Dose considerations in the SO2-exposed exercising asthmatic.

In this study we have demonstrated that by combining data from several recent controlled human exposure studies it is possible systematically to relate increases in airways resistance to the rate of SO2 exposure (Dmin) in the exercising asthmatic. It was determined that the mode of SO2 exposure (oral vs. oronasal) greatly influences the degree of response in the asthmatic. Forced oral breathing consistently produces larger percentage increases in SRaw per unit increase in SO2 exposure rate. We have demonstrated further that while the dose/effect relationship which describes the increases in specific airways resistance (SRaw) versus exposure rate (Dmin) of SO2 is most consistently exponential in character, a linear (more conservative) model also can be used to fit the data. Using both the linear and exponential model, we have constructed a matrix which allows direct estimation of the combined minute ventilation (VE) and SO2 concentration (as ppm or microgram/L) required to achieve various levels of specific airways resistance increase. In this report this matrix is constructed only on subjects breathing in an unencumbered (oronasal) manner. Future reports will explore these relationships in the asthmatic breathing in an encumbered (oral) manner.


Introduction
Based upon data from several recently conducted controlled human exposure studies (1)(2)(3)(4)(5), it has been demonstrated that asthmatic subjects exposed to SO2 respond with an increase in specific airways resistance. It has been demonstrated further that when exposure is combined with exercise, at a light to moderate level, the magnitude of the S02-induced increase is greater.
Based upon what is known concerning the asthmatic and within the context of the clinical definition of this disease, this effect of SO2 exposure, especially when combined with exercise, is not unexpected.
The current results of controlled human exposure studies in which exercising asthmatic subjects were exposed to SO2 during exercise can be divided into two groups: studies of subjects exposed via a mouthpiece which precludes nasal breathing and thus forces SO2 uptake to be exclu-*Colucci and Associates Inc., 17705 I Hale Ave., Suite  sively oral (encumbered breathing) or studies of subjects exposed via a facemask or in a chamber which permits oronasal (unencumbered) breathing.
Under these circumstances, the exercising asthmatic who is exposed exclusively by mouthpiece (encumbered) represents the most severe or "worst case" exposure situation. In the study reported herein, this group will be focused on initially. Their airways resistance responses will then be compared to those observed in asthmatics exposed in an unencumbered manner.

Approach
The approach used in this study is the same as has been reported previously (6)(7)(8). Ib briefly summarize, observed changes in airways resistance expressed as specific airways resistance (SRaw) are calculated as a percentage increase (or decrease) from the control (pre-exposure) value with both individual and sets of subjects serving as their own control. For each data set, individual subject and group mean values for percentage changes in SRaw (%A SRaw) are calculated and represent the "effect" (or dependent) variable. Since the method by which increases in SRaw are presented is not always consistent among the various investigators, for convention we have chosen to utilize a uniform method of calculation which allows direct interexperimental comparisons to be made. Briefly described our method utilizes the pre-and post-exposure SRaw values from each subject or group of subjects exposed to various levels of SO2 as the basis for estimation of %A SRaw* From this %A SRaw is subtracted the percentage of change observed when the same subjects are exposed to air alone (an SO2 concentration of zero). Another means by which %A SRaw can be determined is to utilize the air only (sham) exposure values as the overall baseline for all subjects regardless of day of exposure. However, after investigating this approach, we concluded that it fails to account for the observed daily variations in pre-exposure SRaw which occurs in asthmatic subjects.
The choice of the dose parameter also provided opportunity for investigation. In the first instance it is not possible from any studies conducted thus far to absolutely quantify the dose of SO2 delivered to the target tissue or organ. However, attempts at developing extrapolation methods are currently in progress (8,9) and although they appear promising, none has as yet been empirically confirmed. Consequently, the forms in which SO2 exposure can be expressed are either as concentration alone (ppm, jg/M3, jg/L), the product of concentration and exposure duration (C x T) or the product of concentration and minute ventilatory volume (provided VE is known or estimatable) with or without temporal factors. Since neither the concentration alone, nor the concentration times exposure duration (C x T) expression can account for the influence of changes in minute ventilation (VE) that accompany increased exercise, we elected not to explore them further. Alternatively, the use of several forms utilizing the product of concentration and VE were explored. Remembering that VE is expressed as liters/minute, several equations can be derived which combine SO2 concentration and VE with or without temporal factors. SO2 concentration (jg/m3/1000) x VE (L/min) = jg/min SO2 (1) Since (jg/m3)/1000 = jg/L, SO2 concentration (jg/L) = VE x jg/min SO2 (2) SO2 concentration (ppm) x VE (L/min) = ppm-L/min SO2 SO2 concentration (ppm as jL/L) x VE (L/min) = pL/min SO2 When consideration is given to temporal factors, either jg/min or jL/min can be multiplied by the total minutes of exposure. The resultant products are total jg or total bL. jg/min x minutes of exposure = total jig SO2 (5) jL/min x minutes of exposure = total jL SO2 (6) We have adopted for our purposes Eqs. (2) and (5). This was done because it was concluded that a mass measurement (jg) was preferred to a volumetric (ppm) one, particularly when comparison with other airborne pollutants is desired. We have labeled the resultant of Eq. (2) Dmin (jg/min), and that of Eq. (5) Dt (jg). It should be noted that Dmin is actually an exposure rate, while Dt is a total exposure dose. Also, since VE is normally expressed as L/min, the concentration form of jg/ L was adopted over jig/M3, although this latter form is that in which ambient levels are normally expressed. It was reasoned that the use of jg/L allows a more direct estimate of Dmin by simply combining VE in L/min and concentration as jg/L.
Based on previous studies (6-8) we observed that changes in airways resistance (expressed as either Raw or SRaw) in response to SO2 exposure vary most consistently with the exposure rate (Dmi). In fact, Dt is normally found to correlate very poorly. In the studies reported herein, Dmin (exposure rate) was also found to correlate better with changes in SRaw and thus will be used as the dose (or independent) variable.
For each set of data, Dmin and %A SRaw are calculated and a scatter plot of Din (x axis) versus %A SRaw (y axis) values is prepared. Tb these points a series of curve-fitting equations is applied for the purpose of determining which mathematical relationship best fits these data points and which serve as the basis for prediction of changes in %A SRaw in a broader context.
Since the issue ofthe choice ofthe most applicable mathematical relationship is an important one, we will briefly discuss our approach to making this choice. There are a large number of mathematical relationships (equations) which can be applied to any set of data points, either in the normal or transformed state. Thus, a series of guidelines must be adopted which will assist in selecting the correct form of the equation to be used. We have adopted the guidelines set out by Daniel and Wood (10). The method of fitting equations to data which we have utilized is an adaptation of both the Linwood and non-Linwood leastsquares fitting program which has been widely documented and is available to multiple users.
As a working principle we have adopted the approach of favoring the equation with the least number of constants which provides the best fit. In some cases alternative equations are also chosen to visualize the dose/effect relationship as well. These cases are noted and the rationale for their exploration and use discussed. It should be noted also that we have utilized group mean values of %A SRaw to prepare our scatter plots and as the basis of analysis. In a previous report (8) we have presented data which compare the results obtained utilizing both group mean values and individual subject changes in %A SRaw. Table 1 summarizes the percent changes in SRaw reported in exercising asthmatic subjects exposed to SO2 via a mouthpiece (encumbered) along with group mean VE values. In addition, the author and reference are listed. The broader set of data from which these summary values are derived appear in Table 2. As can be observed, there is a progressive increase in %A SRaw as exposure rate (Dmin) increases. By the application of linear regression analysis (11,12) (see Table 3 for details), the coefficient of correlation r was estimated to be 0.9605, and the coefficient of determination r2 was estimated to be 0.9225. Stated simply, it is observed that in this body of data which relates SO2 Dmin to %A SRaw in the exercising asthmatic that Dmin correlates well with %A SRaw. Figure 1 illustrates the results obtained when both a linear equation and exponential equation are fitted to the data points. It should be noted that the exponential equation provides a better fit to the points than the linear. In this latter case, the exponential coefficients are r = 0.9927 and r2 = 0.9855.

Results
As we discussed previously, a number of equations can be fitted to these data points. In this specific case both an exponential and geometric (power) equation were found to fit the observed data points best. However, the choice of which equation to use for curve fitting requires further distinction.
In general, exponential least squares is favored when a plot of log y (%A SR8W) versus x (Dmin) is linear in form. Alternatively, a geometric (power) least-squares equation is favored when a plot of log y versus log x is linear. A test of both equations revealed that for the appropriately log transformed data the exponential equation provided the better linear fit. Although it contains more constants than the geometric (power) form, Response of the exercising asthmatic to S02 (encumbered breathing): (@) observed; (--) linear best fit curve; (-) exponential best fit curve. Equations: linear, y = a + bx, y = -157 + 6.4x (r2 = 0.923); exponential, y = aebx, y = 14e004x (r2 = 0.986). 0 its use is dictated in this context. As such we conclude that an exponential equation ofthe form y = aebx most accurately describes the relationship between Dmin and %A SRaw in this set of data on exercising asthmatics.
As noted, a simple linear equation can be fitted to these data points as well but displays the relationship less accurately. The decision to include it in Figure 1 and in subsequent calculations was based on the fact that over this range of SO2 exposure rate (Dmin) values it generally predicts a higher %A SRaw per unit increase in Dmin.
Thus it can be used to represent a more conservative or "worst case" model for prediction purposes. In later sections wherein the application of the model is discussed this distinction should be kept in mind.
Even with modeling considerations aside, it is clear that data derived from controlled study of S02 exposures to exercising asthmatics breathing in an encumbered mode indicate a consistent and positive relationship between increases in exposure rate (Dmin) and specific airways resistance (%A SRaw) increases.
As we have stated previously, several other authors have chosen to investigate this relationship in asthmatic subjects allowed to breath in an unencumbered manner, reasoning that it is more reflective of ambient circumstances (2)(3)(4)(5). We have evaluated these data as well, and they are summarized in Table 4 (derived from Table 5). Figure 2 illustrates both the fitted linear and exponential curves. In this case the previous pattern observed with the encumbered breathing subjects is repeated, i.e., the exponential equation most accurately reflects the dose/effect relationship, and Dmin is shown to be highly correlated to %A SRa8 (see Table 6).
There is, however, a crucial and very important difference between the changes observed in the unencumbered breathers and those observed in encumbered breathers. In the case of encumbered breathers, the exercising asthmatic subjects demonstrate a consistently larger %A SRaw increase per unit Dmin increase than the unencumbered breathing subjects. This larger response per unit Din is particularly noted at the higher exposure rate levels and is vividly illustrated in Figures 3  and 4, wherein both the best fit linear and exponential curves are compared as a function of expo-    I   I   I   ,   I   I   I   I   I   I   I   I  I   I   O   5   10  15  20  25  30  35  40  45  50  55  60  65  70  75  80 EXPOSURE RATE (Dmn as ..g/min) FIGURE 2. Response of the exercising asthmatic to SO2 (unencumbered breathing): (.) observed; (--) linear best fit curve; (-) exponential best fit curve. Equations: linear, y = a + bx, y = -74 + 2.74x (r2 = 0.842); exponential; y = aebx, y = 0.24e0°09 (r2 = 0.963). 0 0 226 sure mode. In both figures, A represents the encumbered breathing asthmatic and B the unencumbered. Inasmuch as the differences between the result obtained with the two exposure modes are not trivial, a decision must be made for the future as to which exposure conditions are most adaptable for attempting an extrapolation of these data to the free-living asthmatic. At present we are evaluating this issue and are proceeding to examine results based on data obtained using both exposure modes. An examination of this issue is underway, and preliminary findings are discussed below.

Application of the Model
One key question that data in this form can address is concerned with exploring the interrelationships between minute ventilation (and by association level of activity), ambient SO2 concentration and increased specific airways resistance in the asthmatic. lb examine these interrelationships, we have assembled in Tables 7 and 8  from studies of asthmatics breathing in an unencumbered mode. The data in Table 7 are derived from the Dmin and %A SRaw values obtained from the linear model and those in Table 8 from the exponential (see Fig. 2 (Table 8) would be 0.577 ppm (1.50 jg/L). As can be seen in Table 7, an increase of exercise to a moderate level (VE = 40 L/min) lowers the SO2 concentration required to achieve the 0% A SRaw increase Din value of 27 gg/min to 0.26 ppm (0.675 gg/L). Similar relationships are observed at all Dmin and/or VE values. An examination of Table 8 (values derived from the exponential equation) reveals the same pattern. Namely, as VE increases, the SO2 concentration required to achieve any increase in SRaw (Dmin) decreases.
In Figures 5 and 6 we have plotted a subset of Iw   I   I   I   I   I   I   I   I   I   r   I   I   0   5  10  15  20  25  30  35  40  45  50  55  60  65  70  75  80 EXPOSURE RATE (Dmin as -g/min) FIGURE 4. Comparison of the response of the exercising asthmatic to SO2, encumbered vs. unencumbered breathing; (-) best fit exponential curves: (A) encumbered breathing; (B) unencumbered breathing. S02-EXPOSED EXERCISING ASTHMATICS these data as log (In VE vs. In ppm) transformed values to achieve linearity. In this form the data provide a direct visualization of the VE (level of exercise) combined with SO2 concentration (as ppm) required to achieve any Dmin (%A SRaw) value. Most importantly these figures illustrate the strong interdependence of VE and concentration and thereby serve to underscore another important issue, namely, that regardless of which exposure mode is chosen to extrapolate to the free living asthmatic or, further, no matter which level of specific airways resistance increase is adjudged as adverse to the exercising asthmatic both activity level (VE) and SO2 concentration (ppm or jg/L) must be addressed in the definition of acceptable ambient concentrations.
If, for example, 0.5 ppm SO2 is chosen as that concentration which will be protective ofthe asthmatic, it can be clearly seen that this will be protective under some circumstances and not protective in others. Utilizing the more conservative linear model (Table 7, Fig. 5) if a zero increase in SRaw is desired the asthmatic will only be protected in an atmosphere of 0.5 ppm (1.3 gg/L) SO2 when VE values are at or below 20 L/min. At a 25% A SRaw, the VE value lies between 20 and 30; at 50% A SRaw between 30 and 40, and at 100% A SRaw between 40 and 50 L/min. Stated another way, if it is assumed that a 50% increase in SRaw is the maximal tolerable change, it can be seen that this will be achieved over a wide range of S02 concentrations. However, if it is further required that the majority of subjects be   b/min jg/L ppm gg/L ppm jig/L ppm gg/L ppm gg/L ppm jig/L ppm jig/L ppm jg/L ppm protected when undergoing moderate to heavy exercise (VE equal to 40 L/min), it can be seen (using the linear model) that the maximum SO2 concentration can never be allowed to exceed 0.44 ppm (1.14 gg/L). Using the exponentially derived values ( Table 8, Fig. 6), the SO2 levels change accordingly, but the same principle applies. In this latter case, a 50% increase in SRaw will be prevented at SO2 concentrations below 0.

Discussion
From this study there are a number of observations which can be made regarding increases in specific airways resistance in the exercising asthmatic exposed to SO2 and the means by which these changes are viewed in attempts to establish protective ambient concentrations.
Initially, it can be concluded that there is a very consistent increase in specific airways resistance in these asthmatics as the rate of SO2 expo- sure increases. This relationship has been found to be best described by an exponential equation suggesting that at the higher exposure rate (Dmin) values, SRaw increases more rapidly per unit increase in Dmin than at lower exposure rates. The converse is true at the lower Dmin values. Also, it has been determined that, while a simple linear relationship between Dmin and %A SRaw can be shown to fit the data, it does so less strongly than the exponential equation and provides a generally more conservative model.
An important additional finding is that the observed increases in specific airways resistance that occur in these asthmatic subjects in response to SO2 challenge are different in magnitude (but not in form) depending upon the mode of SO2 exposure. Subjects forced to breath in an exclusively oral manner (mouthpiece with noseclip) demonstrate a consistently greater increase in SRaw per unit increase in SO2 exposure rate than their counterparts allowed to breath SO2 in a less encumbered manner (oronasally). This observa- tion is not surprising, inasmuch as the forced oral (encumbered) breathers would be deprived of the filtering effect of the nose which is known to absorb SO2 from the inhaled air and as such could be reasonably expected to receive a greater mass of SO2 in their upper airways. Thus, although this finding is not surprising, it does pose serious questions as to the choice ofdata for extrapolation in a broader context. Application ofthe model (linear or exponential) suggests also that future attempts to arrive at acceptable ambient levels must consider the influence of exercise level (activity patterns) more closely than in the past. We have shown that exercise level profoundly influences the extent of specific airways resistance increase which will occur at any SO2 concentration. This is particularly true when data collected on small sets of subjects are to be used to provide quantitative insights into the expected changes in specific airways resistance of asthmatics in the general population experiencing changing exposures and manifesting changing activity patterns.
In the past, attempts to arrive at acceptable ambient levels have most commonly defined SO2 exposure in terms of concentration alone (ppm or gg/m3). While this approach may be applicable on singular sets of data obtained under closely controlled laboratory conditions, it is not sufficiently robust to account for the free-living circumstance.
Future attempts should define acceptable ambient levels as a combination of the degree of change in the effect parameters judged as desirable, as well as the concentration of SO2 combined with level of activity which interact to produce this degree of change.

Conclusions
In this study we have demonstrated that by using data from a variety of controlled human exposure studies it is possible to relate increases in airways resistance systematically to the rate of SO2 exposure in the exercising asthmatic. We have illustrated that the mode of exposure (oral vs. oronasal) greatly influences the degree of response in the asthmatic. Forced oral breathing consistently produces larger increases in SRaw per unit increase in SO2 exposure rate.
We have demonstrated further that the dose/ effect relationship which describes the increases in SRaw versus exposure rate (Dmin) of SO2 is most consistently exponential in character, but that a linear (more conservative) model also can be used to fit the data.
Using both the linear and exponential model, we have constructed a matrix which allows direct estimation ofthe combined VE and SO2 concentration (as ppm or gg/L) required to achieve various levels of airways resistance increase. At present we have explored only subjects exposed in an unencumbered (oronasal) manner. Future studies will explore these relationships in the asthmatic breathing exclusively orally.