attainment 

demonstration? How, if at all, should AQS be used to flag such data? 

Answer: (See also Question 19 for a related question regarding PM10.) The question 

reflects a proper understanding that not every natural or infrequent anthropogenic event 

that affects air quality is a true "exceptional event" under the definition of that term in the 

Exceptional Event Rule. Ambient data affected by an event that does not meet the "but 

for" criterion cannot be excluded under the authority of the Exceptional Events Rule even 

if in all other respects the event meets the definition of an exceptional event. When the 

available evidence indicates that there would have been an exceedance of a NAAQS even 

in the absence of the event, for example when a wildfire makes a summer-time ozone 

exceedance worse than it otherwise would have been, the event is not a true “exceptional 

event” under the EER. The Exceptional Events Rule does not address data handling 
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associated with events that are not considered “exceptional” under the EER, and does not 

provide the EPA with authority to exclude such data. Yet as the question points out, this 

event-related concentration could still impact design values. An air agency incorporating 

the event-related concentration in a design value used for a prospective attainment 

demonstration might seem to need more emission reductions to attain the NAAQS by its 

attainment deadline than is actually the case. 

However, the EPA intends to achieve much the same effect as if such data were 

excludable under the Exceptional Events Rule, by addressing this topic in future guidance 

on the preparation of attainment demonstrations in required SIPs for areas designated as 

nonattainment. The first pollutant and NAAQS that the EPA will address this way will be 

the 2008 ozone NAAQS. The EPA plans to more formally describe its intention to 

develop such ozone guidance in the preamble of a soon-to-be-proposed rulemaking on 

SIP requirements for areas designated nonattainment for the 2008 ozone NAAQS. Until 

the planned guidance for a pollutant and NAAQS of interest is issued, air agencies should 

consult with their EPA regional office if they face this situation. To avoid confusion, air 

agencies should use AQS informational-only "I" flags on such data, rather than "R" flags. 

In the remainder of this response to the question, the EPA describes in more detail the 

differences between the event scenario described in the question and a true "exceptional 

event" under the Exceptional Events Rule, for the purpose of clarifying why the planned 

guidance on attainment demonstrations and the SIP approval process, rather than the 

Exceptional Event Rule and the associated AQS data flagging, demonstration submittal, 

and review process, will apply to such an event scenario. 

To illustrate an attainment demonstration scenario using the 2006 24-hour PM2.5 NAAQS 

of 35 μg/m3, assume that the three annual 98th percentile 24-hour PM2.5 concentrations for 

a monitoring site for 2006-2008 are 44, 31, and 37 μg/m3 for each respective year, with a 

resulting 3-year design value of 37 μg/m3, which is a violation. Also, assume that the next 

highest concentration in 2006 below the 44 μg/m3

 was 40 μg/m3. The 44 μg/m3

concentration in 2006 was affected by a one-day wildfire, and the air agency was able to 

show that the concentration would have been 41 μg/m3 without the fire. Because both 44 

μg/m3 and 41 μg/m3

 are exceedances, the event on that day does not meet the “but for” 

test when viewed from an “exceedance” perspective. Moreover, from a “violations” 

perspective, the 2006 value also would not meet the “but for” test, because the “no event” 

concentration value of 41 μg/m3 for the event day in 2006 would still be the 98th

percentile concentration and would still result in a 3-year design value of 36 μg/m3

 which 

is a violation. Thus, the 2006 wildfire does not meet the definition of an exceptional 

event. 

E. General Exceptional Events Rule Applicability and Implementation Issues 

14. Question: The Preamble to the Exceptional Events Rule states that the EPA 

headquarters or the EPA regional office will make its decision on demonstrations public. 

See 72 FR 13574 ("The EPA regional offices will work with the States, Tribes, and local 

agencies to ensure that proper documentation is submitted to justify data exclusion. EPA 
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will make the response and associated explanation publicly available."). What method 

does the EPA plan to use to make the explanation "publicly available?" 

Answer: The EPA posts example demonstration packages and decisions (consisting of 

air agency demonstration submittals, the EPA responses, and the EPA technical support 

documents) on the EPA regional office websites and/or the Technology Transfer 

Network website.5

 In certain instances, the EPA’s concurrence or non-concurrence 

determination may be a factor in a rulemaking that includes a public comment period. In 

these cases, the same information that is posted on the EPA websites, and any additional 

supporting correspondence, will also be posted in the relevant rulemaking docket. 

Further, the EPA plans to make the demonstrations and the EPA’s concurrence decisions 

available to interested parties upon request. 

14a. Question: At what point in the exceptional event development and review process is 

public notice and opportunity for comment required? How does the EPA determine the 

need for public comment? 

Answer: The EER requires that air agencies offer notice and opportunity for public 

comment as part of the demonstration development process (see 40 CFR 50.14(c)(3)(i) 

and 40 CFR 50.14(c)(3)(v)). The EPA must also provide notice and opportunity for 

public comment prior to taking a final Agency action, such as acting on an air agency’s 

request for area redesignation, that may rely upon air quality monitoring data including 

exceptional event claims. In addition, an air agency may need to provide an additional 

opportunity for public comment if the EPA requests and/or if the air agency provides 

supplemental information not included in the original documentation made available for 

public comment. The EPA will make a case-by-case decision regarding supplemental 

opportunities for public comment during the demonstration preparation, submittal, and 

review process. As part of this decision, the EPA may consider potential impact and/or 

expressed public interest in the claimed event, data uncertainty, historical application of 

demonstration approach, etc. 

When the EPA concurs based on the weight-of-evidence that the air agency has 

successfully made the demonstrations referred to in 40 CFR 50.14(a)(2) and (b)(1) to the 

EPA’s satisfaction, the EPA generally will exclude the affected data from the following 

types of calculations and activities: 

 The EPA’s AQS will not count these days as exceedances when generating user 

reports, and will not include them in design values estimates, unless the AQS user 

specifically indicates that they should be included.6

 

 The EPA will accept the exclusion of these data for the purposes of selecting 

appropriate background concentrations for New Source Review (NSR) air quality 

analyses.7

 

5

 http://www.epa.gov/ttn/analysis/exevents.htm 

6

Due to the complexity of the AQS software, inadvertent errors may occur. The EPA asks that agencies provide 

the EPA with information if/when AQS outputs seem inconsistent with the EPA’s intention to exclude 

concurred upon data.
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 The EPA will accept the exclusion of these data for the purposes of selecting 

appropriate background concentrations for transportation conformity hot spot 

analyses.8

 The data will continue to be publically available, but the EPA’s publications and 

public information statements on the status of air quality in the affected area generally 

will not reflect these data in any summary statistic of potential regulatory application, 

unless such inclusion is specifically noted.9 

In addition, some proposed regulatory actions (e.g., proposed designation, classification, 

attainment demonstration, or finding as to whether the area has met the applicable 

NAAQS) will rely on design values that exclude data that the EPA has determined meet 

the exceptional event weight-of-evidence requirements. These regulatory actions require 

the EPA to provide an opportunity for public comment prior to taking a final Agency 

action. If the EPA pursues one of these actions for a given area, the EPA will open a new 

comment period during which the public may comment on the exceptional event 

submission and/or the EPA’s determinations. The EPA must consider and respond to 

received comments before taking final regulatory action. 

15. Question: It is possible for events to affect more than one state. Each state/air agency 

must then submit its own exceptional events demonstration package, which may result in 

redundant work. Could the EPA take on multi-state/agency demonstrations? 

Answer: The primary responsibility for developing demonstrations lies with state, local, 

and tribal air agencies. The EPA encourages states and air agencies to coordinate with 

each other in compiling demonstration packages, and these agencies may submit some of 

the same data and analyses when a single event affects multiple jurisdictions. Each 

NAAQS exceedance, however, will likely have some unique properties (e.g., unique 

monitoring locations, different surrounding and potentially contributing sources with 

varying levels of control, different historical concentration patterns, etc.). States/agencies 

need to address these unique characteristics in individual submittal packages. Similarly, 

where a single event results in exceedances of multiple NAAQS (e.g., annual and 24-hour 

PM), the submitting agency needs to address the unique features of each NAAQS 

exceedance or violation (e.g., potentially different monitoring locations, different 

historical concentration patterns). An air agency could submit a single demonstration 

package for a single event affecting multiple NAAQS provided the air agency clearly 

identifies the unique characteristics of each NAAQS. 

 7

 If the EPA is the permitting authority, the EPA will propose permits on this basis. If the EPA is commenting 

on another permitting authority’s proposed action, the EPA’s comments will be consistent with the 

determinations in this guidance document and any applicable NSR permitting and/or modeling guidance. 

8

Applicable only to PM10 and PM2.5. See “Transportation Conformity Guidance for Quantitative Hot-spot 

Analyses in PM2.5 and PM10 Nonattainment and Maintenance Areas,” EPA-420-B-10-040, US EPA Office of 

Transportation and Air Quality, December 2010, page 98. 9

These data may be included in statistics intended to describe current status and trends in actual air quality in 

the area for public information purposes including reporting of the Air Quality Index. 
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For example, if multiple states or jurisdictions are affected by a Saharan dust plume, they 

could collaborate and submit a common demonstration component (e.g., the same or very 

similar information in multiple submittals) for the “not reasonably controllable or 

preventable” and “human activity unlikely to occur or natural event” elements. Because 

the actual event-related exceedance would have been measured by different monitors 

located in different regions with possibly different contributing factors (e.g., rural 

monitor affected by both dust from feedlots and Saharan dust and urban monitor affected 

by both nearby industrial sources and Saharan dust), the “clear causal relationship,” “but 

for,” and “historical fluctuations” elements are likely to differ from one submittal to 

another. 

16. Question: Does the EER address scenarios in which temporary activities (e.g., multimonth or multi-year road construction / demolition projects) significantly influence 

measured concentrations at a long-sited monitor such that the nature of the monitor 

changes from “area-wide” to “unique”? 

Answer: Generally, all monitoring data, if meeting applicable CFR regulations, are 

comparable to the NAAQS. There are special provisions applicable only to the PM2.5

NAAQS, which provide that monitors must be representative of area-wide air quality to 

be comparable to the annual NAAQS, and that monitors representative of unique microor middle-scale impact sites are comparable only to the 24-hour PM2.5 NAAQS. See 40 

CFR 58.30. In the provided example, the affected air agency may believe that site meets 

the criteria for data to be comparable only to the 24-hour PM2.5 NAAQS for the period of 

the construction. The affected air agency could request this type of change through 

updates to its annual monitoring network plan or in a separate request, subject to review 

and approval by the EPA regional office. 

The EER does not specifically address temporary, but multi-day or multi-year, 

anthropogenic emission sources such as construction projects. However, neither does the 

EER explicitly place a limit on the duration of a single event. A submitting agency could 

make a showing that a claimed event (e.g., a multi-year road construction project) is not 

likely to recur at the location in question. If the remaining exceptional event criteria and 

demonstration criteria are met, including the requirement that the event (including the 

emissions from the project) is not reasonably controllable, the activity might qualify as 

being an exceptional event. 

Air agencies not wishing to develop exceptional event demonstration packages for the 

described scenario can request agreement from the EPA regional office to relocate a 

monitor that no longer meets monitoring objectives. This process is, however, time 

consuming and resource intensive, so air agencies usually "monitor through" the 

disruption or ask their regional offices to support a temporary shut-down. When the EPA 

regional office approves a temporary shut-down, the operating air agency should assign a 

Null Data Code in AQS for “construction/repairs in area” (AC) to identify and invalidate 

data associated with periods of local construction. 
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16a. Question: Are policy relevant background (PRB) ozone concentrations and exceptional 

events related? 

Answer: PRB ozone concentrations and exceptional events can include partially 

overlapping concepts. The 2007 Staff Paper10 defines policy relevant background ozone 

“as the distribution of [ozone] concentrations that would be observed in the U.S. in the 

absence of anthropogenic (man-made) emissions of precursor emissions (e.g., VOC, 

NOx, and CO) in the U.S., Canada, and Mexico.” In the current ozone review process, 

the EPA has more broadly considered background ozone by assessing three separate 

definitions of background: natural, North American, and U.S. background.11 As before, 

each background is defined as the ozone that would be observed in the absence of 

specific categories of emissions. For example, North American background (NAB) is 

equivalent to PRB. An exceptional event is a natural event (excluding stagnations, 

inversions, high temperatures, or precipitation) or an anthropogenic event that is unlikely 

to recur in the same location. Both exceptional events and North American background 

can involve emissions from natural events like forest wildfires or stratospheric ozone 

intrusions. However, exceedances due to natural emissions that occur every day and 

contribute to policy relevant background, such as biogenic emissions, do not meet the 

definition of an exceptional event and are thus not eligible for exclusion under the EER. 

Routine anthropogenic emissions outside of the U.S. contribute to policy relevant 

background, but are not exceptional events. Air agency preparation of a demonstration 

package and the EPA’s subsequent review of the demonstration package is case-by-case 

based on a weight-of-evidence approach and does not explicitly consider whether the 

event type might contribute to North American background, or any other background 

definition. However, if a natural event that contributes to background ozone causes an 

observed concentration that meets the statutory definition of an exceptional event and 

fulfills all of the exceptional event criteria, the EPA would consider the event to be an 

exceptional event. 

17. Question: Volcanoes on Hawaii are causing 1-hour and 24-hour SO2 exceedances, 

which are clearly volcanic exceptional events. Section 319 of the Clean Air Act and CFR 

require the EPA to provide air agencies with a method to flag and petition the EPA for 

exclusion of exceptional events data. When will the EPA provide the method for SO2? 

Answer: AQS has been modified to allow flags on all criteria pollutant data. The 

specific schedule for exceptional event flagging and documentation submission for data 

to be used in designations decisions is identified in the final primary SO2 NAAQS rule 

 

10 Environmental Protection Agency, Review of the national ambient air quality standards for ozone: assessment 

of scientific and technical information. OAQPS staff paper. (Updated Final) July 2007. Research Triangle Park, 

NC: Office of Air Quality Planning and Standards. EPA-452/R-07-007, available online at: 

http://epa.gov/ttn/naaqs/standards/ozone/s_o3_cr_sp.html. 11 Environmental Protection Agency, Integrated Science Assessment for Ozone and Related Photochemical 

Oxidants. (Third External Review Draft) June 2012. Research Triangle Park, NC: National Center for 

Environmental Assessment – RTP Division, Office of Research and Development. EPA-600/R-10-076C, 

available online at: http://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=242490#Download. 
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(see preamble at 75 FR 35585-35586 and regulatory text at 75 FR 35592). The correct 

flag to use for a volcanic eruption event is “RS.” 

17a. Question: The EPA acknowledges that extreme exceptional events may justify more 

limited demonstration packages. How might the EPA decide whether to consider a 

particular high wind dust event “extreme” when reviewing a limited demonstration 

package? 

Answer: While many dust storms could qualify as exceptional events, the EPA believes 

that most events that are conventionally referred to as “dust storms” should not be 

considered “extreme” events for this purpose. The National Weather Service (NWS) 

defines a “dust storm” as a severe weather condition characterized by strong winds and 

dust-filled air over an extensive area, but does not include any quantified criteria for the 

spatial extent or the concentration of the dust. In contrast, a haboob is of the magnitude 

that could be considered an extreme event. Haboobs are often caused by severe weather 

(e.g., severe thunderstorm activity, cold frontal passages) and are typically characterized 

as “solid walls” of dust that can rise up to 2,000 meters and travel hundreds of miles. 

Generally, the EPA would consider sustained wind speed, spatial extent, visibility, and 

PM concentrations in determining whether an event is an extreme event. An example of 

an event that could be considered an exceptional event but not an extreme event would be 

the Santa Ana winds blowing at 25-30 mph, creating an exceedance at one monitor, with 

maximum hourly PM10 levels of less than 800 μg/m3

. In contrast, a haboob that occurred 

in Phoenix in 2011 had downburst winds of 70 mph, with a wall of dust moving at 30-40 

mph for 150 miles; hourly PM10 levels of 50,000 μg/m3

 were monitored during this 

event. Both of these events could be considered for exclusion under the EER. The South 

Coast Air Quality Management District prepared a 49-page demonstration package (plus 

an appendix with additional supporting information) for the Santa Ana winds event, parts 

of which have been used as examples in the High Winds guidance document. However, 

the EPA anticipates that much more limited documentation for an event like the haboob 

would be sufficient to convince the EPA (and all other parties) that the event meets the 

several criteria for data exclusion (clear causal connection, not reasonably controllable or 

preventable, etc.). 

18. Question: Carbon monoxide (CO) flags are in AQS for exceedances caused by fires, but 

the CO NAAQS (40 CFR 50.8) does not reference the Exceptional Event Rule. What is 

the EPA’s approach for the treatment of CO data affected by exceptional events? 

Answer: CO flagging, including the option for the EPA’s concurrence, has been enabled 

in AQS. CO flags from structural fires and wildfires that qualify as exceptional events 

have been allowed in historic EPA guidance. The EER Preamble (72 FR 13563) explains 

the EPA’s position with respect to exceptional event flagging for pollutants for which the 

statement of the NAAQS in 40 CFR part 50 does not explicitly reference the Exceptional 

Events Rule: “In the interim, where exceptional events result in exceedances or violations 

of NAAQS that do not currently provide for special treatment of the data, we intend to 

use our discretion as outlined under section 107(d)(3) not to redesignate affected areas as 
Interim Exceptional Events Rule Frequently Asked Questions 

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nonattainment based on these events.” Therefore, air agencies may flag CO data in AQS 

and the EPA may apply the same process and approval criteria as in the Exceptional 

Events Rule. 

On August 12, 2011, the EPA issued a decision to retain the current suite of CO standards 

without revision (see 76 FR 54294). Because the EPA made no revisions to the CO 

standards, it promulgated no related changes to the Exceptional Events Rule. 

19. Question: The limited maintenance plan requirements for PM10 require a demonstration 

that the area design value is less than or equal to 98 g/m3

. Flagging of values between 

98 g/m3 and the NAAQS are therefore relevant for this regulatory decision. Can air 

agencies flag and request/receive the EPA’s concurrence on these values, which are not 

exceedances and do not contribute to violations? 

Answer: Yes. The May 7, 2009, memorandum from William T. Harnett to Regional Air 

Division Directors states the following regarding the PM10 limited maintenance plan 

option: “In determining eligibility for the limited maintenance plan option, the EPA will 

treat 24-hour average air quality data between 98 g/m3 and 155 g/m3 in a manner 

analogous to the treatment of exceedance data under the Exceptional Events Rule, 

provided the impacted data meet the general definition and criteria for exceptional events 

(natural event, or exceptional event that is not reasonable controllable or expected to 

recur).” This memorandum is posted on the EPA website at 

http://www.epa.gov/ttn/oarpg/t1/memoranda/lmp_final_harnett.pdf 

19a. Question: What does the EPA mean when we say we will review exceptional event 

demonstration submittals using a “weight-of-evidence” approach? 

Answer: In using the term “weight-of-evidence,” the EPA believes we should consider 

all relevant evidence and qualitatively “weigh” this evidence based on its relevance to the 

EER criterion being addressed, the degree of certainty, its persuasiveness, and other 

considerations appropriate to the individual pollutant and the nature and type of event. 

 

20. Question: Exactly which section(s) of the preamble to the final Exceptional Event Rule 

has been declared a “legal nullity” by the court, and what does that mean? 

Answer: In NRDC v. EPA, No. 07-1151 (D.C. Cir. 3/20/09), the DC Circuit Court states 

that 

“In one section of the preamble, EPA refers to its ‘final rule concerning high wind 

events’, which ‘states that ambient particulate concentrations due to dust being raised 

by unusually high winds will be treated as due to uncontrollable natural events’ when 

certain conditions apply (72 Fed. Reg. 13576). There is no such final rule. The final 

rule [language in 40 CFR 50 and 40 CFR 51.930] does not mention high wind events 

or anything about ‘ambient particulate matter concentrations.’ EPA calls this a 

drafting error. In light of the error, the high wind events section of the preamble is a 

legal nullity.” 
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The EPA considers the “high wind events section of the preamble” to which the court 

referred to be the section titled “B. High Wind Events” beginning on 72 FR 13576. This 

does not necessarily mean that these passages do not reflect the EPA’s interpretation of 

what might be appropriate under the EER. Rather, it means that implementing air 

agencies and other stakeholders should rely on other parts of the preamble and other EPA 

guidance instead of statements in these passages of the final rule preamble, which should 

be treated as not having been published. 

20a. Question: What fire-related definitions should air agencies use in their exceptional 

event documentation? 

Answer: Land Management Agencies modified their fire-related definitions after the 

EPA promulgated the Exceptional Events Rule. The EPA is using the following firerelated terminology in the interim exceptional events guidance documents to ensure 

consistency: 

Prescribed fire - Any fire intentionally ignited by management under an approved plan to 

meet specific objectives. 

Wildfire – Any fire started by an unplanned ignition caused by lightning; volcanoes; 

unauthorized activity; accidental, human-caused actions; and escaped prescribed fires. 

20b. Question: How should air agencies support a claim that emissions from wildfires are 

“not reasonably controllable or preventable”? 

Answer: The Clean Air Act and the EER apply the “not reasonably controllable or 

preventable” requirement to any event that an air agency wishes to be treated as an 

exceptional event, and thus it applies to wildfires. The current United States Forest 

Service (USFS) definitions of “wildfire” and “prescribed fire” define these events in 

terms of purpose and deliberateness of ignition (See definitions in response to Question 

20a). Based on the USFS definitions, a wildfire is a fire that has started from an 

unintentional ignition or an unintentional escape of a prescribed fire. The initiation of a 

wildfire is thus by definition unplanned, but the concepts of reasonable prevention and 

control should not be overlooked in an exceptional event demonstration. The EPA 

recognizes that wildfires and emissions from wildfires are generally not reasonable to 

prevent or control. 

When documenting the “not reasonably controllable or preventable” criterion in their 

wildfire exceptional event demonstration submittal, air agencies should identify the 

origin and evolution of the wildfire, describe local efforts to prevent fires due to 

unauthorized activity or accidental human-caused actions (if relevant given the origin of 

the fire)12, and explain how any efforts to limit the duration or extent (and thus the 

 

12 Prevention/control efforts could include posting High Fire Danger signs to make people more careful and 

prevent accidental fires, and/or taking reasonable action to contain a fire once it has started. 
Interim Exceptional Events Rule Frequently Asked Questions 

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emissions) from the wildfire were reasonable. During wildfires, fire management 

resources deployed to the fire event give first priority to protecting life and property. 

Because wildfires are, by definition, unplanned and unwanted, fire management 

resources often have limited advance notice of ignition and location, which generally 

limits preparation time and reasonable efforts to limit the duration or extent of a wildfire. 

In light of these considerations, the EPA believes that it will generally be sufficient for air 

agencies to provide a statement such as the following to document the “not reasonably 

controllable or preventable” criterion for wildfires: “Based on the documentation 

provided in [section X] of this submittal, [lightning] caused the unplanned, unwanted 

wildfire event. The responsible agencies did their reasonable best to control the extent of 

and extinguish the fire by taking the following actions [insert list or description of actions 

taken]. Therefore, emissions from this wildfire were ‘not reasonably controllable or 

preventable.’” For fires that could have been suppressed or contained but which fire 

management officials allowed to burn for resource management purposes, air agencies 

can generally reference or paraphrase a previously adopted resource management plan to 

support the “not reasonably controllable or preventable” criterion. 

21. Question: The Exceptional Event Rule allows for exclusion of data affected by a 

prescribed fire if the usual requirements of the rule are satisfied and if the air agency has 

adopted and is implementing a Smoke Management Program (SMP) or if the air agency 

has ensured that the burner employed basic smoke management practices. Are there 

minimum requirements for a Smoke Management Program? What are “basic smoke 

management practices?” 

Answer: The preamble to the Exceptional Events Rule at 72 FR 13567 describes an 

SMP as establishing a basic framework of procedures and requirements for managing 

smoke from a prescribed fire managed for resource benefits. Further, the EPA’s “Report 

to Congress on Black Carbon”13describes the intent of SMPs as “mitigat[ing] the public 

health and welfare impacts from prescribed fires and promot[ing] communication and 

coordination of prescribed burning among land owners.” The Report to Congress also 

states that basic smoke management practices could “...include, among other practices, 

steps to minimize air pollutant emissions during and after the burn, evaluate dispersion 

conditions to minimize exposure of sensitive populations, and identify procedures to 

ensure that burners are using basic smoke management practices.” The EPA intends to 

develop separate guidance to address this issue, which will be issued at a later date 

following an opportunity for stakeholder input. 

22. Question: Is there a tie between the requirements of 40 CFR 51.930 Mitigation of 

Exceptional Events and the EPA’s approval for exclusion of data affected by an 

exceptional event? 

 

13 Report to Congress on Black Carbon, EPA-450/R-12-001, US EPA, March 2012, page 230. Available at 

http://www.epa.gov/blackcarbon/. 
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Answer: The EPA encourages the submittal of mitigation measures with the 

demonstration package, particularly for those events likely to recur. The Exceptional 

Events Rule was promulgated pursuant to Section 319 of the Clean Air Act which 

contains a provision that each air agency “must take necessary measures to safeguard 

public health regardless of the source of the air pollution...” This provision was the basis 

for the mitigation requirements in 40 CFR §51.930 and the requirement in the EER at 40 

CFR §50.14(c)(1)(i) that all air agencies must “notify the public promptly whenever an 

event occurs or is reasonably anticipated to occur which may result in the exceedance of 

an applicable air quality standard.” The language at 40 CFR §51.930 requires that: 

“(a) A State requesting to exclude air quality data due to exceptional events must take 

appropriate and reasonable actions to protect public health from exceedances or 

violations of the national ambient air quality standards. At a minimum, the State 

must: 

(1) Provide for prompt public notification whenever air quality concentrations exceed 

or are expected to exceed an applicable ambient air quality standard; 

(2) Provide for public education concerning actions that individuals may take to 

reduce exposures to unhealthy levels of air quality during and following an 

exceptional event; and 

(3) Provide for the implementation of appropriate measures to protect public health 

from exceedances or violations of ambient air quality standards caused by exceptional 

events.” 

Although the language at 40 CFR §51.930 does not require air agencies to prepare or 

submit a mitigation plan, it does require that air agencies develop and implement 

processes and measures that could easily become the elements of a formal, written plan. 

The mitigation criteria focus on specific measures and actions to protect public health, 

rather than on measures that control or prevent emissions associated with a specific event. 

So, a mitigation plan may include measures that apply to emissions sources in general 

(e.g., dust suppression or covering techniques for mineral processing) rather than those 

measures or controls that might be discussed in the “not reasonably controllable or 

preventable” portion of an event demonstration (e.g., controls/measures X, Y, and Z were 

in place on sources A, B, and C during the time of the event). A mitigation plan may also 

include procedures and responsibilities for public alerts and sheltering advisories. 

Because having a mitigation plan in place will help air agencies meet the EER 

requirements at 40 CFR §50.14(c)(1)(i) related to public notification more systematically, 

the EPA encourages the development and submittal of a mitigation plan with the 

demonstration package if one has not already been adopted. 

 

23. Question: Need a state (or tribe) make an argument or submit evidence about control 

measures for events that took place in other states or countries, on federally-owned and 

managed land, or on tribal (or state) lands not subject to state (or tribal) regulation? 
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Answer: Under the Clean Air Act, the EPA generally considers a state (not including 

areas of Indian country) to be a single responsible actor. Accordingly, neither the EPA 

nor the Exceptional Events Rule provides special considerations for intrastate scenarios 

when an event in one county affects air quality in another county in the same state, 

assuming that the event occurs on land subject to state authority (versus tribal 

government authority). For cases involving intrastate transport, the state or local air 

agency should evaluate whether emissions from neighboring (or contributing) counties 

are not reasonably controllable or preventable. As discussed in greater detail in the 

overview guidance document and the interim High Winds Guidance document, the 

assessment of “not reasonably controllable or preventable” is based on the existing level 

of required control, attainment status, and, for high wind dust events, wind speed and 

other factors. States and tribes should consult with their EPA regional office early in the 

development of an exceptional event demonstration package if they believe that 

emissions from sources on federally-owned and managed land (e.g., national parks within 

the state) have been affected by an event in a way that raises issues of reasonable control. 

Interstate and international transport events are different than intrastate events. The EPA 

believes that generally it is not reasonable to expect the downwind state (i.e., the state 

submitting the demonstration) to require the upwind country or state to have implemented 

controls on sources sufficient to limit event-related air concentrations in the downwind 

state. As with any demonstration submittal, the submitting (downwind) state should 

sufficiently identify all natural and anthropogenic contributing sources of emissions (both 

in-state and out-of-state) to show the causal connection between an event and the affected 

air concentration values. A submitting state may provide a less detailed characterization 

of sources in the upwind state or country than of sources within its jurisdiction. After 

completing the source characterization, the submitting state should assess whether 

emissions from sources within its jurisdiction (i.e., in-state sources) were not reasonably 

controllable or preventable. Although the submitting state should also provide available 

information on the status of control measures for emissions from out-of-state sources, the 

submitting state may determine based on available information that the “not reasonably 

controllable or preventable” criterion is satisfied in light of the state’s inability to require 

controls of the upwind state. When assessing emissions transported from other states or 

countries, the submitting state can say that it characterized the out of state sources, 

determined that these sources contributed to the noted exceedance or violation, and 

determined, based on jurisdictional boundaries and other available information, that 

contributing emissions from the upwind state or country were not reasonably controllable 

or preventable. Submitting states are further required to submit evidence/statements 

supporting the other exceptional event criteria (i.e., clear causal relationship, but for, 

human activity unlikely to recur or a natural event, affects air quality, and historical 

fluctuations). 

The EPA recommends a similar approach to significant out-of-state anthropogenic 

sources in the case of a mixed natural/anthropogenic event that the submitting state 

wishes to consider a natural event of the grounds that all significant anthropogenic 

sources were reasonably controlled. 
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As with all exceptional event demonstrations, the EPA will evaluate the information on a 

case-by-case basis based on the facts of a particular exceptional event including any 

information and arguments presented in public comments received by the state in its 

public comment process or by the EPA in a notice-and-comment regulatory action that 

depends on the data exclusion. This response is not intended to discourage states from 

working cooperatively to plan and apply controls on both sides of a state boundary for 

their mutual benefit. 

In addition to the provisions in the EER, the Clean Air Act provides mechanisms in 

sections 110(a)(2)(D) and 126 to address interstate transport issues and mechanisms in 

section 179(b) to address international transport issues. 

24. Question: Need an air agency make an argument or submit evidence about control 

measures for air quality impacts from wind-blown dust from desert land in its natural 

state? 

Answer: While the EPA’s position is generally that impacts from wind-blown dust from 

undisturbed natural deserts are inherently not reasonable to control, the air agency would 

need to state this and provide appropriate supporting documentation in its demonstration 

package. The supporting documentation could include descriptions of the geographic area 

(with maps or available visuals) and a discussion of the historical land use, including 

prior disturbances, water diversions and other historical practices which may have 

occurred on the land, even if the land seems or is considered to be “undisturbed” at 

present. Submitting agencies should also identify all sources contributing to an event and 

identify appropriate control strategies for each anthropogenic source. 

25. Question: Is there a template or example for preparing a demonstration document? 

Answer: The guidance document, “Interim Guidance on the Preparation of 

Demonstrations in Support of Requests to Exclude Ambient Air Quality Data Affected by 

High Winds Under the Exceptional Event Rule” (the High Winds guidance document)

provides this type of advice for demonstrations for high wind dust events. While the High 

Winds guidance document speaks specifically to high wind dust events, the EPA believes 

that many of the principles discussed therein to extend to all types of exceptional events. 

The EPA has also developed a presentation entitled, “Presenting Evidence to Justify Data 

Exclusion as an Exceptional Event: Ideas based on how the EPA has recently 

documented events to support regulatory decisions.” Interested parties can download this 

presentation from the following site: 

http://www.epa.gov/ttn/analysis/docs/IdeasforShowingEEEvidence.ppt. Additionally, the 

EPA is currently developing separate guidance to address the preparation of 

demonstrations to support wildfire-related ozone event claims. 

26. Question: Where can an air agency find examples of demonstrations from other air 

agencies that have been approved by the EPA? 
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Answer: The EPA has posted examples of approved demonstrations at 

http://www.epa.gov/ttn/analysis/exevents.htm. 

27. Question: How quickly will the EPA review the demonstration document and provide 

feedback to the air agency on the approval, or on any suggested improvements? 

Answer: The EPA generally intends to conduct its initial review of a submitted 

exceptional event demonstration package within 120 days of receipt. Following this 

initial review, the EPA will generally send a letter to the submitting agency that includes 

a completeness determination and/or a request for additional information, a date by which 

the supplemental information should be submitted (if applicable)14, and an indicator of 

the timing of the EPA’s final review. The EPA will generally prioritize exceptional event 

determinations that affect near-term regulatory decisions.15 

To promote early communication, the EPA suggests that air agencies provide a letter of 

intent to submit a demonstration package for flagged data in AQS as soon as possible, if 

possible within 12 months from the event occurrence, after the agency identifies the 

event(s) as being significant. A letter of intent is an option for the air agency to use in 

situations where it may help communication and prioritization.16 This initial notification 

can assist both the air agency and the EPA in the planning and prioritization process. The 

EPA intends to respond to such a letter within 60 days of receipt. The EPA response will 

provide the regional office’s best assessment of the priority that can be given to the 

submission once received and any case-specific advice the EPA may have to offer for the 

preparation of the demonstration. 

The EPA intends to make a decision regarding concurrence with an air agency’s flag as 

expeditiously as necessary if required by a near-term regulatory action, but no later than 

18 months following submittal of a complete package. The EPA intends to communicate 

with the submitting agency, as needed, during the demonstration review period. 

Submitting air agencies that believe their demonstration packages are tied to near-term 

regulatory actions should submit their demonstration packages well in advance of the 

regulatory deadline. Air agencies should also identify the relationship between the 

exceptional event-related flagged data and the anticipated regulatory action in the cover 

letter that accompanies their initial submittal package to the reviewing EPA regional 

office. 

 

14 The EPA will generally ask that air agencies provide supplemental information within 60 days from receipt of 

the letter from the EPA. The EPA recognizes that air agencies may need more than 60 days to prepare and 

submit some types of supplemental information. The EPA is willing to work with agencies on supplemental 

timeframes; however, the mandatory timing of the EPA’s actions may limit the response time the EPA allows. 

15 “Regulatory decisions” include findings as to whether the area has met the applicable NAAQS, classification 

determinations, attainment demonstrations, the development of Limited Maintenance Plans, clean data findings. 16 The Letter of Intent is an optional step and the EPA recognizes that air agencies may need additional time to 

prepare and submit demonstration packages particularly where the basis of the exclusion is violating an annual 

standard or a 3-year design value. Similarly, an air agency could consider submitting an annual letter of intent if 

annual submittal makes sense for resource planning or for historically seasonal events. 
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28. Question: Will the EPA ever perform and consider additional data analysis itself before 

deciding whether to approve an air agency-submitted demonstration in support of data 

exclusion? 

Answer: In general, the EPA does not prepare analyses or additional arguments for 

inclusion in a submitted demonstration package or to support the EPA’s concurrence on a 

demonstration package. Rather, the EPA will recommend demonstration package 

improvements to the submitting agency. However, if a demonstration package is 

associated with an imminent regulatory action and the public interest will be best served 

by the EPA’s preparing and/or considering additional analyses, the EPA may either assist 

with or independently prepare supporting analyses that could become part of the 

submission package or an EPA-prepared technical support document. Analyses prepared 

by the EPA could support either approval or disapproval of an air agency’s request for 

concurrence on flagged data. 

28a. Question: Does the Exceptional Events Rule contain a dispute resolution process that 

air agencies can use to resolve disagreements regarding non-concurrence on submittal 

packages? 

Answer: Several mechanisms currently exist that air agencies can use at various points in 

the exceptional events process: 

 Engage in early dialogue with the appropriate EPA regional office. 

 Submit requests for reconsideration to the official who made the determination if a 

request identifies a clear error or if information submitted by the agency was 

overlooked 

 Elevate the concern within the EPA’s chain of command. 

 Participate in the public notice and comment process (see Question 14a). 

 Challenge in an appropriate court the regulatory decision subsequently made that is 

based on the EPA’s exceptional event determination. 

In addition, for complex exceptional events claims or those with significant regulatory or 

other impacts (e.g., those claims that directly influence proposed designation or 

redesignation, classifications, and attainment determinations), the EPA regional office 

staff will generally seek input from other EPA regional offices and/or the EPA 

headquarters staff. 

28b. Question: Can air agencies use data from non-regulatory monitors in exceptional events 

analyses? 

Answer: Yes, air agencies can use data from non-regulatory monitors to support their 

exceptional event demonstrations. Generally, monitoring data used for NAAQS 

regulatory purposes are collected from Federal Reference Method (FRM), Federal 

Equivalent Method (FEM), and/or Approved Regional Method (ARM) monitors that are 

sited and operated in accordance with 40 CFR Part 58. Exceedances or violations 
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identified as exceptional events originate from these same data from FRM/FEM/ARM 

monitors. The AQS, the EPA's repository of ambient air quality data, stores data from 

more than 10,000 monitors, about 5,000 of which are currently active. Although not all of 

these monitors are FRM/FEM/ARM-approved, data from non-FRM/FEM/ARM monitors 

can be used in exceptional event analyses. For example, air quality data summaries from 

non-FRM/ FEM/ARM monitors may be helpful in defining the duration and geographic 

extent of the event, including the area of exceedance/violation and the area containing 

sources that contribute to the exceedances/violations. Similarly, chemical speciation data 

from monitor samples can help characterize the nature of the violation and identify 

contributing emissions sources.

F. Exceptional Event Data Flagging for Air Quality Concentrations that Could 

Contribute to an Exceedance or Violation of the National Ambient Air Quality 

Standards 

29. Question: Each criteria pollutant except PM10 now has multiple NAAQS in effect that 

differ by averaging period, and/or there is an “original” and a lower “revised” NAAQS 

level each of which has regulatory significance. If the EPA approves a measurement 

value for exclusion for one particular NAAQS averaging period and level, does the EPA 

automatically exclude the same value for all the other NAAQS for that pollutant? 

Answer: No. Air agencies should request and support the exclusion of a measured air 

concentration separately for each NAAQS that applies to the pollutant. The EPA will 

similarly provide separate concurrences. 

When initially flagging data, an air agency does not need to identify the specific NAAQS 

for which it seeks to exclude a measured concentration. The EPA’s ambient air quality 

database, AQS, is designed to allow an air agency to apply a single flag to a measured 

concentration value, which merely indicates the agency’s interest in excluding that value 

with respect to one or more of the applicable NAAQS. Later, in the request for data 

exclusion (i.e., the demonstration), the air agency can indicate the specific NAAQS for 

which it seeks exclusion and for which the demonstration addresses the Exceptional 

Events Rule criteria. When the EPA makes a decision regarding concurrence with an air 

agency’s flag, it will generally identify in its approval/disapproval letter (or other official 

notice) all of the NAAQS for which the EPA has concurred on the flag. The EPA will 

also generally set a flag in AQS indicating concurrence with respect to a specific single 

NAAQS or a specific combination of NAAQS for that pollutant (e.g., in the case of 

PM2.5, the 24-hour NAAQS only, the annual NAAQS only, or both the 24-hour and the 

annual average NAAQS). The EPA does this by associating one or more “pollutant 

standard ID” value with the concurrence. 

Air agencies preparing demonstrations to support requests to exclude 24-hour average 

values for PM2.5 and PM10 should flag all 24 1-hour values within a given day. If 

concurred upon, flagging all 1-hour values will ultimately result in the same available 

remaining data for regulatory analysis and calculation regardless of whether the 24-hour 

PM2.5 or PM10 measurement data are collected from filter-based or continuous monitoring 
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instruments.17 The EPA believes flagging all 24 hourly values is appropriate because 

flagging only peak or selected hours could result in the remaining hourly values still 

meeting the data completeness requirements. Exclusion of only the high hourly 

concentrations could result in AQS calculating a valid low (or, potentially high) biased 

24-hour concentration under the rules for data interpretation.18

 

The EPA concurrence flags entered into AQS prior to the March 2010 re-engineering of 

AQS to accommodate the Exceptional Events Rule did not indicate the specific single 

NAAQS or the specific combination of NAAQS for which the exclusion was approved. 

These “legacy” concurrence flags have been converted to the new approach using the 

following defaulting scheme: 

 For ozone, all legacy flags were treated as applying to both the 0.08 ppm 8-hour 

NAAQS and the 0.12 ppm 1-hour NAAQS. This default was chosen because as of 

March 2010, designations under the 2008 NAAQS of 0.075 ppm had been 

suspended pending reconsideration of that NAAQS, and AQS staff were not 

aware of any concurrences already granted with respect to the 0.075 ppm 

NAAQS. 

 For PM2.5, all concurrences on events with dates prior to January 1, 2005 

(meaning the date of the concentration, not the date of the EPA’s concurrence) 

were presumed to be applicable only to the annual PM2.5 NAAQS. This default 

was chosen because prior to the revision of the 24-hour PM2.5 NAAQS in 2006, 

violations of the 1997 24-hour NAAQS were extremely rare. 

 For PM2.5, all concurrences on events with dates of January 1, 2005 through 

March 2010 were presumed to be applicable only to the 24-hour NAAQS because 

there were no revisions to the annual PM2.5 NAAQS during this timeframe, so 

designations to nonattainment for the annual PM2.5 standard were extremely rare. 

This 24-hour PM2.5 NAAQS default was chosen because it was possible for 

designations under the 2008 24-hour NAAQS to be based on data as early as 

2005. 

 For PM10, all concurrences were presumed to apply to the 24-hour NAAQS, as 

the annual PM10 NAAQS was revoked in 2006.19 

 

17 Filter based instruments typically record a single value within a 24-hour period while continuous monitors 

typically collect 24 1-hour measurements. Because AQS can calculate a valid 24-hour average concentration 

with as few as 18 hours, it may be necessary to exclude hours not actually affected by the event to ensure the 

same data exclusion outcome as if the measurement had been made with a 24-hour filter. 

18 The form of the 24-hour PM2.5 NAAQS of 35 μg/m3 is 98th percentile averaged over 3 years. The form of the 

primary annual PM2.5 NAAQS of 12 μg/m3 is an annual mean averaged over 3 years. The form of the 24-hour 

PM10 NAAQS of 150 μg/m3 is not to be exceeded more than once per year on average over 3 years. Biased 

concentrations can potentially skew the determination of the 98th percentile and/or the annual mean for PM2.5 

and the averages for PM2.5 or PM10 calculated to determine compliance with the relevant NAAQS. 19 The EPA realizes that many of the defaulted EPA concurrences for pre-2006 PM10 concentrations that were 

below the level of the 24-hour PM10 NAAQS actually were applicable to the annual PM10 NAAQS, but this 

approach was the most practical way to ensure that all other concurrences originally intended to be applicable to 

the 24-hour NAAQS were preserved. Because concentrations below the level of the 24-hour NAAQS have no 

effect on attainment determinations for the 24-hour NAAQS, no error can come from treating such values as 
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 For CO, all concurrences were presumed to apply to both the 1-hour and the 8-

hour NAAQS. This default was chosen to ensure that the concurrence applied to 

whichever NAAQS had been exceeded and was the basis for the exclusion 

request. 

 For SO2, all concurrences were presumed to apply to both the 24-hour and the 

annual NAAQS. This default was chosen to ensure that the concurrence applied to 

whichever NAAQS had been exceeded and was the basis for the exclusion 

request. No flags were assumed to apply to the 1-hour NAAQS because the 1-

hour SO2 standard was not promulgated until June of 2010, after the AQS reengineering. 

 For Pb, all concurrences (if any existed) were presumed to apply to the quarterly 

average NAAQS of 1.5 μg/m3. This default was chosen because March 2010 was 

prior to the EPA issuing final designations under the 2008 Pb NAAQS of 0.15 

μg/m3

. 

 For NO2, all concurrences were presumed to apply to the annual NAAQS because 

the 1-hour NO2 standard was not promulgated until February of 2010. 

For concurrences on events with dates after the March 2010 re-engineering of AQS, the 

EPA will specify the NAAQS to which the concurrence applies. If this defaulting scheme 

does not properly represent the actual concurrence action that was taken by the EPA 

regional office, the regional office should revise and correct the concurrence flags, if it 

has not already done so. 

 

Air agencies can find detailed information on the use of events flags in AQS in a tutorial 

posted at http://www.epa.gov/ttn/airs/airsaqs/manuals/ExceptionalEventTutorial.pdf. The 

tutorial discusses concurrence flags on page 20. 

30. Question: For a NAAQS that is defined for a multi-hour or multi-day averaging time, 

but for which concentrations are measured, reported, and flagged on the basis of a shorter 

time period, what comparisons between measurements and the NAAQS level should air 

agencies prepare to satisfy the “but for” test? 

Answer: One requirement for data exclusion under the Exceptional Events Rule is that 

there would have been no exceedance or violation of the NAAQS “but for” the event. In 

AQS, flagging and concurrence are done for each individual reported measurement. 

When the averaging period for the NAAQS is the same as the measurement duration 

period, individual measurements that have event flags attached can be compared directly 

to the level of the NAAQS. This is the case for the 1-hour ozone, 1-hour CO, 1-hour SO2, 

and 1-hour NO2 NAAQS. This is also the case when 24-hour filter-based PM10 or PM2.5

concentrations are compared to the respective 24-hour NAAQS.20 However, a difference 

 having been concurred. Nevertheless, the EPA regional office may choose to update these concurrence flags as 

time permits. 

20 Air agencies have for many years reported SO2 concentrations as hourly averages. While some air agencies 

have also voluntarily reported 5-minute average concentrations also, either for each of the 12 5-minute blocks in 

an hour or for the maximum 5-minute average concentrations (block or running) during an hour, it is the hourly 

concentration averages that should be compared to the 1-hour SO2 NAAQS. Under a change in SO2 monitoring 
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exists for the following NAAQS between the time period for reporting concentrations and 

the averaging period to which the level of a NAAQS applies. 

 Ozone, CO, NO2, and SO2 are reported to AQS as 1-hour measurements, but all 

three have NAAQS defined for longer averaging periods (3-hours, 8-hours, 24-

hours, and/or annual). The longer-period concentration values that are compared 

to these NAAQS are calculated from the submitted hourly values within AQS and 

cannot have event flags attached to them. 

 Pb is reported as 24-hour measurements, but the old and new NAAQS are both 

for three-month averages (quarterly averages and 3-month rolling averages, 

respectively). The quarterly and 3-month concentration values that are compared 

to these NAAQS are calculated from the submitted 24-hour measurements within 

AQS and cannot have event flags attached to them. 

 When using automated/continuous monitoring equipment, PM2.5 and PM10 data 

are reported as 1-hour measurements but there are PM2.5 and PM10 NAAQS with 

24-hour averaging periods and a PM2.5 NAAQS with an annual averaging period. 

The 24-hour and annual values compared to the NAAQS are calculated within 

AQS and cannot have event flags attached to them. As described in more detail in 

the response to Question 29, to ensure the same data exclusion outcome 

regardless of whether PM2.5 and PM10 measurements are made with filter-based or 

continuous monitoring equipment, the EPA intends to exclude all 24 1-hour 

measurements in a given day whenever the “but for” criterion (and other 

exceptional event criterion) are satisfied for that day even if an event only 

affected discrete hours of the day. The EPA will be able to do this only if the air 

agency has applied “R” flags to each of those hours.21 

 When using filter-based monitoring equipment, PM2.5 and PM10 are reported as 

24-hour measurements but there is a PM2.5 NAAQS with an annual averaging 

period. The annual values used in comparisons the NAAQS are calculated within 

AQS and cannot have event flags attached to them. 

 requirements that accompanied the promulgation of the 1-hour SO2 NAAQS, the EPA now requires that air 

agencies report the maximum 5-minute block average concentration, as well as the hourly concentration (see 40 

CFR § 58.12(g)). Air agencies may satisfy the 5-minute reporting requirement by submitting all twelve 5-

minute block averages or by reporting only the maximum 5-minute block average concentration. The EPA’s 

AQS retains the hourly concentration as submitted; AQS does not use 5-minute data to replace the submitted 

hourly concentration. While 5-minute concentrations may play a role in evaluating whether Exceptional Event 

criteria are satisfied for a given hour and event, for example to establish a clear causal connection, they are not 

to be compared to the level of the 1-hour (or any other) NAAQS for SO2 as part of a “but for” demonstration 

and should not be flagged for exclusion under the EER. Air agencies may, however, use “I” series flags 

(Information only) with 5-minute SO2 data. 21 Because AQS can calculate a valid 24-hour average concentration with as few as 18 hours, it may be 

necessary to exclude hours not actually affected by the event to ensure the same data exclusion outcome as if 

the measurement had been made with a 24-hour filter. Exclusion of only the high hourly concentrations could 

result in AQS calculating a valid low (or, potentially high) biased 24-hour concentration under the rules for data 

interpretation. 
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The mismatches of time periods make this a question with a complex answer. The 

following paragraphs, summarized in Table Q30-1, explain the general rationale behind 

the pollutant and NAAQS-specific entries in Table Q30-2. 

To satisfy the “but for” criterion, there must have actually been an exceedance or 

violation of the NAAQS in a time period overlapping with the event and its effects on air 

quality, and which would not have occurred “but for” the effects of the event.22 By 

definition, an exceedance necessarily involves a comparison between an air 

concentration, averaged over a time period equal in length to the averaging time of the 

NAAQS, and the level of the NAAQS. For example, it does not make sense to compare 

an individual 1-hour ozone concentration to the level of the 8-hour NAAQS as part of a 

test of whether the “but for” criterion is met, because the outcome of the comparison for a 

single hour does not indicate whether an exceedance or violation of the 8-hour NAAQS 

occurred, or whether it would not have occurred “but for” the event. Instead, air agencies 

should consider whether the event made a “but for” difference in the average 

concentration over the period that is the same as the averaging period for the NAAQS. 

That is, air agencies making a “but for” argument should compare the average 

concentration, rather than the individual concentrations comprising the average, to the 

identified NAAQS.23 Air agencies should, however, identify in their exceptional event 

submission those particular measurements that caused the elevated average. 

The preamble to the Exceptional Events Rule provides one exception from this formal 

definitional approach. The preamble states that in the particular case of PM2.5, the direct 

comparison of a single 24-hour average concentration (determined from a single filterbased measurement or by averaging 24 1-hour measurements from a continuous 

equivalent instrument) to the level of the annual NAAQS can be the basis for meeting the 

 

22 The EPA interprets the Exceptional Event Rule and its preamble to mean “exceedance or violation” each time 

that “exceedance” or “violation” occurs in the text, consistent with the obvious intent of the Clean Air Act 

amendment requiring the EPA to promulgate the Rule. An “exceedance” occurs each time the concentration in 

the air for the averaging period applicable to the NAAQS is higher than the level of the NAAQS. Most NAAQS 

allow some such occurrences in a 1-year or 3-year time period (depending on the NAAQS). A “violation” of the 

NAAQS occurs when there have been enough high-concentration episodes that the statistical form of the 

particular NAAQS indicates a failure to meet the NAAQS. 23 A scenario could exist in which the effect of an event on one or more 24-hour PM2.5 concentration creates a 

“but for” difference on the annual concentration even though the actual 24-hour concentration(s) on the day(s) 

of the event was below the level of the annual NAAQS. This implies that the EPA could concur with the 

exclusion request for the 24-hour concentration value. However, the Exceptional Events Rule preamble makes 

clear that only 24-hour PM2.5 concentrations that are above the level of the annual NAAQS maybe excluded. 

Similarly, the EPA generally does not intend to concur with respect to any NAAQS on a flag for a 1-hour NO2

and SO2 concentration that is below the level of the respective annual NAAQS, regardless of the outcome of 

“but for” tests based on comparison of 24-hour or annual average concentrations to their same-period NAAQS. 

Also, the EPA generally does not intend to concur on flags for a 24-hour Pb measurement below the level of the 

old (fixed quarterly average) Pb NAAQS or the new (rolling 3-month average) Pb NAAQS. The EPA believes 

that it is generally appropriate to use the similar restriction for PM2.5 stated and explained in the preamble to the 

Exceptional Event Rule. Moreover, it is highly unlikely that even several hourly concentrations below the level 

of the annual NO2 NAAQS of 53 ppb could include an event contribution that when summed with all other 

hourly concentrations and then divided by 8760 (24 hours times 365 days), could result in the annual average 

NO2 concentration crossing from below the level of the annual NAAQS to above the level of the annual 

NAAQS. 
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“but for” criterion for exceedances or violations of the annual NAAQS.24 In context, it is 

clear that based on this comparison, a 24-hour concentration can be excluded from the 

calculation of the annual PM2.5 NAAQS design value, if other rule criteria are also met. It 

is therefore not necessary to show that the annual average PM2.5 concentration was above 

12 or 15 μg/m3

 with the event and would have been below 12 or 15 μg/m3 “but for” the 

single event at issue. Such a concentration can also be excluded from the calculation of 

the design value for the 24-hour PM2.5 NAAQS, although this is likely to make a 

difference to meeting the NAAQS only if the actual measured concentration were close 

to or above 35 μg/m3

. This special case is reflected in Table Q30-2. 

In light of this departure in the preamble from a formal definitional approach in the case 

of a 24-hour PM2.5 measurement and the annual PM2.5 NAAQS, Table Q30-2 also 

provides a parallel special approach for similar comparisons involving Pb, NO2 and SO2 

that the EPA generally intends to apply. The EPA believes applying this interpretation for 

Pb, NO2, and SO2 is consistent with the interpretation in the preamble for PM2.5 and is 

consistent with the EPA’s intent in drafting the Exceptional Events Rule. That is, a 24-

hour average concentration of Pb, NO2, or SO2 can be compared to the NAAQS level 

defined for a longer period, for purposes of meeting “but for” with respect to both the 24-

hour NAAQS, if applicable, and the NAAQS with the longer averaging period. 

Table Q30-1. Principles for General Approach to Satisfying the “But For” Test 

Note: The principles identified in this table are presented from the more general and/or selfevident to the more specialized and/or derivative. 

 Principle Application to Specific NAAQS Exceptions 

1 A single measurement may be 

compared directly to the level of the 

NAAQS if the averaging times are 

the same. 

 1-hour NAAQS for CO, SO2, NO2, 

and ozone. 

 24-hour filter-based PM2.5 or PM10

measurements vs. 24-hour 

NAAQS. 

 

24 When the EPA promulgated the Exceptional Events Rule in 2007, the level of the annual PM2.5 NAAQS was 

15 μg/m3

. On December 14, 2012, the EPA promulgated a revised annual PM2.5 NAAQS of 12 μg/m3 (78 FR 

3086). Because both standards apply, an air agency can choose the appropriate level of the annual NAAQS (i.e., 

either 12 μg/m3

 or 15 μg/m3) as the basis for meeting the “but for” criterion. For example, an air agency 

developing an exceptional events demonstration package that may influence an attainment demonstration for 

the annual PM2.5 NAAQS of 15 μg/m3

 would likely use 15 μg/m3 as the basis for meeting the “but for” criterion 

while an air agency preparing a demonstration package that may influence initial area designation status for the 

2012 annual PM2.5 NAAQS of 12 μg/m3

 would likely use 12 μg/m3 as the basis for meeting the “but for” 

criterion. 
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 Principle Application to Specific NAAQS Exceptions 

2 When the measurement time is 

shorter than the averaging time of 

the NAAQS (e.g., 1-hour O3

measurements and the 8-hour O3

NAAQS), air agencies can compare 

the average of the multiple 

measurements within the averaging 

period of the NAAQS to the level of 

the NAAQS (e.g., compare the 

average of eight 1-hour 

measurements to the 8-hour 

NAAQS). If this comparison shows 

that the average is more than the 

NAAQS but would have been 

below the NAAQS in the absence of 

the event, then the “but for” test will 

have been met for those individual 

measurements in the longer 

averaging period that were affected 

by the event. Air agencies should, 

however, identify in their 

exceptional event submission those 

particular measurements that caused 

the elevated average. 

 1-hour ozone measurements vs. 8-

hour NAAQS. 

 1-hour CO measurements vs. 8-

hour NAAQS. 

 1-hour SO2 measurements vs. 3-

hour, 24-hour, and annual 

NAAQS. 

 1-hour NO2 measurements vs. 

annual average .NAAQS. 

 1-hour PM2.5 measurements vs. 24-

hour and annual average NAAQS. 

 1-hour PM10 measurements vs. 24-

hour average NAAQS. 

 24-hour PM2.5 measurements vs. 

annual average NAAQS. 

 24-hour Pb measurements vs. 

quarterly average NAAQS. 

 24-hour Pb measurements vs. 

rolling 3-month average NAAQS. 

If a measurement value 

is below the level of the 

quarterly, rolling 3-

month, or annual 

average NAAQS, it 

generally will not be 

considered for 

exclusion regardless of 

the outcome of 

comparing the longer 

period average to the 

NAAQS level. 

3 When the PM2.5 or Pb measurement 

time is 24 hours (and when hourly 

PM2.5 measurements are used to 

calculate a 24-hour concentration), 

it is also permitted to compare the 

24-hour concentration to the annual 

average PM2.5 NAAQS or the 

quarterly or rolling 3-month Pb 

NAAQS. 

 24-hour PM2.5 concentrations vs. 

the annual average NAAQS 

(expressly permitted in the 

preamble to the Exceptional 

Events Rule). 

 24-hour Pb filter measurements vs. 

the quarterly average and rolling 3-

month average NAAQS (suggested 

by this guidance as a consistent 

with the intent of the PM2.5

provision in the preamble). 

4 1-hour SO2 measurements may be 

averaged to 24-hour periods and 

then compared to the annual 

average NAAQS. If the “but for” 

test is supported by this comparison, 

the showing supports a “but for” 

finding with respect to the 24-hour 

NAAQS for those individual 1-hour 

measurements in the 24-hour 

averaging period that were affected 

by the event. 

 A comparison of 1-hour SO2

measurements vs. the annual 

average NAAQS (where the 30 

ppb annual SO2 NAAQS still 

applies) is recommended in this 

guidance to create a reasonable 

benchmark for judging the 

excludability of 1-hour SO2

measurements for the purpose of 

the annual NAAQS, for cases 

when the event did not affect the 

annual average enough to make a 

“but for” difference relative to the 

annual average NAAQS. 

5 When there is no NAAQS for the 

24-hour averaging period, 1-hour 

measurements may be compared 

directly to the annual NAAQS. 

 A comparison of 1-hour NO2

measurements vs. annual average 

NAAQS is recommended in this 

guidance to create a reasonable 
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 Principle Application to Specific NAAQS Exceptions 

benchmark for judging the 

excludability of 1-hour NO2

measurements for the purpose of 

the annual NAAQS, for cases 

when the event did not affect the 

annual average enough to make a 

“but for” difference relative to the 

annual average NAAQS. 

6 Otherwise, single 1-hour 

measurements generally may not be 

compared to the level of the annual 

average NAAQS. 

 Single 1-hour SO2 measurements 

generally may not be compared the 

annual average NAAQS (because 

there is a 24-hour NAAQS for SO2 

with a defined averaging 

methodology). 

 Single 1-hour PM2.5 measurements 

generally may not be compared to 

the annual average NAAQS 

(because there is a 24-hour 

NAAQS for PM2.5 with a defined 

averaging methodology). 

Table Q30-2 identifies the comparisons and conclusions that generally would help satisfy 

the “no exceedance but for” test for each pollutant, for each current NAAQS. Note that 

for completeness Table Q30-2 addresses some situations that may be very unlikely to 

actually occur – for example, that a single event might cause an exceedance of the annual 

average NO2 NAAQS. Also, note that Table Q30-2 addresses only the “no exceedance 

but for” question. As indicated in the answer to Question 31, even if an event cannot be 

demonstrated to make a “but for” difference in whether an exceedance occurred, it is 

possible that it makes a “but for” difference in whether a 3-year violation of the NAAQS 

occurred, for the NAAQS that are defined based on a 3-year average design value 

concentration. The logic behind Table Q30-2 applies to a “no violation but for” test also. 

Air agencies may request assistance from the EPA regional office on applying this logic 

when performing a “no violation but for” test. 

 
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Table Q30-2. General Approaches for Satisfying the “No Exceedance But For” Test 

 Pollutant Specific Case: 

NAAQS level 

NAAQS averaging period 

Measurement period

General Approach

1 Ozone 0.12 ppm 

1-hour averaging period 

1-hour measurement 

 If a 1-hour measured concentration was above 

0.124 ppm but would have been 0.124 ppm or less 

in the absence of the event, the 1-hour ozone 

concentration value meets the “but for” test for 

purposes of comparison to the 1-hour NAAQS. If 

other criteria are also met for that hour (e.g., there 

was a clear causal relationship between the event 

and that hour’s ozone level, among other criteria), 

then the hour can be flagged and concurred for 

exclusion. 

2 Ozone 0.08 ppm 

8-hour averaging period 

1-hour measurement 

 If the daily maximum 8-hour average of 

measured concentrations was above 0.084 ppm 

but would have been 0.084 ppm or less in the 

absence of the event, those 1-hour concentration 

values that were affected by the single event meet 

the “but for” test for purposes of comparison to 

the 0.08 ppm 8-hour ozone NAAQS. 

The exclusion of some or all hours of the 8-hour 

period that was originally the daily maximum 8-hour 

period may cause another 8-hour period to become the 

daily maximum. The “but for” comparison can be 

repeated for this new 8-hour period, which may result 

in flagging and concurrence for more 1-hour values. It 

is also possible for additional hourly concentrations 

that were not included in the original 8-hour block to 

be excluded as part of a second 8-hour block. 

3 Ozone 0.075 ppm 

8-hour averaging period 

1-hour measurement 

 If the daily maximum 8-hour average of 

measured concentrations was above 0.075 ppm 

but would have been 0.075 ppm or less in the 

absence of the event, those 1-hour concentration 

values that were affected by the single event 

meet the “but for” test for purposes of comparison 

to the 0.075 ppm 8-hour ozone NAAQS. 

The exclusion of some or all hours of the 8-hour 

period that was originally the daily maximum 8-hour 

period may cause another 8-hour period to become the 

daily maximum. The “but for” comparison can be 

repeated for this new 8-hour period, which may result 

in flagging and concurrence for more 1-hour values. It 

is also possible for additional hourly concentrations 

that were not included in the original 8-hour block to 

be excluded as part of a second 8-hour block. 
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Table Q30-2. General Approaches for Satisfying the “No Exceedance But For” Test 

 Pollutant Specific Case: 

NAAQS level 

NAAQS averaging period 

Measurement period

General Approach

4 PM2.5 35 μg/m3 

24-hour averaging period 

1-hour measurement 

(Note: Air agencies can use either 

15.0 μg/m3 or 12.0 μg/m3

 as a 

basis for comparison.) 

 If the 24-hour average concentration based on 1-

hour measurements was above 35.4 μg/m3 (after 

truncating after the first decimal digit, per 40 

CFR 50 Appendix N section 3.0(c)) but would 

have been 35.4 μg/m3 or less in the absence of 

the event, those 1-hour concentration values that 

were affected by the single event meet the “but 

for” test for purposes of comparison to the 35 

μg/m3 24-hour PM2.5 NAAQS. 

 Also, if the 24-hour average concentration based 

on 1-hour measurements was above 12.0 / 15.0 

μg/m3 (after truncation after the first decimal 

digit) but would have been 12.0 / 15.0 μg/m3 or 

less in the absence of the event, those 1-hour 

concentration values that were affected by the 

single event are eligible to be considered for 

exclusion for purposes of comparison to the 35 

μg/m3 24-hour PM2.5 NAAQS. 

5 PM2.5 12.0 μg/m3 

Annual averaging period 

1-hour measurement 

(Note: Air agencies preparing 

demonstrations involving PM 

concentrations for comparison 

against the 1997 annual PM2.5

standard of 15.0 μg/m3 should 

substitute 12.0 μg/m3

 with 15.0 

μg/m3 in the “General Approach” 

steps in the next column.)

 If the annual average PM2.5 concentration was 

above 12.0 μg/m3 but would have been equal to 

or less than 12.0 μg/m3 (after rounding to one 

decimal digit) in the absence of the single 

event’s effect on one or more hours, those 1-hour 

concentration values that were affected by the 

single event meet the “but for” test for purposes 

of comparison to 12 μg/m3 annual PM2.5

NAAQS. 

 Also, if the 24-hour average concentration based 

on 1-hour measurements was above 12.0 μg/m3 

(after rounding to one decimal digit, per 40 CFR 

50 Appendix N section 4.3(a)) but would have 

been equal to or less than 12.0 μg/m3

in the 

absence of the event, those 1-hour concentration 

values that were affected by the single event 

meet the “but for” test for purposes of 

comparison to 12 μg/m3 annual PM2.5 NAAQS. 

However, an hourly value must be part of a 24-hour 

average concentration that is above 12 μg/m3

 (after 

rounding to one decimal digit) to be excluded from an 

annual NAAQS calculation. 
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Table Q30-2. General Approaches for Satisfying the “No Exceedance But For” Test 

 Pollutant Specific Case: 

NAAQS level 

NAAQS averaging period 

Measurement period

General Approach

6 PM2.5 35 μg/m3 

24-hour averaging period

24-hour measurement 

(Note: Air agencies can use either 

15.0 μg/m3 or 12.0 μg/m3

 as a 

basis for comparison.) 

 If the 24-hour average concentration was above 

35.4 μg/m3 (after truncating after the first 

decimal digit, per 40 CFR 50 Appendix N 

section 3.0(b)) but would have been 35.4 

μg/m3

or less in the absence of the event, the 24-

hr concentration value meets the “but for” test 

for purposes of comparison to 35 μg/m3 24-hour 

PM2.5 NAAQS. 

 Also, if the 24-hour average concentration was 

above 12.0 / 15.0 μg/m3 (after truncating after the 

first decimal digit, per 40 CFR 50 Appendix N 

section 3.0(b)) but would have been 12.0 / 15.0 

μg/m3

or less in the absence of the event, the 24 

average concentration meets the “but for” test for 

purposes of comparison to 35 μg/m3 24-hour 

PM2.5 NAAQS. 

7 PM2.5 12 μg/m3 

Annual averaging period 

24-hour measurement 

(Note: Air agencies preparing 

demonstrations involving PM 

concentrations for comparison 

against the 1997 annual PM2.5

standard of 15.0 μg/m3 should 

substitute 12.0 μg/m3

 with 15.0 

μg/m3 in the “General Approach” 

steps in the next column.) 

 If the annual average PM2.5 concentration was 

above 12.0 μg/m3 (after rounding to one decimal 

digit per 40 CFR 50 Appendix N section 4.2(a)) 

but would have been equal to or less than 12.0 

μg/m3

 in the absence of the single event’s effect 

on one or more days, those 24-hour 

concentration values that were affected by the 

single event meet the “but for” test for purposes 

of comparison to 12 μg/m3 annual PM2.5

NAAQS. 

 Also, if the 24-hour average concentration from 

the filter-based sampler was above 12.0 μg/m3 

(after truncating after the first decimal digit, per 

40 CFR 50 Appendix N section 3.0(b)) but 

would have been equal to or less than 12.0 

μg/m3

in the absence of the event, the 24-hour 

value meets the “but for” test for purposes of 

comparison to 12 μg/m3 annual PM2.5 NAAQS. 

8 PM10 150 μg/m3 

24-hour averaging period 

1-hour measurement 

 If the 24-hour average concentration based on 1-

hour measurements was above 150 μg/m3 (after 

rounding to the nearest 10 μg/m3

, per 40 CFR 50 

Appendix K section 1.0(b)) but would have been 

equal to or less than 150 μg/m3

 in the absence of 

the event, those 1-hour concentration values that 

were affected by the single event meet the “but 

for” test for purposes of comparison to the 150 

μg/m3 24-hour PM10 NAAQS. 
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Table Q30-2. General Approaches for Satisfying the “No Exceedance But For” Test 

 Pollutant Specific Case: 

NAAQS level 

NAAQS averaging period 

Measurement period

General Approach

9 PM10 150 μg/m3 

24-hour averaging period 

24-hour measurement 

 If the 24-hour average concentration from the 

filter-based sampler was above 150 μg/m3 (after 

rounding to the nearest 10 μg/m3

, per 40 CFR 50 

Appendix K section 1.0(b)) but would have been 

equal to or less than 150 μg/m3

 in the absence of 

the event, the 24-hour value meets the “but for” 

test for purposes of comparison to the 150 μg/m3 

24-hour PM10 NAAQS. 

10 CO 35 ppm 

1-hour averaging period 

1-hour measurement 

 If a 1-hour measured concentration was above 

35.0 ppm (after rounding to one decimal digit 

per 40 CFR 50.8(d)) but would have been 35.0 

ppm or less in the absence of the event, the 1-

hour CO concentration value meets the “but for” 

test for purposes of comparison to the 1-hour 

NAAQS. 

11 CO 9 ppm 

8-hour averaging period 

1-hour measurement 

 If an 8-hour average of measured concentrations 

is one of the two highest non-overlapping 8-hour 

periods of the year and was above 9.0 ppm (after 

rounding to one decimal digit per 40 CFR 

50.8(d)) but would have been equal to or less 

than 9.0 ppm in the absence of the event, those 

1-hour concentration values that were affected 

by the single event meet the “but for” test for 

purposes of comparison to the 9 ppm 8-hour CO 

NAAQS. 

The exclusion of some or all hours of the 8-hour 

period that was originally one of the two highest nonoverlapping 8-hour periods of the year may cause 

another 8-hour period to become one of two highest 

non-overlapping 8-hour periods of the year. The “but 

for” comparison can be repeated for this new 8-hour 

period, which may result in flagging and concurrence 

for more 1-hour values. It is also possible for 

additional hourly concentrations that were not 

included in the original 8-hour block to be excluded as 

part of a second 8-hour block. 
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Table Q30-2. General Approaches for Satisfying the “No Exceedance But For” Test 

 Pollutant Specific Case: 

NAAQS level 

NAAQS averaging period 

Measurement period

General Approach

12 Pb 1.5 μg/m3 

Quarterly averaging period

24-hour measurement 

 If the quarterly mean was above 1.5 μg/m3 (after 

rounding to one decimal digit) but would have 

been equal to or less than 1.5 μg/m3 in the 

absence of the single event’s effect on some 

day(s), the 24-hour value(s) affected by the 

single event meets the “but for” test for purposes 

of comparison to the 1.5 μg/m3 quarterly average 

Pb NAAQS. (Note that given the 1-in-6 

sampling schedule for Pb, it will be unusual for a 

single event to affect multiple sampling days.) 

 Also, if the 24-hour average concentration from 

the filter-based sampler was above 1.5 μg/m3 

(after rounding to one decimal digit) but would 

have been equal to or less than 1.5 μg/m3

in the 

absence of the event, the 24-hour value meets the 

“but for” test for purposes of comparison to 1.5 

μg/m3 quarterly average Pb NAAQS. 

 

A 24-hour Pb concentration that is equal to or less 

than 1.5 μg/m3 will generally not be considered for 

exclusion. 

13 Pb 0.15 μg/m3 

Rolling 3-month averaging period

24-hour measurement 

 If a 3-month mean was above 0.15 μg/m3 (after 

rounding to two decimal digits) but would have 

been equal to or less than 0.15 μg/m3

 in the 

absence of the single event’s effect on some 

day(s), the 24-hour value affected by the single 

event meets the “but for” test for purposes of 

comparison to the 0.15 μg/m3 quarterly average 

Pb NAAQS. (Note that given the 1-in-6 

sampling schedule for Pb, it will be unusual for a 

single event to affect multiple sampling days.) 

 Also, if the 24-hour average concentration from 

the filter-based sampler was above 0.15 μg/m3 

(after rounding to two decimal digits per 40 CFR 

50 Appendix R section 5(b)) but would have 

been equal to or less than 0.15 μg/m3

 in the 

absence of the event, the 24-hour value meets the 

“but for” test for purposes of comparison to the 

0.15 μg/m3 quarterly average Pb NAAQS. 

A 24-hour Pb concentration that is equal to or less 

than 0.15 μg/m3 will generally not be considered for 

exclusion. 
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Table Q30-2. General Approaches for Satisfying the “No Exceedance But For” Test 

 Pollutant Specific Case: 

NAAQS level 

NAAQS averaging period 

Measurement period

General Approach

14 NO2 100 ppb 

1-hour averaging period 

1-hour measurement 

 If a 1-hour measured concentration was above 

100 ppb (after truncating to a whole number per 

40 CFR 50 Appendix S section 4.2(c)) but would 

have been equal to or less than 100 ppb in the 

absence of the event, the 1-hour NO2

concentration value meets the “but for” test for 

purposes of comparison to the 1-hour NAAQS. 

15 NO2 53 ppb 

Annual averaging period 

1-hour measurement 

 If the annual average of all the measured 1-hour 

concentrations in a year was above 53 ppb (after 

rounding to a whole number per 40 CFR 50 

Appendix S section 4.1(b)) but would have been 

53 ppb or less in the absence of the event, those 

1-hour values that were affected by the single 

event meet the “but for” test for purposes of 

comparison to the 53 ppb annual average NO2

NAAQS. 

 If the 1-hour concentration was above 53 ppb 

(after truncating to a whole number per 40 CFR 

50 Appendix S section 4.2(c)) but would have 

been equal to or less than 53 ppb in the absence 

of the event meets the “but for” test for purposes 

of comparison to annual NAAQS. 

However, a 1-hour NO2 concentration that is below 53 

ppb (after rounding to a whole number) will generally 

not be considered for exclusion. 

16 SO2 75 ppb 

1-hour averaging period 

1-hour measurement 

 If a 1-hour measured concentration was above 75 

ppb (after rounding to a whole number per 40 

CFR 50 Appendix T section 4(c)) but would 

have been equal to or less than 75 ppb in the 

absence of the event, the 1-hour SO2

concentration value meets the “but for” test for 

purposes of comparison to the 1-hour SO2

NAAQS. 
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Table Q30-2. General Approaches for Satisfying the “No Exceedance But For” Test 

 Pollutant Specific Case: 

NAAQS level 

NAAQS averaging period 

Measurement period

General Approach

17 SO2 140 ppb 

24-hour averaging period 

1-hour measurement 

 If the 24-hour average concentration based on 1-

hour measurements was above 140 ppb (after 

rounding to the nearest 10 ppb per 40 CFR 

50.4(b)) but would have been equal to or less 

than 140 ppb in the absence of the event, those 1-

hour concentration values that were affected by 

the single event meet the “but for” test for 

purposes of comparison to 140 ppb 24-hour SO2

NAAQS. 

 Also, if the 24-hour average concentration based 

on 1-hour measurements was above 30 ppb (after 

rounding to the nearest 10 ppb per 40 CFR 

50.4(b)) but would have been equal to or less 

than 30 ppb in the absence of the event, those 1-

hour concentration values that were affected by 

the event meet the “but for” test for purposes of 

comparison to the 140 ppb 24-hour SO2 NAAQS. 

18 SO2 30 ppb 

Annual averaging period 

1-hour measurement 

 If the annual average of measured 1-hour 

concentrations was above 30 ppb (after rounding 

to a whole number per 40 CFR 50.4(a))) but 

would have been 30 ppb or less in the absence of 

the event, those 1-hour values that were affected 

by the single event meet the “but for” test for 

purposes of comparison to the 30 ppb annual 

average SO2 NAAQS. 

If the 30 ppb annual SO2 NAAQS still applies in the 

affected area, a 1-hour concentration equal to or below 

30 ppb (after rounding to a whole number per 40 CFR 

50.4(a)) will generally not be considered for 

exclusion. 

19 SO2

(secondary) 

500 ppb 

3-hour averaging period 

1-hour measurement 

 If the 3-hour average of measured 1-hour 

concentrations was above 500 ppb (rounded to 

the nearest 100 ppb per 40 CFR 50.5(a)) but 

would have been equal to or less than 500 ppb in 

the absence of the event, those 1-hour values that 

were affected by the single event meet the “but 

for” test for purposes of comparison to the 3-

hour average secondary SO2 NAAQS. 

31. Question: When is it appropriate for air agencies to flag concentration values that are 

less than the level of the relevant NAAQS? Under what circumstances will the EPA 

concur on such flags? 
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May 2013

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Answer: (Please read Q30 before reading this response.) 

AQS currently allows an air agency to flag any measured concentration values it chooses, 

including values below the level of the relevant NAAQS. The EPA does not plan to 

implement any new technical restrictions through the AQS software. Also, the 

Exceptional Events Rule does not prohibit air agencies from flagging values below the 

level of the NAAQS. However, the EPA does not intend to review data flags in AQS for 

concurrence until the air agency submits its evidence/analysis package demonstrating that 

exclusion of the flagged values is consistent with the criteria in the Exceptional Events 

Rule, including the “but for” analysis at 40 CFR 50.14(c)(3)(iv)(D). Air agencies wishing 

to flag values for informational purposes should use the “I” series flags in AQS. 

Air agencies may see an advantage in flagging all values they believe were affected by an 

event (and contribute to a violation of the NAAQS), for purposes of being able to later 

identify historical data that have not been affected so that “normal” concentration patterns 

can be presented as part of meeting the “in excess of historical fluctuations” prong of the 

exclusion criteria. AQS does not prevent such flagging, but air agencies should be aware 

that agency flagging by itself does not establish that the concentrations were in fact 

affected by an event and should be excluded from the “normal” baseline. 

Of the flagged cases that appear in both AQS and in demonstration packages, the EPA 

may find it appropriate to concur with flags for concentrations that are below the NAAQS 

only in five very narrow conditions described below. If the EPA determines that a flag on 

a value less than the level of the NAAQS cannot meet the “but for” test, it is likely the 

EPA would nonconcur or leave the default/null value of the AQS concurrence flag 

(indicating no EPA action) in place. 

Except in cases involving PM10 limited maintenance plans25, the EPA intends to prioritize 

events that result in a violation or exceedance of a NAAQS or those that otherwise 

impact a regulatory decision. As described below and in the response to Question 30, 

there may be specific instances where individual measurements fall below a NAAQS but 

still contribute to a violating design value. There may also be instances where a shorter 

averaging time measurement (e.g., 1-hour O3 measurement of 100 ppb) is not above the 

level of that averaging time NAAQS (e.g., 1-hour O3 NAAQS of 120 ppb), but is above a 

longer averaging time NAAQS (e.g., 8-hour O3 NAAQS of 80 ppb) and contributes to a 

violation of the longer averaging time NAAQS. In such cases, although the individual 

measurement may not exceed the level of the shorter-term NAAQS, it may be possible 

for air agencies to present sufficient evidence to satisfy the “but-for” criterion for a 

longer-term NAAQS. 

(See Questions 8, 9, 13, and 19 for additional information.) 

 

25 See May 7, 2009 policy memorandum from William T. Harnett to Regional Air Division Directors at 

http://www.epa.gov/ttn/oarpg/t1/memoranda/lmp_final_harnett.pdf that allows PM10 values between 98 and 154 

μg/m3 (inclusive) to be flagged, concurred, and excluded for purposes of qualifying an area for reliance on only 

a limited maintenance plan. 
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First, PM10 values between 98 and 154 μg/m3 (inclusive) may be flagged, concurred, and 

excluded for purposes of qualifying an area for reliance on only a limited maintenance 

plan (see footnote 24). Because of the expected exceedance form of the PM10 NAAQS, 

concentrations in this range cannot possibly affect whether a site actually meets the 

NAAQS, so there is no reason for flagging them except when the acceptability of a 

limited maintenance plan is an issue. The normal AQS flagging and concurrence 

procedures may be used in this situation.26

A second scenario in which the EPA may find it appropriate to concur with flags for 

concentrations that are below the NAAQS is indicated at 72 FR 13570. If (i) an event has 

affected air quality on multiple consecutive days, (ii) at least one measured concentration 

during the episode can be found to meet the “but for” test using the relevant comparison 

specified in Table Q30-2, and (iii) the air quality impact on each day is “exceptional,” 

measurements for the entire period are eligible for data exclusion regardless of how they 

compare to the level of the NAAQS. In the context of this provision, “exceptional” 

encompasses all the requirements of the Exceptional Events Rule other than the “but for” 

test (e.g., clear causal connection, “in excess of normal historical fluctuations, including 

background,” not reasonably controllable or preventable). 

Scenarios in which the measured concentration is greater than a NAAQS with a longer 

averaging time but less than the level of a NAAQS with a shorter averaging time

Third, applying Table Q30-2 may result in qualifying a 24-hour PM2.5 measurement that 

is greater than the 12 or 15 μg/m3 annual PM2.5 NAAQS but not greater than the 35 μg/m3

24-hour PM2.5 NAAQS for exclusion for the purposes of the 24-hour PM2.5 NAAQS. 

This is the result if the actual 24-hour concentration was between 12 or 15 and 35 μg/m3

but would have been below 12 or 15 μg/m3 but for the effect of the event. It should be 

noted that an exclusion made under this very specific provision for the 24-hour PM2.5

NAAQS will only affect the outcome of an attainment determination for the 24-hour 

NAAQS if the concentration value in question is one of the few highest daily 

concentrations during the year, because only then could it have affected the 3-year design 

value. When a 24-hour value below the level of the 24-hour NAAQS does affect the 3-

year design value, the application of the guidance for the fourth situation (below), which 

is applicable to all four NAAQS pollutants with multi-year design values, would get to 

the same result as application of this paragraph. 

Fourth, assuming that all other Exceptional Events Rule requirements and conditions are 

met, the EPA may concur with flags for ozone, PM2.5, 1-hour NO2, and 1-hour SO2 that 

are “less than the level of the NAAQS” if adjusting the flagged concentrations for the 

estimated contribution from the event would change the 3-year design value from being 

 

26 Values in this range can potentially affect the design value for PM10, but these design values are primarily 

informational and are not likely to influence designations or regulatory determinations of attainment. The 

procedure for determining a PM10 design value in units of μg/m3 is given in section 6.3 of the EPA guidance 

document “PM10 SIP Development Guideline,” June 1987, posted at 

http://www.epa.gov/ttn/oarpg/t1/memoranda/pm10sip_dev_guide.pdf. 
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above the NAAQS to being equal to or below the NAAQS. However, as indicated in 

footnote 21, concentrations below certain values generally will not be excluded. 

Fifth, a 1-hour measurement of a pollutant that is below the level of the 8-hour, 3-hour, 

24-hour, or quarterly NAAQS for that pollutant can be excluded if (1) the event affected 

the 1-hour measurement, and (2) taking into account the event’s effect on all the hours in 

the longer period the effect of the event on the longer averaging period’s concentrations 

satisfies the “but for” criterion. These situations are described in Table Q30-2 (rows 3, 4, 

8, 11, 12, 13, 17, and 19). However, as indicated in Table Q30-2, concentrations below 

certain values generally will not be excluded. 

The following NAAQS-specific discussions provide further explanations regarding some 

of the situations in which a concentration less than the level of the NAAQS may qualify 

for exclusion. These discussions are not exhaustive and do not obviate the need to refer to 

Table Q30-2. 

24-hour PM2.5 

Assume for illustration that the three annual 98th percentile 24-hour PM2.5 concentrations 

for a monitoring site for 2006-2008 are 41, 31, and 37 μg/m3 for each respective year with 

a resulting 3-year design value of 36 μg/m3

 which is a violation of the 24-hour PM2.5 

NAAQS of 35 μg/m3. Also, assume that the next highest concentration in 2007 below the 

31 μg/m3

 was only 20 μg/m3. The 31 μg/m3 concentration in 2007 was affected by a oneday wildfire. The air agency has been able to show that the concentration would have 

been 17 μg/m3 without the fire. Because neither 20 μg/m3 nor 31 μg/m3

 exceed the 

NAAQS, the event on that day does not meet the “but for” test when viewed from an 

“exceedance” perspective. However, the effect of the fire on the 2007 value determines 

whether the 3-year design value passes the 24-hour NAAQS. Had there been no fire, the 

98th percentile concentration in 2007 would have been 20 μg/m3 which would result in a 

3-year design value of 33 μg/m3

 (i.e., less than the 24-hour PM2.5 NAAQS of 35 μg/m3). 

Therefore, the 2007 value of 31 μg/m3 meets the “but for” test when the focus in on 

NAAQS violations rather than individual exceedances. Assuming other requirements are 

met, the 31 μg/m3

 concentration would be approved by the EPA for exclusion from the 

2006-2008 design value. Note that in doing a “violations-based” “but for” analysis, one 

does not simply substitute the “no event” concentration for the original 98th percentile 

day into the design value calculation. Rather, one must re-select the 98th percentile day, 

which sometimes will result in a different day’s actual measured value being used in the 

design value calculation.27 

It is conceivable that the effect of an event on a given day is not enough to satisfy the 

“but for” test with regard to the “violation” perspective explained in the preceding 

 

27 Note that exclusion of this 24-hour value from design values for the annual average NAAQS is a separate 

question, the likely answer to which is that the value is not excludable. If the event did not make the 24-hour 

concentration change from below 12 or 15 to above 12 or 15 μg/m3

 the event does not meet the first condition 

specified in row 7 of Table Q30-2. It is also very improbable that an event affecting a single day would meet the 

second condition in row 7 of Table Q30-2. 
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paragraph for one three-year period, but that it does satisfy it for an earlier or later 3-year 

period when it is combined with one or two different concentrations to calculate a 3-year 

design values, since the outcome of the “violations” analysis may change. After the EPA 

has approved the exclusion of a concentration based on a “violations” analysis for one 3-

year period, the EPA will also exclude that concentration when calculating design values 

and attainment for the other two 3-year periods that include that same year. 

For the 24-hour PM2.5 NAAQS, it is possible that multiple days with concentrations 

below the NAAQS within one year are flagged. Excluding just one of these 

concentrations may not change the annual 98th percentile concentration enough to cause 

the 3-year design value to change from “violating” to “complying,” but excluding several 

of them may. The outcome for the design value may also depend in part on whether 

exclusion is granted for some other concentrations that are above the level of the 

NAAQS. In such cases, the exclusion decisions should first be made for each of the 

flagged concentrations that are above the NAAQS. All remaining flagged concentrations 

(those meeting all other requirements and conditions of the Exceptional Events Rule) 

should then be considered in progressively larger groups ranked by concentration. That 

is, if excluding the highest one of the flagged concentrations below the level of the 

NAAQS would cause a switch in whether the 3-year design value violates the NAAQS 

then if the EPA determines that value is to be excluded then there is no impact to 

retaining all others and, thus, no need to make determinations for those others. If 

excluding the two highest such concentrations causes a switch, then there is no impact to 

determining whether others beyond those two should be retained. 

However, the preamble to the Exceptional Events Rule explicitly states that PM2.5

concentrations below the level of the annual NAAQS cannot be excluded for purposes of 

comparisons to the annual NAAQS. (72 FR 13570, bottom of middle column) Even if the 

conditions described in the preceding paragraph are met, values below 12 or 15 μg/m3 

cannot be excluded. 

Annual PM2.5

The preamble to the Exceptional Events Rule explicitly states that PM2.5 concentrations 

below the level of the annual NAAQS cannot be excluded for purposes of comparisons to 

the annual NAAQS. (72 FR 13570, bottom of middle column) 

Ozone (0.075 ppm 8-hour NAAQS)

Assume for illustration that the three annual 4th highest daily 8-hour ozone values in 

2006-2008 are 0.077, 0.076, and 0.075 ppm respectively. The 0.075 ppm value in 2008 

was affected by an exceptional event. The 3-year average would be 0.076 ppm, a 

NAAQS violation. If the 0.075 ppm value for 2008 were to be excluded and if, as a 

result, 2008’s new 4th highest value was 0.074 ppm or less, the 3-year average (after 

Appendix P truncation) would be 0.075 ppm, which is not a NAAQS violation. The 0.075 

ppm value may be excluded under these circumstances even though it is not itself an 

exceedance. Furthermore, the exclusion also applies to the use of this value when 
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calculating the 2007-2009 and 2008-2010 design values, regardless of whether such 

exclusion causes those design values to switch from violating to complying with the 

NAAQS. 

For ozone, as for 24-hour PM2.5, it is possible that an air agency could flag multiple days 

within one year with concentrations below the NAAQS. Excluding just one of these 

concentrations may not change the annual 4th highest concentration enough to cause the 

3-year design value to change from “violating” to “complying,” but excluding several of 

them may. Also, the outcome for the design value may depend, in part, on whether 

exclusion is granted for some other concentrations that are above the level of the 

NAAQS. In such cases, the exclusion decisions should first be made for each of the 

flagged concentrations that are above the NAAQS. All remaining flagged concentrations 

(those meeting all other requirements and conditions of the Exceptional Events Rule) 

should then be considered in progressively larger groups ranked by concentration. That 

is, if excluding the highest one of the flagged concentrations below the level of the 

NAAQS would cause a switch in whether the 3-year design value violates the NAAQS 

then if the EPA determines that value is to be excluded, all others can be retained without 

impact. If exclusion of the two highest such concentrations causes a switch, then the EPA 

may focus first on whether only those are to be excluded. 

PM10

The only current PM10 NAAQS is the 24-hour NAAQS based on the expected number of 

exceedances over a 3-year period. Since a concentration below the level of the NAAQS 

would not be an exceedance and cannot affect compliance with the NAAQS in any way, 

a concentration below the level of the NAAQS usually cannot be excluded. However, 

under an EPA policy memo, for the purpose of the EPA approval of a limited 

maintenance plan PM10 values as low as 98 μg/m3 can be concurred for exclusion when 

determining whether an area is eligible for a limited maintenance plan. (See May 7, 2009 

memorandum from William T. Harnett to Regional Air Division Directors, 

http://www.epa.gov/ttn/oarpg/t1/memoranda/lmp_final_harnett.pdf). Because 

concentrations less than 98 μg/m3 would appear to have little regulatory significance, the 

EPA discourages the flagging of such data. 

Pb 

The legacy 1.5 μg/m3 and current 0.15 μg/m3 NAAQS for lead are both based on a 

maximum three-month average concentration. The 1.5 μg/m3 standard is based on the 

maximum quarterly average, while the 0.15 μg/m3

 NAAQS is based on the highest 

rolling 3-month average during a 3-year period. As previously explained, the EPA is not 

likely to concur on the exclusion of a 24-hour concentration value that is below the level 

of the NAAQS, and we discourage air agencies from flagging such values. 

 
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NO2 

As previously explained, the EPA is not likely to concur on the exclusion of a 1-hour 

NO2 concentration that is below the level of the annual NO2 NAAQS, and we discourage 

air agencies from flagging such values. 

SO2

As previously explained, the EPA is not likely to concur on the exclusion of a 1-hour 

SO2 concentration that is below the level of the annual SO2 NAAQS, and we discourage 

air agencies from flagging such values.