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4- 79 -2n 

-T - 265 

STAFF ASSESSMENT OF RECENT COMMENTS ON THE 

SHORT-TERM SULFUR DIOXIDE EXPOSURE ANALYSIS 

STRATEGIES AND AIR STANDARDS DIVISION 

OFFICE OF AIR QUALITY PLANNING AND STANDARDS 

U.S. ENVIRONMENTAL PROTECTION AGENCY 

RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711 

July 1986 
, p.2 

Disclaimer 

This report has been reviewed by the Office of Air Quality Planning and 

Standards, U.S. Environmental Protection Agency, and approved for publication. Mention of trade names or commercial products is not intended to 

constitute endorsement or recommendation for use. 

1 
Table of Contents 

Page 

I. Introduction - 1 

II. Findings 3 

1) NEM and Air Quality Modeling Errors 4 

2) Definition of Exposures of Concern 5 

3) Meteorology and Modeling 9 

4) Populations at Risk to Short Term/Low Persistance Events 12 

5) Monitoring Data Review 14 

6) Size of the Sensitive Population 17 

III. Implications for the Exposure Analysis 18 

References 

a. 3 
STAFF ASSESSMENT OF RECENT COMMENTS ON THE 

SHORT-TERM SULFUR DIOXIDE EXPOSURE ANALYSIS 

I. INTRODUCTION 

Analyses of population exposures to short-term peaks of SO2 have been 

a key element in the review of the sulfur oxides National Ambient Air Quality 

Standards. Recent health studies have shown effects in asthmatics exposed 

to short-term peaks. Knowledge of these effects led to questions reyardiny 

the degree to which the current SO2 NAAQS protected against such peaks. Preliminary analyses of short-term SO2 air quality were prepared for the Staff 

Paper in 1982 (Strategies and Air Standards Division, 1982). Following 

closure on the Staff Paper, an expanded effort examining short-term 

ambient air quality levels and the potential for population exposures to 

short-term peaks was initiated. This effort was concerned with determining 

population exposure to peak levels of SO2 (generally levels > 0.5 ppm). 

To do this, the NAAQS Exposure Model (NEM) was modified to calculate the 

probability of population exposures around utility power plants. The 

decision to focus the exposure analysis on power plants was guided by the 

earlier air quality studies which showed that they were the source cateyory 

most likely to produce high short-term SO2 levels in populated areas. Analyses 

were made for both current conditions and those that would obtain if the 

current NAAQS were just met. To simplify the problem the analysis focused 

on what the staff defined as an Exposure of Concern (E0C). Based on the 

clinical studies avallable . in1982 the EOC was defined as an exposure to 

> 0.5 ppm for 10 minutes while at a ventilation rate > 35 1/minute. The 

methodology and results of the exposure analysis are described in several 

papers and reports (U.S. EPA 1984 and Biller et al. 1984). 

p.4 
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2 

Recently, however, concerns were raised by several environmental and 

public interest groups over the relevance and completeness of the exposure 

analysis. This paper summarizes recent efforts to assess the implications 

of the scientific and technical issues which were raised. In some cases, 

this assessment provides definitive answers, but in many instances this 

assessment can only define the bounds for these concerns. 

In November 1985 representatives of the American Lung Association 

(ALA) met with the Administrator and raised a number of questions concerned 

with the exposure analysis (Godar, 1985). On behalf of ALA Dr. Dean Sheppard 

expressed the view that 0.4 ppm for 5 minutes would be a more appropriate 

definition of a exposure of concern than that used in the exposure analysis. 

Or. Sheppard suggested that the sensitive population was larger than just 

diagnosed asthmatics and should include individuals with "twitchy airways." 

ALA also expressed judgments regarding the seriousness of responses of 

asthmatics to such exposures. 

The Natural Resources Defense Council, Inc. (NROC) in a November 8, 

1985 letter to the Administrator raised still other issues related to the 

exposure analysis (Hawkins, 1985). NRDC asserted that since the exposure 

analysis relied on standard dispersion models it failed to account for 

peaks due to certain low persistance meterological events (e.g. building 

downwash, looping plume, and fumigation). They also pointed out that 

peaks > 0.5 ppm due to such events could occur around numerous small 

sources not addressed in our analysis. NRDC asserted that EPA's monitor 

siting criteria and guidelines on spanning of instruments have resulted 

in a systematic underestimate of hourly exceedances of 0.5 ppm. 

In an August, 1985 petition to the Administrator the Environmental 

Defense Fund (EDF) presented evidence of population exposures to very 
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3 

high concentrations (> 1 ppm) in the vicinity of three smelters none of 

which were in complete compliance with the current NAAQS (Yuhnke, 1985). 

In subsequent submittals EDF presented evidence of individuals requiring 

hospitalization as a result of exposure. Although EDF's immediate concern 

focused on smelters, their information prompted us to consider again 

exposures to very high short-term levels of SO2 (> 1 ppm) around power 

plants. 

A preliminary staff review of the facts found that there is considerable 

scientific uncertainty surrounding all of these issues, nonetheless there 

is some reason to believe that the present exposure analysis might in 

fact underestimate exposure. In addition, it is also possible that some 

of the underlying analytic assumptions may have been incorrect. Therefore, 

this staff assessment was undertaken to clarify the issues as much as 

possible and to determine the implications for the exposure analysis. In 

the process of completing this staff assessment, a programming error in 

the SO2 version of NEM was discovered. Further review of the earlier 

exposure analyses revealed other problems with the air quality modeling. 

The impact of these errors on the previously reported results is also 

discussed below. 

II. FINDINGS 

This assessment focused on six areas of inquiry: 1) NEM and Air 

Quality Modeling Errors, 2) definition of an "exposure of concern," 

3) meteorology and modeling of very short-term peaks, 4) estimates of 

the populations risking exposure due to low persistance events, 5) review 

of monitoring data, and 6) size of the sensitive population. Each of 

these six areas is addressed below. 
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4 

1) NEM AND AIR QUALITY MODELING ERRORS 

As a part of this assessment, the results of the earlier exposure 

analysis were carefully reviewed. In the process, an apparent inconsistency 

was found in the reported output for one of the power plants. In tracking 

down the source of this inconsistency, a programming error was discovered 

in the SO2 variant of NEM. This error resulted in some non-zero probabilities 

of exposure being treated in the model as zero probabilities, implying 

a systematic downward bias in the previously reported results. Preliminary 

results with the corrected version of NEM indicate that exposure counts 

may increase by some 40 - 50%. Having found the one error, a thorough 

examination of the entire SO2 NEM code was undertaken. The examination 

revealed no further problems in the code. Nevertheless, we have decided 

to make the model code available for review to parties outside EPA. 

Finally, it should be noted that the coding error does not imply a flaw 

in the underlying methodology or logic. Our assessment indicates that the 

underlying methodology is still sound. 

Since it was planned to examine a number of new air quality scenarios, 

a review of the previous air quality modeling was made. Around several of 

the power plants studied in the 1984 report, it was determined that more 

representative estimates of background could be made. At one power plant, 

Gallagher, it was found that an error had been made in determining the 

compliant fuel for the current NAAQS. This error has been corrected in 

analyses now underway. Finally, changes were made to the meteorological 

data used at Portage des Sioux and at Gallagher. 

In summary, this staff assessment resulted in the discovery of several 

errors in the 1984 exposure analyses. The correction of these errors 

will lead to higher estimated exposure rates. nespite this, the staff 
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5 

assessment indicates the errors were very technical in nature and do not 

lead to the conclusion that the underlying methodology is flawed. A revised 

SO2 exposure analysis report is planned for later this year. 

2) DEFINITION OF EXPOSURES OF CONCERN 

A full assessment of risk associated with alternative SO2 standards 

would combine exposure analysis with appropriate dose response information. 

For the bronchoconstrictive effects of 502, however, the dose-response 

relationship is a complex function of SO2 concentration, exercise level 

and breathing pattern of suhjects, sensitivity of suhject, air temperature, 

humidity, and duration of exposure. To simplify the analysis, we developed 

a benchmark termed an "exposure of concern" (EOC), which fixed some of 

these variables through appropriate assumption. Using studies summarized 

in the 1982 criteria document and staff paper, the EOC was defined as 

follows: 

SO2 concentration - > 0.5 ppm 

%ration - 10 minutes 

Exercise Level - Producing > 35 1/min ventilation 

Sensitive - Exercising Asthmatic 

Population 

Other Factors - Assumed comparable to those studied, 

room temperature, humidity 

Based on the limited set of studies available in 1982, it appeared that such 

exposures could produce moderate to large changes in pulmonary function 

with perceptible symptoms in 20 to 70% of mild asthmatics exposed. The 

data available in 1982 did not provide evidence for function changes accompanied 

by symptoms at lower concentrations. The EOC was not, however, intended as 

a threshold of response, but a level where a significant fraction of 

asthmatics so exposed might experience potentially adverse effects. 
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6 

Level 

The ALA concerns with the original EOC were based on results from 

studies published after closure on the criteria document and staff Raper. 

ALA suggested that a level of 0.4 ppm for 3 to 5 minutes better reflects 

the current concensus of where adverse effects would begin for exercisiny 

asthmatics. In particular, data from Linn et. al. (1983) do show moderate 

group mean functional changes with symptoms that in at least one individual 

were clearly significant with 5 minute exposures to 0.4 ppm at 48 to 50 

1/minute. Other studies suggest that at higher ventilation rates 

bronchoconstriction can occur at even lower levels, but the physiological 

significance of the changes and the nature of any symptoms are unclear. 

While these studies add to the information on the effects of SO2 

below 0.5 ppm, still other recent studies (Bethel et al., 1983; Schacter 

et al., 1984; . Roger et al., 1985) expand on our understanding of the 

extent of effects at 0.5 ppm and above. These tend to confi - 7 our original 

assessment that only a portion of these asthmatics respond to such exposures 

with siynificant functional changes and symptoms, with exercise rate 

playiny a major role. At 35 to 42 1/minute and 5 to 75 minute exposures 

to 0.5 ppm SO2 these studies found no yroup mean increase in symptoms and 

in two of the three studies no significant change in lung function. 

Based on a reanalysis of Roger et al., the draft criteria document addendum 

(EPA 1986) found approximately 25% of the asthmatics exhibited functional 

changes suggestive of potential clinical significance (100% increase in 

airway resistance) following such exposures. At higher exercise rates 

( 60 1/minutes), however, even group mean functional changes were large. 

At the lower concentration (0.4 ppm) with high exercise ( 50 1/min), on 
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7 

the other hand, only 1 of 23 asthmatics selected for sensitivity to SO2 

experienced a clearly significant response (Linn et al. 1983). Thus 0.5 ppm 

remains a useful "benchmark" for assessing exposures which could result 

in effects in a significant fraction of asthmatics. 

Clinically significant responses in individuals are important and 

lead us to believe that some exposure analyses of 0.4 ppm should be 

conducted. However, when the level of the EOC is reduced to 0.4 ppm, the 

analysis will need to change the minimum "high" exercise rate from 35 

1/min to 45 to 50 1/min and/or note the small fraction of potential 

responders. Unfortunately, the structure of the activity files used in 

the NEM analysis make it extremely difficult to change the definition of 

"high" exercise. Therefore, the interpretation of results from model 

runs at 0.4 ppm will need to recognize the smaller percentage of asthmatics 

potentially responding. In summary, although we are currently planning 

to add limited analyses of 0.4 ppm to the exposure study, the 0.5 ppm 

level remains a useful benchmark and will be employed for most analyses. 

Duration 

The current exposure analysis already includes a sensitivity analysis 

for 5 minute vs. 10 minute exposures, which shows an increase in the percentage 

exposed for the shorter averaging period. However, the American Luny 

Association pointed out the results of a mouthpiece study which produced 

effects after 3 minute exposures. It is always difficult to relate results 

from mouthpiece studies to free breathing subjects. Nevertheless this and 

a second similar study do suggest that there may be reason for concern at 

shorter averaging periods. At this stage there are many questions regarding 

the extent of response at the shorter averaging periods. The review of a 

recent paper by Huber and Pooler (Huber et al. I985a) and recent analyses 
p. 11 

8 

of monitoring data, which show substantial peaks lasting up to 3 minutes 

increase the need to resolve these questions. 

In an effort to resolve these uncertainties, the problem was discussed 

with three different groups that conduct SO2 exposures. The EPA laboratory 

in Chapel Hill has obtained approval of a study protocol addressing this 

problem and has already begun work. Their protocol includes exposure 

periods from 30 seconds up to 3 minutes. Dr. Hackney and Dr. Sheppard 

also expressed interest in pursuing the question. Thus although there is 

currently some uncertainty over the extent of response for periods < 3 

minutes, it should be partially resolved later this year. In the meantime, 

we will plan to shift our focus from analysis of 10 minute exposures to the 

5 minute period. Once the clinical studies addressing shorter periods have 

been completed and reviewed, we may wish to consider analyses of periods 

less than 5 minutes. 

Benchmarks for Severe Effects 

Our exposure work has generally focused on populations exposed to 

concentrations > 0.5 ppm 10-minute without regard to how high the concentrations 

might be. Use of the 0.5 ppm 10-minute threshold simplified the analysis 

and permitted the examination of a number of different power plants as well as 

the effect of alternative NAAOS. Nonetheless, exposures to significantly 

higher concentrations are of concern. In several cases population exposures 

around utility power plants to concentrations > 1 ppm have been estimated. 

The results derived from recent runs of the revised NEM model are shown 

in Table 1. As can be seen, the number of individuals exposed and the 

maximum probability of exposure at this concentration level are significant. 

An analysis of measured data around 3 TVA power plants tends to support the 

modeled air quality data shown in Table 1. The TVA data also show that the 
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peak concentrations can reach 2.0 ppm. Due to the way these data were stored 

it is impossible to say how long these peaks lasted. However, given what 

is known regarding instrument response time it is likely the peak persisted 

for 2-4 minutes. A limited analysis of the 1.0 ppm benchmark has been 

prepared (Thomas, 1986). 

3) METEOROLOGY AND MODELING 

The basis for many of the concerns raised by NRDC is an analysis 

completed by Alan Huber and Fran Pooler of the Atmospheric Sciences 

Research Laboratory (ASRL), and referenced in the Stack Heights regulation 

(Huber et al. 1985a). That analysis indicated that extremely high 

concentrations could result from short-term/low persistance events not 

explicitly modeled in EPA's dispersion models. The Huber/Pooler paper 

presented its conclusions in the form of dilution ratios and expected 

annual occurence rates for two model facilities. Their data indicated 

peaks lasting 30 seconds to 2 minutes could occur on the order of 1000 

times per year at any location within 0.5 kilometers of a short stack 

plant (less than GEP) due to building downwash. For looping plumes 

similar peaks could occur on the order 10 times per year at any given 

location within 3 kilometers of any source. Huber and Pooler point out 

in their paper that such calculations are subject to considerable uncertainty 

and are only accurate as order of magnitude estimates. 

To place these numbers in perspective we did several things. In the 

first place, Huber's procedures were used to calculate the dilution 

factors due to building downwash around a number of actual facilities 

(not model facilities as in the ASRL work). The dilution factors obtained 

were not radically different from those in Huber's paper (see Table 2). 

In addition, we obtained stack gas concentration estimates for various 
10 

fuels in order to calculate ground level concentrations. Both stack and 

ground level concentrations are shown in Table 2. Given the assumptions 

of this analysis all but the very cleanest fuels are capable of producing 

very short term levels > 0.5 ppm as a result of building downwash. Thus, 

concentrations >0.5 ppm for very short periods may occur around small, 

less than GEP stack facilities burning relatively clean fuel. Our earlier 

screening studies (which used standard dispersion modeling assumptions 

and therefore did not address building downwash, looping plume, etc.) had 

led us to believe that such facilities would not be a problem. 

Although it is clear that the exposure analysis does not account 

for exposures due to building downwash, there is reason to believe that 

phenomena such as looping plume and limited mixing are addressed. In the 

exposure analysis the probability of exceeding 0.5 ppm for 10 minutes 

relied on the hourly averages predicted by CRSTER and an empirically 

derived distribution of peak-to-mean ratios. The distribution of ratios 

was obtained through an analysis of measured data around the Kincaid 

power plant. The Kincaid network was designed to pick-up high concentrations 

due to phenomena such as looping plume and limited mixing. In discussing 

this with Dr. C.S. Burton, at Systems Applications, Inc., we found that 

the 20 highest observations were taken under strongly convective boundary 

layer conditions. These conditions would lead to what observers would 

identify as looping plume. Thus it is reasonable to conclude that, 

except for building downwash, the peak-to-mean ratios used is our analysis 

do reflect the occurrence of the atmospheric phenomena cited by Huber and 

Pooler. [Given the stack height and plume temperature at Kincaid as well 

as the network design, we can be certain that building downwash did not 

occur and would not have been measured if it had.] In fact the exceedance 

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11 

rates of 0.5 ppm estimated in our modeling studies are consistent with 

the frequency of occurence numbers calculated by Huber and Pooler for all 

conditions except building downwash. 

In addition to the work described above, a number of meetings were 

held with ASRL staff to determine if a more precise characterization of 

these phenomena were possible. As noted above, ASRL staff felt their 

earlier work gave only an order of magnitude estimate. After our 

meetings and a review of the subject, ASRL determined that more precise 

estimates were impossible without further research. Although no detailed 

plans were made, their initial review and planning indicated that the 

research could take several years to complete and require a budget in 

excess of $1 million (Huber et al. (1985b). However, the meetings with 

ASRL as well as discussions with other meteorologists, both inside and 

outside EPA, revealed strong agreement that the phenomena described in 

the paper by Huber and Pooler do in fact produce high short-term concentrations 

of S02. However, there was some disagreement as to the frequency of 

occurrence and dilution factors. 

In summary, the initial review of the meteorological and modeling 

aspects of the concerns raised by NRDC indicates that levels > 0.5 ppm 

may well be occurring around facilities never addressed in our earlier 

exposure analysis. The peak concentrations are likely to occur within 

0.5 km to 3 km and reach levels well over 0.5 ppm. Although it is 

certain these phenomena exist, there is considerable uncertainty over 

their extent, frequency, and magnitude. Moreover, it appears that many 

such peaks are of very short duration (30 seconds to 2 minutes). As noted 

above, the effect of such short peaks on exercising asthmatics is the 

subject of an ongoing clinical study. Additional research would he 
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needed to produce better methods for calculating ground level concentrations 

and frequencies resultiny from these low persistance events. 

4) POPULATIONS AT RISK TO SHORT TERM/LOW PERSISTANCE PEAKS 

In the short period of this initial assessment it was not possible 

to analyze actual population exposures associated with phenomena such as 

building downwash and looping plume. However, an effort was made to better 

define the dimensions of the problem. This initial effort focused on 

trying to determine the total population living around sources which 

might produce these peaks. It must be stressed that this effort aimed at 

determining the population at risk of an exposure and not the population 

actually exposed. The first difficulty in assessing the population at 

risk of exposure was simply determining which sources miyht produce 

short-term peaks due to' downwash. Since data on building dimensions are 

not available on NEDS it was impossible to determine which sources have 

stacks less than GEP. As a surrogate we focused on sources with stacks 

less than 60 m. It should be stressed that for some sources stacks < 60 

m would represent GEP, while for others > 60 m would not be GEP. The 60 

m cutoff is, therefore, an extremely rough surrogate. 

A program was developed to count the population centroids lying 

within specified distances of facilities which had at least one stack 

< 60 m. In our first attempts at this we found that double counting of 

centroids was a significant problem. Therefore, the final version of 

the program calculated the populations within a specified distance of 

1,2,3,...,n facilities. The results of this analysis are shown in 

Table 3. As can be seen, approximately 9 million individuals live within 

0.5 km of such a facility (where we presume building downwash might occur). 

If the radii are extended to 3 km (area within which looping plume impacts 
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might be observed) the count increases to 84 million. It should be kept in 

mind that those individuals within 0.5 km of these facilities are at risk 

from an event which may occur approximately 1000 times per year, while 

those between 0.5 km and 3 km are at risk from an event which may occdr 

only approximately 10 times per year. This suggests that for any individual 

selected at random the probability of exposure due to looping plume is much 

lower than for building downwash. In our earlier exposure analyses around 

power plants, we attempted a very rough national estimate of the population 

at risk of exposure. At that time, we estimated that approximately 25 

million lived around power plants capable of producing hourly SO2 levels > 

0.5 ppm. The estimates shown in Table 3 thus bracket our earlier estimate 

of the total population at risk. This implies that if all the sources 

discussed above in fact produced levels > 0.5 ppm the total population at 

risk could remain roughly the same or potentially increase by more than 

a factor of two. 

Several additional cautions are in order regarding the data in Table 3. 

In the first place, erroneous UTM coordinates led to approximately 17% 

of the facilities being discarded from the analysis. This would imply that 

the population totals shown underestimate the "true" figure. moreover, 

it became apparent in the course of the analysis that the census data 

frequently did not provide resolution down to 0.5 km. That is, the block 

yroup enumeration districts covered areas much larger than 0.5 km. It is 

not possible to determine in which direction this might have biased the 

results. Finally, the totals in Table 3 reflect only the populations 

residing within the specified distance of a facility. The totals do not 

reflect populations residing outside the specified distance, but commuting 

into the area. This also might lead to an underestimation of the population 

risking exposure. 
14 

In summary, it was not possible to complete a true exposure analysis 

in this initial study. However, an extremely rough estimate of the 

population at risk was prepared. Considering downwash alone, the estimate 

was 9 million, increasing to 84 million if looping plume were considered. 

The risk of exposure is very different for these two groups because the 

frequency of occurrence of the two phenomena are so radically different. 

At present, we have no way of estimating the percent of those living near 

such a facility who might actually suffer an exposure. 

5) MONITORING DATA REVIEW 

The review of monitoring data addressed several different concerns. 

In the first place, an assessment was made of raw data submitted by UARG 

during the Stack Height rule-making which purportedly showed evidence of 

building downwash. Secondly, an effort was made to locate any other data 

which might give evidence of these low persistance events. Finally, an 

effort was made to determine the extent to which our SAROAD data was biased 

due to spanning of the instruments. 

UARG Data Analysis 

The review of the UARG data tends to confirm Alan Huher's calculations. 

Table 4 is a portion of the strip chart taken from one of the UARG sites. 

As can he seen, repeated peaks > 0.5 ppm were recorded with each peak 

appearing to last no more than about a minute. Data taken around another 

facility where the instrument was located 1.7 km away, show peaks lasting 

5-10 minutes. Although one can not be sure, the distant location of the 

instrument argues that these latter data show some condition other than 

building downwash. 

The review of these strip charts by OAQPS and ASRL staff showed 

several other things. In the first place, the instruments were spanned 

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at 1 ppm and frequently pegged out. This means that the true peak can 

not be readily estimated but was presumably in excess of 1 ppm. However, 

despite this "peak lopping", we conclude that the hourly averages are 

unlikely to be significantly underestimated. This conclusion is based on 

the observation that the area under the curve at the peak is relatively 

small and thus unlikely to significantly change the average for the entire 

hour. Clearly, for peaks of longer duration (> 5-10 minutes), "peak lopping" 

would lead to a significant underestimate of the hourly average. Peaks of 

at least 5 -10 minutes were seen around another facility in the UARG data 

set and around some TVA facilities. In such cases, it is possible that 

hourly averages may be underestimated due to spanning. "Peak lopping", 

if it occurs, would also bias any analysis of peak to mean ratios. We 

determined that this was not a problem with the Kincaid data set discussed 

above. However, any future analysis of peak to mean ratios would need to 

address this problem. Secondly, it was determined that even if monitors 

were set out to capture peaks due to downwash and properly spanned, instrument 

response time would lead to an underestimate of peak concentrations. This 

would occur because many instruments now in wide use require 4-5 minutes to 

reach 95% of scale. Thus, if the actual peak lasts only 30 seconds to I minute, 

most instruments would not respond fast enough to register the true peak. 

Other Data Sets 

Our second area of inquiry was to determine if other short-term data 

sets existed which would help to characterize the frequency and magnitude 

of peaks due to downwash and looping plume. The Monitoring and Reports 

Branch polled the Regional Offices and found at least 40 sites where 

instruments had been located near facilities with less than GEP stack 

height and where very short term data (< 1 hour) was also availahle. 
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However, in many instances the data were not readily available and/or were 

not in a usable form. Therefore, until the clinical studies discussed 

above are completed and we have a better understanding of the effect of 

<3 minute exposures, we have held off on requesting the raw data. In 

addition to requesting Regional Office data, we also contacted ORD. As a 

result, we found that 2 or 3 of the RAPS sites (sites not analyzed in 

Argonne's earlier study) miyht show evidence of looping plume. Ralph 

Larsen, ASRL is in the process of analyzing the data from those and other 

RAPS sites. Preliminary analyses at a single site indicate very limited 

exceedances of 0.5 ppm 5 minute average. At our request TVA staff provided 

a full year of data recorded around each of three power plants. The data 

include hourly averages and the single highest peak within each hour. 

The TVA data were used in analyzing peaks > 1 ppm discussed above and 

also show significant occurrences of peak values > 0.5 ppm. 

Effect of Spanning on Earlier Studies 

Finally, we examined the possibility that spanning of instruments 

might have biased earlier studies of hourly monitoring data completed by 

Neil Frank and Tony Thrall (Frank et al. 1982). The data set used in the 

earlier analysis was no longer readily available. Therefore, more recent 

data from the National Air Data Bank were examined. The object was to determine if a significant number of instruments were spanned to 0.5 ppm which 

might then result in an underestimate of hourly averages > 0.5 ppm. A true 

determination of this would require a site-by-site review to ascertain how 

each instrument was spanned and the frequency with which it went to full 

scale. Such an effort was beyond the scope of this assessment, but a simplier 

analysis did seem possible. Based on our earlier studies of peak-to-mean 

ratios, we determined that sites measuring > 0.25 ppm 1-hour average could 
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well have experienced > 0.5 ppm as a shorter term peak. Therefore, of the 

some 800 sites available, we focused on those that showed at least 1 exceedance 

of 0.25 ppm 1-hour in a 3 year period. In total, there were 210 such 

sites, of which 100 showed at least 1-hourly average > 0.5 ppm and therefore 

were almost undoubtedly spanned > 0.5 ppm. However, the remaining 110 

sites recorded hourly peaks between 0.25 ppm, and 0.5 ppm. It is, therefore, 

possible that at these sites hourly averages > 0.5 ppm may have occurred 

but not been recorded due to spanning. The extent to which hourly averages 

may have been underestimated is still unknown, since as noted above the 

very short term peaks would have to persist for the hourly average to be 

underestimated. These 110 sites would, however, be candidates for a siteby-site review if a more detailed study were to be undertaken. 

In summary, the review of the monitoring data did not conclusively 

answer the question of whether or not hourly averages might be underestimated 

due to spanning. The review did, however, convince us that very shortterm peaks (i.e., > U.5 ppm for 30 seconds to 2 minutes) do occur and have 

been measured. Since the peaks in question are of such short duration, 

hourly averages may not be underestimated. On the other hand, our review 

clearly demonstrated that the peaks themselves may be underestimated because 

of both spanniny and instrument response time. Moreover, with peaks of 

longer duration, hourly averages could be underestimated due to spanning 

alone. A search by the Regional Offices and others turned up a number of 

other sites where these phenomena may have been observed. Apart from the 

limited review of the RAPS. data, we have no further analyses underway at 

this time. 

6) SIZE OF THE SENSITIVE POPULATION 

ALA in their meeting with the. Administrator stressed that they 

believed that the sensitive population should be defined to include more 
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18 

than just diagnosed asthmatics. In particular, they pointed out that there 

might be up to 10% of the national population affected, if one included individuals with sensitive airways. It is, of course, entirely possible that 

such individuals might respond to an exposure of concern. Such individuals 

are likely to be a subset of atopics and in the staff paper were included 

in the original sensitive population. The exposure analysis report tended 

to focus on just asthmatics as the sensitive population. However, the size 

of the sensitive population is not critical to the exposure analysis. The 

NEM analysis uses the entire population of an area in its calculations. In 

this work, the measure we have tended to focus on is the percent of the population exposed while at exercise (defined as > 35 1/min). A change in the size of 

the sensitive population will not chanye the percent exposed. The absolute 

number of individuals is also presented and this would change if the size of the 

sensitive population were altered. Thus, although ALA may be right on this 

point, it should not alter any of the general conclusions we have reached. 

III. IMPLICATIONS FOR THE EXPOSURE ANALYSIS 

Clearly the coding errors found in NEM indicate that the estimates 

presented in our 1984 reports (U.S. EPA, 1984, Biller et al., 1984) 

are not accurate. These errors have been corrected and exposures are being 

re-calculated. At this stage it seems reasonable to conclude that the 

revised exposure estimates will be reliable for situations around large 

GEP sources. "Reliable" must be understood in the context of the caveats 

outlined in the exposure report. In addition, it should be emphasized that 

the revised estimates apply to power plants with GEP stack heights. It is 

now clear that numerous smaller sources of SO2 are also likely to be causiny 

very short-term exceedances of and exposures to 0.5 ppm. This latter point 

is at variance with our earlier conclusions on this subject. Although it 
19 

is clear that 5 minutes is a more appropriate period to use in defining an 

EOC, it is not clear whether periods < 3 minutes should be considered. If 

clinical studies do show effects at these shorter periods, it is uncertain 

to what degree the additional exposures around smaller sources would add 

to the totals already estimated. 

The first conclusion regarding the reliability of the revised power 

plant analyses rests in part on the results of dispersion model evaluation 

studies. The Kincaid and Clifty Creek studies indicate that CRSTER predicts 

the distribution of 1-hour values reasonably well; implying that the 1-hour 

valuec in the exposure analysis are reliable. This despite the fact that 

CRSTER does not explicitly treat some low persistance phenomena. In the 

exposure analysis the hourly estimates from CRSTER were used as one input 

to the calculation of 10-minute air quality. The other input was a 

distribution 10-minute peak to hourly mean ratios derived from measured 

data around the Kincaid power plant. As discussed above, the distribution 

of ratios does reflect the occurrence of phenomena such as looping plume 

and limited mixing. Therefore, it seems reasonable to conclude that the 

exposure analysis around GEP power plants already accounts for most of the 

phenomena discussed by Huber and Pooler. 

The general conclusion regarding the soundness of the power plant 

analysis does not extend to power plants with significantly less than GEP 

stack height. At such plants downwash probably leads to appreciably 

higher exceedances rates of 0.5 ppm; although most of the peaks would 

likely be of very short duration (30 seconds to 2 minutes). As discussed 

above, the effect of such short-term exposures on asthmatics is, at present, 

still uncertain. 

The second conclusion is that, contrary to our earlier assumptions 

regarding smaller sources, it now appears that small sources with less than 

p.22 
p.23 

20 

GEP stack height can cause exceedances of and exposures to > 0.5 ppm. Most 

of these exceedances are due to building downwash, occur in most cases 

within 0.5 km of the stack, and are of very short duration (less than 2 

minutes). By Huber's calculations and by inspection of the UARG data, it 

appears that such exceedances may occur on the order of 1000 per year at a 

fixed location. At this point we have not been able to accurately determine 

how many sources may be subject to downwash nor the populations who may be 

at risk of an exposure. Preliminary, but very rough, calculations indicate 

that the numbers may be quite significant. In addition, small sources 

regardless of stack height may also produce very short-term (< 2 minute) 

exceedances of U.5 ppm due to looping plume. These exceedances would 

likely be found within 3 km of the stack and occur on the order of 10 times 

per year. This conclusion rests on both Huber and Pooler's work as well as 

our own analysis of peak-to-mean ratios. Smaller sources were not addressed 

in our earlier exposure analysis and their inclusion would only increase 

the estimated exposures. 

The very short averaging period of the peaks due to building downwash 

raises the question of the appropriate definition for an exposure of concern. 

Based on data now available, it is possible to conclude that 5 minutes 

is a more appropriate averaging period than 10 minutes. Whether or not 

shorter periods should be considered will depend on the results of clinical 

studies now underway. We also conclude that > 0.5 ppm is still an appropriate 

concentration benchmark. Data are available showing functional changes 

with symptoms at 0.4 ppm. However, when this lower concentration is used, 

the results will have to be interpreted differently reflecting the fact a 

smaller percent of asthmatics miyht respond given such an exposure. For 

these reasons, 0.5 ppm still seems an appropriate benchmark concentration 

for most purposes, but 0.4 ppm will be used in sensitivity analyses. 
21 

It is important to stress that an analysis of smaller sources would he 

an almost intractable analytic problem. Since building downwash would be 

central to the problem, one would need to determine the number and type of 

sources subject to downwash. However, downwash depends on the specific 

stack/building/nearby obstacle geometry; With each site being different. 

This means it would be impossible to draw a "representative" sample of 

sources and any analysis would be anecdotal. Furthermore, with only one 

exception, our present dispersion models do not address even hourly 

averages due to building downwash. ISCST does have a downwash algorithm; 

however, it produces only hourly estimates and, similar to Huber, assumes 

the plume is totally downwashed for less than GEP stacks. Even if hourly 

estimates were available one would still need to estimate the 5-10 minute 

peaks. Rough calculations (similar to Huber's) are possihle, but only for 

the simplest situations. More typical urban situations with numerous 

sources, street canyons, and terrain would he impossible. Finally, the 

geographic scale of the problem does not match the scale of census data. 

Our initial analyses, described above, indicate that we would have serious 

difficulties preparing population data for such an analysis. If better 

methods to calculate peaks due to downwash were developed, the air quality 

data issue would be largely resolved. However, the other issues would 

remain. 

In summary, now that it has been revised to account for coding errors 

the analysis of exposures around power plants should be reasonably accurate 

for conditions around GEP power plants. However, the analysis does neglect 

potential exposures around numerous smaller facilities and some power 

plants with less than GEP stack heights. At present it is not possible to 

state conclusively whether the additional exposures around smaller sources 

p.24 
p.25 

22 

would add significantly to the totals already estimated nor whether such 

exposures would result in a significant response in exercising asthmatics. 

However, our preliminary work on the question indicates that large numbers 

of individuals may he at risk of an exposure. Clinical studies are underway 

to determine if the response to very short exposures is significant or not. 

Thus, although two years ago we concluded that the power plant analysis 

accounted for most potential exposures > 0.5 ppm, the recent information 

regarding smaller sources/low persistance events call this conclusion into 

doubt. Unfortunately, with our present data bases and analytic tools, a 

satisfactory analysis of smaller sources is not possible. 
•Or 

p.26 

Table 1 

Modeled Occurrences > 1 ppm and Estimated Exposures 1 

(Current Emissions) 

Asthmatics 

Expected Expected Asthmatics Exposed Exposed 2_ 1 ppiri 

Exceedances Exceedances > 1 ppm 5-minute 5-minute Any 

Plant 1 ppm 1-hour 1 ppm 5-minute 'High" Exercise (%) 2 Exercise (%) 

Gallagher 0.71 6.4 570 (2.8%) 2300 (12%) 

Portage Des Sioux 0.66 5.4 140 (0.85%) 590 (3.5%) 

1) Results based on revised 1986 exposure model 

2) Defined as > 35 l/min 
0.003 

10 

0.3-3 

0.5-2 

•- 

p.27 

Table 2 

Estimated Peak Concentrations Due to Low Persistance Events 

(Ppm) 

Coal Sulfur 

(1b/MMBtu) 

Stack Concentration 

(Ppm) 

Range for Downwash 

Hypothetical Range for 

Plants Actual Plants 

Looping 

Plume 

38.7 

77.4 

116.1 

154.8 

193.8 

0.65 12.13 0.06 

1.29 24.25 0.12 

1.93 36.38 0.18 

2.60 48.50 0.25 

3.20 60.63 0.31 

0.5 

1.0 

1.5 

2.0 

2.5 

258 

516 

774 

1032 

1290 

0.8 

1.5 

2.3 

3.1 

3.9 

Dilution Ratio 0.15 1 

Estimated Frequency of 

Occurrence 

Distance from Stack (km) 

Period of Event (min.) 

1000 

<0.5 

0.5-2 

.0025 2 0•047 30.000244 

1000 

<0.5 

0.5-2 

1) Wind Speed: 5 m/s; Bldg Height: 25 m 

2) Wind Speed: 10 m/s; Bldg Height: 50 m 

3) Wind Speed: 5 m/s; Bldg Height: 45•7m, Stack Height: 61m 

4) Wind Speed: 20 m/s; Bldg Height: 57m, Stack Height: 34m 
Table 3 

Population Residing Within Specified Distayces of Short Stack 

(< 60 m) Sources of S02 1 

(000s) 

Number of Sources 2 0.5 km 1 km 2 km 3 km 5 km 

1 6,843 15,831 27,273 33,021 35,794 

2 1,413 4,878 10,528 15,156 19,432 

3 430 1,990 5,835 8,565 12,506 

4 218 888 3,736 5,184 8,086 

5 101 521 2,390 3,702 5,962 

Any 9,186 25,761 57,783 84,114 120,956 

1) Based on use of 1980 U.S. Census data. Total considered is 226,546,000. 

2) Total number of sources is 14,694. 

p.28 
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'44 t 
References 

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4 w . .i" 

2 

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