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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 andStandards, 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 reyardinythe 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

. in 1982 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.4P. 5

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 veryp. 

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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.P. 7

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 staffp. 

## 8

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.1 3
. 9

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), onp.10

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 analysesp. 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 thep.12

9 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

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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 p.13p.14



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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 hep.15

12
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 impactsp.16
13
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 p.17p.18

15
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.p.19

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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 recentdata 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 couldp. 20

17 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 beunderestimated. 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 underestimateddue 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 morep.21
18 than just diagnosed asthmatics. In particular, they pointed out that theremight 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. Inthis 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 it19

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.22p.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.24p.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.•O r 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/min0.003

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

0.15 1 Dilution Ratio
Estimated Frequency of

Occurrence Distance from Stack (km) Period of Event (min.) 1000 <0.5 0.5-2 .0025 2 0•047 3 0.00024 4

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.28rt.



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# '44 tReferences

Bethel, R. A., Erle, D. J., Epstein, J., Sheppard, D., Nadel, J.A., and Boushey, H.A. (1983b). Effect of Exercise Rate and Route of Inhalation* on Sulfur Dioxide-Induced Bronchoconstriction in Asthmatic Subjects. Am. Rev. Respir. Dis. 128: 592-596. Biller, W. F. et al. (1984) Estimation of Short-Term Sulfur Dioxide Population Exposures. Paper presented at 77th Annual Meeting of the Air Pollution Control Association, San Francisco. Frank, N.H. et al. (1982) Relationship Among SO2 Averaging Times and Ambient Standards. Monitoring and Data Analysis Division, U.S. EPA, Research Triangle Park, N.C. Godar, Thomas, J. (1985) Letter to Lee M. Thomas, Administrator, U.S. Environmental Protection Agency, November 13, 1985. Hawkins, David G. (1985) Letter to Lee M. Thomas Re: Review of National Ambient Air Quality Standards for Sulfur Oxides, November 8, 1985. Horstman, D.H., Roger, L. J., Kehrl, H., and M. J. Hazucha (unpublished manuscript). Airway Sensitivity to Sulfur Dioxide. HERL, U.S. EPA, RTP, NC.Huber, A.H. et al. (1985) Comments on Peak Ground-Level Concentrations Due to Building Downwash Relative to Peak Concentrations Under Atmospheric Dispersion Processes. (Unpublished paper) Atmospheric Sciences Research Laboratory, U.S. EPA, Research Triangle Park, N.C. Huber, A.H. et al. (1985) Memorandum to Joseph Tikvart, Subject: Comments of June 1985 Analysis of Peak Concentrations. Atmospheric Sciences Research Laboratory, U.S. EPA, Research Triangle Park', N.C. Linn, W. J., Vent, T. G., Shamoo, D. A., Valencia, L. M., Anzac, V. T., Spier, C. E., and J. D. Hackney (1983). Respiratory Effects of Sulfur Dioxide in Heavily Exercising Asthmatics. Am. Rev. Respir. Dis. 127:278-283. Roger, L.J., Kemrl, H.R., Hazucha, M., and D.H. Horstman. Bronchoconstriction in Asthmatics Exposed to Sulfur Dioxide During Repeated Exercise. 5 App. Phys. 59: 784-791. Schachter, E. N., Witek, T. J., Beck, G. J., Hosein, H. R., Colige, G., Leaderer, B. P., and W. Cain (1984) Airway Effects of Low Concentrations of Sulfur Dioxide: Dose Response Characteristics. Arch. Environ. Health 39: 34-42. Strategies and Air Standards Division (1982) Review of the National Ambient Air Quality Standards for Sulfur Oxides: Assessment of Scientific and Technical Information OAOPS Staff Paper. Strategies and Air Standards Division, U.S. Environmental Protection Agency, Research Triangle Park, N.C., November 1982. p.30p.31 

## 4 w

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Thomas, Henry C. (1986) Memorandum to Bruce Jordan, Subject: Occurrence of and Exposure to SO2 Levels > 1.0 ppm. Strategies and Air Standards Division, U.S. EPA Research Triangle Park, N.C. June 24, 1986.U.S. Environmental Protection Agency (1984) Estimation of ShortTerm Sulfur Dioxide Population Exposures. Office of Air Quality Planning and Standards, Research Triangle Park, N.C.. U.S. Environmental Protection Agency (1986) Second Addendum to Air Quality Criteria for Particulate Matter and Sulfur Oxides (1982): Assessment of New Available Health Effects Information. Office of Health and Environmental Assessment, Research Triangle Park, N.C. July 1986.