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estimates are developed, with focus primarily on reduc- tions in ground-level ozone resulting from the control of volatile organic compounds; to a lesser extent, particulate control also is considered. Proposed controls are evaluat- ed for both the nation as a whole and for the Los Angeles metropolitan area, where violations of air quality stan- dards are most frequent and severe. Subject to a number of uncertainties, the costs of proposed new controls are found to exceed the benefits, perhaps by a considerable margin.
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EVNVIRONMENTAL REGULATION IS IMPORTANT TO OUR health and well-being and also is quite expensive. For these reasons, we must look carefillly at our environmental laws and regulations to see what they will accomplish and at what cost.
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Recently, three major changes were made to the Clean Air Act. First, over the next decade electric power plants must make sharp reductions in emissions ofsulfur dioxide (S02)-10 million tons per year measured from their 1980 level. Second, most major sources of what are called hazardous air pollutants (less ubiquitous but still potentially harmful substances such as benzene, acrylonitrile, beryl- lium, and coke oven emissions) must install state-of-the-art emis- sions control equipment and, eventually, further reduce any residual emissions that pose unacceptably high health risks. Third, a number of new measures have been enacted to improve air quality in areas where the national ambient air quality standards (NAAQS) are currently being violated (1).
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This third problem, referred to as nonattainment with regard to the NAAQS, is by far the most difficult to solve. According to the Environmental Protection Agency (EPA), in 1989 more than 66 million people in the United States lived in counties where the ozone (03) standard was being exceeded at one or more monitors (2). Another 27.4 million lived in areas violating the particulate standard; the corresponding totals were 0.1 million for SO2, 33.6 million for carbon monoxide (CO), and 8.5 million for nitrogen dioxide (NO2).
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Most of the inexpensive pollution control measures were imple- mented during the last 20 years, so that future reductions in emissions are likely to be more expensive than earlier ones. Further- more, the measures required to address remaining pollution prob-522 lems will fall increasingly on individuals rather than on the large industrial facilities (called point sources) and motor vehicles, which shouldered most of the burden of the initial clean-up (3). More and more, air pollution problems are those associated with wood stoves, small dry-cleaning and degreasing operations, painting shops, bak- eries, and other decentralized sources. As a result, the burden of pollution control will become more obvious to the public.
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To help focus debate about the best use of society's resources, it is important to have estimates of the benefits and costs of further improvements in air quality. In this article we develop such esti- mates, focusing primarily on reductions in ground-level 03 resulting from the control ofvolatile organic compounds (VOCs) (4); we also consider particulate control. We evaluate proposed efforts both at the national level and in the Los Angeles metropolitan area, where violations of air quality standards are most frequent and severe. In both cases, we first present point estimates of benefits and costs and then discuss uncertainties in a subsequent section. A brief back- ground precedes this analysis.
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Benefit-cost analysis. Benefit-cost analysis is used by economists to identify, quantify, and weigh the advantages and disadvantages of public policies designed to increase society's overall well-being.
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Originally used by the Army Corps ofEngineers to judge alternative water resources projects, it has become an integral part of policy analysis at all levels and types of government.
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The quantification of benefits and costs rests on the idea that an action has value if someone is willing to pay for it, and each individual is held to be the best judge of how a policy affects him or her (5). If an individual assigns a high or a low value to something about which others feel different, that value must be counted nonetheless. The overall value to society of a proposed policy change is measured by the sum of the individual valuations. Thus benefit- cost analysis is unabashedly anthropocentric in that things have value only to the extent that they provide well-being to individuals. These effects, however, are not restricted to purely financial gains and losses. They include the reduced well-being that results from aesthetic degradation, for instance, as well as that felt directly in the pocketbook.
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The values that individuals would be willing to pay for the favorable impacts of a policy-whether increased agricultural output or intangibles such as cleaner air, purer drinking water, or the removal of hazardous wastes-often are difficult to ascertain, but economists have devoted much effort to developing valuation techniques consistent with the underlying principles of welfare economics (6). Costs are generally measured as the expenditures that private firms, governments, and individuals must make to comply with regulations. Analogous to benefits, however, costs should be measured by the amount of money required to compensate individ- uals for the unfavorable effects associated with a regulatory or other public policy; these might take the form of higher prices, reduced incomes, the inconvenience of forced car-pooling, or some other welfare-reducing policy (7). Because models frequently are not available to estimate costs in the preferred way, pollution control expenditures are usually used as proxies. That is the approach taken here.
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In principle, benefit-cost analysis includes the value of even the most intangible effects of a policy. For instance, if everyone in the United States was willing to pay $4 per year to preserve a particular wetland area, annual benefits of its preservation would be about $1 billion; similarly, to the extent that people are willing to pay something for the preservation of species diversity, whether for commercial or philosophic reasons, it is counted as an economic benefit. It is important to understand that benefit-cost analysis is not restricted to goods bought and sold in private exchange.
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Finally, although benefit-cost analysis is a technique for identify- ing efficient policies, economic efficiency is surely not the only basis on which policy decisions should be made. Distributional consider- ations, legal mandates, and ethical concerns are also of great importance, and benefit-cost analysis is generally (but not always) silent about such matters (8).
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Air quality regulation. When the Clean Air Act was amended in 1970, Congress directed the administrator of the new EPA to establish air quality standards to protect human health with an adequate margin ofsafety. The first standard for 03 was set in 1971, the basis ofwhich was total photochemical oxidants: The daily high 1-hour reading was not to exceed 0.08 part per million (ppm) on more than 1 day per year. In 1979 the EPA changed the basis of the standard from total photochemical oxidants to 03 and relaxed the 1-hour standard to 0.12 ppm, not to be exceeded on more than 3 days in any 3-year period (in other words, the reading on the fourth highest day, called the design value, determines attainment status). The 03 standard remains the same today.
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In spite of the 1979 relaxation, violations of the 03 standard are frequent. For instance, 101 metropolitan areas failed to meet the standard in 1988; 31 of these had more than 10 violations, and 3 (Los Angeles, Fresno, and Bakersfield) had more than 20 violations. Of these 3, Los Angeles led the pack with 148 days on which the 0.12-ppm 1-hour standard was exceeded at least one monitor.
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In many of the areas where violations occur, the standard is exceeded only slightly. This is not the case everywhere, however. In Houston, New York, Chicago, and Philadelphia, for instance, the 1988 design values were about 0.22 ppm. In Los Angeles, the design value for 1988 was 0.33 ppm, nearly three times the level of the standard (9). Thus both the frequency and the severity of violations of the 03 standard are fueling concern about the nonat- tainment problem nationwide.
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To evaluate the benefits and costs of reducing ambient 03 concentrations, one must first estimate the VOC reductions expect- ed in various areas and the 03 improvements that they imply. Then the costs of the measures to be used to obtain the VOC reductions and the benefits associated with the 03 improvements can be estimated.
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In 1989, the Office of Technology Assessment (OTA) released a major study of air quality problems in the United States (10). The study estimated the changes in emissions of VOCs and, subsequent-
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10-Xo82 0 0 0 /P 0 6. o4 1994 .2 E 5 10 15 20 25 30 35 O
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Cumulative percent emisdons reductions from 1985 levels Fig. 1. Escalation of cumulative annual cost above a 30% reduction in VOC emissions, using control methods analyzed by OTA (nonattainment cities only). [Adapted from (10)] ly, reductions in 03 design values that would result in the years 1994 and 2004 from the application of all currently available VOC control technologies in nonattainment areas and some added control in clean areas (11). No transportation control plans or additional controls on nitrogen oxides (NO_) were included.
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As estimated by OTA, by the year 2004 these control measures would reduce total annual emissions of VOCs in nonattainment areas from about 11 million to about 7 million tons, representing a 35% reduction. Depending on the particular urban area in question, the annual VOC reduction would vary from 20 to 50% (12). Our benefit and cost estimates pertain to this predicted change in air quality.
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These estimated emission reductions for each city were then passed through a set of EPA trajectory models, which predict peak ambient concentrations of 03 (13). The VOC controls that OTA considered were projected to bring 31 of the 94 areas in mild violation of the 03 standard in 1985 into attainment by 2004. Areas such as Los Angeles, however, with design values greater than 0.15 ppm, were predicted to remain in violation even after controls were implemented, although the predicted design values would be re- duced somewhat. For instance, in Los Angeles the highest single reading was predicted to fall by about 20% by the year 2004 as a result of the controls that OTA considered.
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Costs. According to OTA, the annualized cost associated with this ambitious set of measures would be $6.6 billion to $10.0 billion in the nonattainment areas alone, or about $1800 to $2700 per ton of VOC reduced there. Adding in the costs that would be borne in attainment areas raises the estimated total to $8.8 billion to $12.8 billion per year.
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Of all the control measures examined, reducing the volatility of gasoline accounts for the greatest emission reduction (about 14%) and would also be the most cost-effective control technology because this reduction would cost between $120 and $740 annually per ton of VOC reduced. At the other extreme, OTA found that using methanol (an 85% blend) to power fleet vehicles would be an expensive measure: $8,700 to $51,000 per ton of VOC reduction (14). A ranking of individual approaches by cost effectiveness shows that marginal costs increase sharply for obtaining any more than a 30% reduction in VOC emissions (Fig. 1).
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Benfits. How does one ascertain the amount individuals would be willing to pay for the air quality improvements that OTA projects? We concentrate on acute health benefits because protecting health is the primary justification for setting air quality standards under the Clean Air Act and because only acute health effects have been linked convincingly to 03 concentrations. Other benefits could accrue in the form of reduced damage to exposed materials, crops, and other vegetation and possibly reductions in the preva- lence of chronic illness.
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To determine the acute health benefits associated with the esti- mated 35% reduction in VOC emissions in nonattainment areas, we used a county-level model developed for this purpose (15). The model predicts reduced baseline 03 concentrations in each area for each day of the 03 season on the basis of the percentage reductions in 03 design values obtained from OTA (16). These air quality changes must then be mapped into improved human health. To do so, we combined area-specific data on air quality improvements and population with dose-response functions based on epidemiologic and clinical (controlled laboratory) studies relating ambient 03 concentrations to various symptoms and other adverse human health effects (17). Thus, for example, the predicted air quality improvement in a particular urban area in the year 2004 can be translated into fewer asthma attacks, reduced incidence of coughing and chest discomfort, reduced number of days of restricted activity, and the like on the basis of predicted population of that area in that year. For example, the clinical dose-response function we used to predict the reduced incidence of coughs for an entire population at a given reduction in ambient 03 concentrations is
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where AC is the change in number of cough episodes for a 2-hour period; XO is the average concentration for a 2-hour period, baseline; X1 is the average concentration for a 2-hour period, postcontrol; y = 1.742; p = 14.100 per parts per million; and 0 is the percentage of time that the population is engaged in exercise.
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Because clinical and epidemiologic studies provide quite different types of information for use in estimating health effects, we com- puted two separate estimates of these physical improvements on health. Clinical studies are relatively precise in quantifying relation- ships between 03 exposure in the laboratory and either respiratory symptoms or changes in lung function. The most useful clinical studies estimate dose-response functions by observing the presence or absence of specific symptoms in a small group (from 20 to 135 individuals) of heavily but intermittently exercising young adults (generally men) exposed to 03 for 2 hours at various carefully controlled concentrations. Extrapolating from these clinical studies to determine health benefits to the general population from reduced exposures to ambient 03 is difficult. Among other things, it requires adjusting for the exposures and the time that people spend indoors and outdoors as well as the time that they spend either at rest or exercising at low rates. Information about such behavior, particularly at alternative 03 levels, is sparse.
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Epidemiologic studies have the advantage of not requiring these extrapolations and adjustments. Nevertheless, they can show only statistical associations that may not have causal connections between pollution concentrations or exposure and adverse health effects (18). The studies that we used associate ambient 03 concentrations, measured at fixed monitoring stations located close to a person's home, to daily or 2-week records of illness, symptoms, or days of restricted activity that the person experienced.
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We estimated the reduced incidence of the quantifiable adverse health effects in the year 2004 accompanying a 35% reduction in emissions of VOCs for an estimated 129 million people living in the 94 metropolitan areas (322 counties) predicted to be in nonattain- ment in 2004 (19, 20). For each metropolitan area, we made separate calculations on the basis of the predicted change in air quality there and then aggregated these estimates to obtain the 524 national estimates.
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From the epidemiologic studies, we found that the average asthmatic will experience about 0.2 fewer days per year on which he or she has an asthma attack and that the average nonasthmatic will experience about 0.1 fewer minor restricted activity days per year because of reduced VOC emissions and subsequently improved air quality (21). In addition, nonasthmatics will experience other minor health benefits as well in the form of reduced number of symptom days.
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To convert these predicted changes in physical health into economic benefits, it is necessary to ascertain individuals' willingness to pay for a reduced incidence of illness and adverse symptoms. To do so, we drew on a number of studies designed to uncover these values, primarily through questioning ofboth healthy and infirm respondents with supplemental data on the out-of-pocket medical costs and lost income that may be associated with illness or symptomatic effects (22).
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These studies have found an average value of $25 for each asthma attack prevented, $20 for a reduction ofone restricted activity day (on which an individual is neither bedridden nor forced to miss work but must alter his or her usual pattern in some way), and $5 for one fewer day of occasional coughing. When reduced incidence is combined with these values, the predicted aggregate dollar benefits across the United States from these improvements in individuals' acute health status amount to $250 million per year.
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By using clinical rather than epidemiologic studies to estimate health benefits, we arrive at a somewhat larger value for acute health benefits. For example, we predict that the number ofcoughing spells of 2 hours' duration would be reduced by as much as 2.5 episodes per person per year. Also, fewer episodes of shortness of breath and pain on deep inspiration are predicted to occur. Both are important consequences of air pollution control. We estimate that the annual monetary benefits associated with these improvements in health would be on the order of $800 million annually (23). Comparison. To summarize, according to OTA, the costs associ- ated with a 35% reduction in nationwide emissions of VOCs in nonattainment areas will be at least $8.8 billion annually by the year 2004 and could be as much as $12 billion. Yet the acute health improvements that we predict to result from these changes are valued at no more than $1 billion annually and could be as little as $250 million. The high estimate relies on the most generous of the four clinical studies that the EPA sanctioned in its staff paper on the health effects associated with 03 and other photochemical oxidants (24). We also assumed that exercise rates would be high in the exposed population (which increases health benefits), and we in- cluded benefits even for those engaged in light or moderate exercise. Subject to the caveats discussed below, total health benefits are still relatively small. In contrast to, say, the removal of lead from gasoline, for which estimated benefits are well in excess of costs (25), the benefit-cost comparison for national 03 control is unfavorable. The reasons for this are, in part, the relatively small improvements in ambient 03 levels that the controls effect (which in turn imply fairly small benefits) as well as the high costs of control.
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What about air pollution control efforts in Los Angeles, the nation's most notably polluted metropolis? In 1989, the supervisors of the South Coast Air Quality Management District (SCAQMD) approved an ambitious new plan designed to bring the four-county district into attainment with the NAAQS (26). The South Coast plan is designed to reduce ambient concentrations of particulates (such as sulfates), NO, SO2, and CO in addition to 03. To effect these reductions, the plan envisions three tiers of controls. Tier I consists mainly of the wider application of known pollution control technologies. The 120 measures identified in Tier I include such things as installing pollution control devices on equipment used in the manufacture of rubber products, substituting less polluting solvents in degreasing operations and in auto refinishing facilities, and adding new controls on electric power plants. Tier I also contains a number of less conventional pollution control measures, such as the banning of bias-ply tires (to improve mileage and to reduce particulate levels) and even restricting fuels used in backyard barbecues.
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The 37 measures in Tier II of the South Coast's plan all require some advancement or extension of current pollution control tech- niques. They call for such measures as additional control of dust blown from roads and parking lots, incentives to reduce residential and industrial fuel consumption, and restrictions on automobile usage. The South Coast authorities envision implementing these measures over the next 10 to 15 years. Tier III controls are more speculative still and are designed to bring about major technologic breakthroughs. The South Coast plan does not provide an explicit list ofcontrol measures for Tier III, but it does identify programs that together aim to eliminate almost all hydrocarbons from solvents, coatings, and motor vehicles and to convert all the region's vehicles to low-emitting vehicles. According to South Coast officials, enactment of all three sets of controls will, by the year 2010, bring the area into attainment with the NO2 and CO standards and into virtual attainment for particulates and 03.
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Costs. The annual costs associated with 58 ofthe control measures in Tier I are estimated to be $3.4 billion per year and will obtain about one-third of the total emissions reductions under the whole plan (26). These control measures include those designed to reduce 03 concentrations as well as particulates and CO. By itself, this estimate provides some insight into the sweep of the South Coast plan: In 1988, total spending to comply with all federal air pollution control regulations across the entire United States was approximately $30 billion (27).
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The costs of the other elements of the plan to residents of the South Coast basin are difficult to assess because, among other things, they involve valuing time losses and inconvenience, such as those experienced in car-pools and those that require changes in driving, shopping, and even living habits. Although some of these measures involve relatively small out-of-pocket outlays, they will be costly in an economic sense if they increase inconvenience, waste time, or reduce well-being in other ways.
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A recent study provides an indication of the potential costs of the entire Los Angeles air quality plan (28). The overall cost was estimated on the assumption that the per-ton costs of the control measures for which no cost data were provided were equal to the per-ton costs for the measures for which cost data were provided. The estimated annual cost of the entire plan is about $13 billion per year, or about $2700 per household in the Los Angeles basin.
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Pollution control in the South Coast appears to be far more expensive on a per-ton basis than for the rest ofthe nation. For VOC control alone, the same study (28) estimated that the average cost is about $11,000 per ton compared to OTA's estimate of $1,800 to $2,500 per ton nationally (or about $9 billion to $12 billion per year). These differences occur mainly because the South Coast has already implemented far more stringent control sources than other parts of the country (29).
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Benefits. Are the benefits valued at more than $10 billion to $13 billion annually? The South Coast authorities have made two sets of estimates of the benefits of the plan. The first, which was based on a study sponsored by the California Air Resources Board in 1985 (30), covered various health benefits (mortality and morbidity from 26 APRIL 1991 particulates and acute morbidity from 03) as well as materials, agricultural, and visibility effects from meeting the air quality standards in the basin in 2010. It estimated annual health benefits of $2.4 billion (a point estimate) with an upper bound of $6.4 billion annually and total benefits of $3.7 billion to $7.7 billion annually. A more recent estimate, sponsored by the South Coast authorities in 1989 (31), only addressed the above health effects, finding a best conservative estimate of $9.4 billion annually and health benefits as high as $20 billion or as low as $5 billion per year. Perhaps it is surprising, given the association of Los Angeles with 03 problems, that $6 billion of the $9.4 billion in health benefits is for reduced mortality risks from meeting the particulate standards, whereas only $2.4 billion is for 03-related benefits.
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Because of the availability of newer studies on the health effects of air pollution, a number of serious methodologic problems with the second study (32), and the wide disparity in estimates issued by the South Coast, we have reexamined the benefits. On the basis of dose-response functions drawn from a recent epidemiologic study (33) and the South Coast's estimate that annual average ambient concentrations of sulfate (a proxy for acid aerosols) would be reduced by more than 7 ,ug/m3, we estimate that premature mortality in the region might be reduced by 2000 cases per year with full implementation of the South Coast plan. Using results from several studies designed to ascertain the values that individuals place on reduced risk of premature mortality, we assign a value of $1000 to each reduction of 0.001 in annual mortality risk (34,35).
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Combining the reduced mortality risk per individual, the expected population of the South Coast in the year 2010 (16 million), and the valuation of the risk reduction, we estimate possible mortality benefits of $2 billion annually associated with the pollution control plan.
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To estimate benefits in the South Coast in the form of reduced acute morbidity, we use a procedure similar to that described in the discussion of nationwide pollution control benefits. That is, we translate the predicted reductions in airborne concentrations of 03 and particulates into reduced illness and reduced frequency of respiratory symptoms by using epidemiologic and clinical studies. These are valued by means of willingness-to-pay estimates drawn from sources described above.
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On the basis of this approach, we estimate that reduced 03 concentrations will effect an annual reduction of 22 million person- days on which adverse respiratory or other symptoms will be experienced by South Coast residents. In dollar terms, these benefits amount to about $300 million annually. According to the South Coast officials, reduced ambient particulate concentrations will result in $700 million annually in reduced morbidity; reduced particulate loadings will provide $700 million annually in reduced materials damage, and another $130 million in materials damage will be saved as a result of reductions in ambient SO2. We take these at face value.
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In all, annual benefits to human health are predicted to be $3 billion ($2 billion in premature mortality, $0.3 billion in 03-related morbidity, and $0.7 billion for particulate-related morbidity). Ifone includes the South Coast's estimates of materials damage, total annual benefits rise to about $4 billion. This is far short of the $13 billion per year that the plan may cost.
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To this point, we have presented benefits and costs as point estimates, but there are clearly great uncertainties in making such estimates. It is essential to understand them and to bear them in mind in interpreting the findings above.
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With respect to our national comparison, OTA's estimate of control costs has a number oflimitations. For instance, OTA did not estimate the cost ofthe mandatory introduction of alternative motor vehicle fuels (methanol, ethanol, or reformulated gasoline) such as is called for in the new Clean Air Act amendments. This will add approximately $3 billion to annual costs. Also, OTA did not anticipate the second round ofvehicle emissions reductions that will almost surely be required under the amendments; this will add another $5 billion annually (36). Finally, no attempt was made to estimate nonpecuniary costs. For instance, OTA estimates that an enhanced motor vehicle inspection and maintenance program will cost about $50 per vehicle annually, including fees, administrative costs, and repair costs. The opportunity cost of people's time is ignored, however, even though the time spent can be significant. If this time were properly priced, it could add up to $7 per vehicle (37).
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There is also great uncertainty about the costs of the South Coast plan. Marginal costs generally begin to rise sharply at higher levels of control, and VOCs have been controlled longer and more stringent- ly in Southern California than in any other part of the country. For this reason, the controls envisioned in the South Coast plan could prove to be more expensive than anticipated. Also, we have little experience in the United States with stringent transportation control measures. If they are implemented and prove to be quite inconve- nient to those affected, costs could be higher than those projected here. It is impossible to provide anything approaching statistical confidence intervals for either the national or the South Coast plan.
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There are several respects in which costs could be much lower than those forecast here for both national and Los Angeles area air pollution control. For example, the cost of vehicle emissions con- trols are based on modest extensions of proven control technology (the catalytic converter). If, however, the electrically heated catalyst can be perfected and produced relatively inexpensively, control costs may be overstated here. Similarly, breakthroughs in reformulated gasoline or other alternative motor vehicle fuels could bring costs down considerably. Likewise, if the pace of technologic innovation accelerates sharply for VOC control from stationary sources, the same conclusion would apply. Finally (and particularly in Los Angeles), if driving restrictions eventually are imposed, and if commuters easily adapt to them, 03 control costs may be lower than projected here.
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Perhaps it is not surprising that uncertainties are greater concern- ing the benefit estimates presented here. These uncertainties arise from several sources, primarily the prediction of physical effects and the attribution of dollar values to them. For instance, ifwe had used the analysis ofWhittemore and Korn (38) instead ofthat ofHolguin et al. (17) to predict changes in asthma attacks, estimated benefits to asthmatics would be less than half those included above.
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The largest such uncertainty concerns the link between particulate matter at current ambient concentrations and premature mortality. The statistical associations that epidemiologists and others have found between city mortality rates and annual particulate levels do not offer convincing evidence of the existence and magnitude of such effects; for instance, these effects become insignificant with minor changes in sample composition and model specification, and even the best of these studies uses a poor proxy (sulfates) for the particles now thought to be the causal agents (acid aerosols) (39). Because the total number of deaths from lung disease in the South Coast is 4000 annually, attributing a reduction of 2000 premature deaths to the South Coast's plan seems likely to be optimistic.
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In monetizing the reduced frequency of respiratory symptoms or disease, a range of values could have been used. In the literature, the range cited for an asthma attack is $10 to $40; for a restricted- activity day, the corresponding range is $10 to $30; for a symptom day, it is $3 to $10.
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The choice among epidemiologic and clinical studies, and among values to assign to physical effects, can have an important effect on estimated benefits. If we had used only upper bound estimates to predict each type of acute health effect from 03 and, correspond- ingly, to attribute dollar values to reduced incidence of each, acute health benefits nationwide of a 35% VOC reduction would be $2 billion annually, and acute health benefits in the South Coast (of meeting only the 03 standard) would be $2.4 billion per year. Ifwe had used lower bound estimates, on the other hand, benefits would be 3% of our upper bound estimate.
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There is another important caveat to be attached to the benefit estimates presented above, one that can only impart a downward bias to them. Specifically, we excluded certain types of benefits for which it was impossible to predict physical effects or to make reasonable dollar attributions. For instance, some animal toxicologic studies suggest that prolonged exposure to 03 can permanently reduce the elasticity of the lung and, hence, initiate chronic respira- tory illness (40). Although there is no convincing epidemiologic evidence for this potential effect in humans to date, such a finding would affect any benefit-cost analysis of efforts to control ground- level 03 either nationally or locally (41). Similarly, we excluded in our estimates ofnational as well as South Coast benefits any improve- ments in forests or agricultural output in rural regions that might result from VOC control in urban areas because of the difficulty of translating emission reductions in urban areas into reduced ambient concentrations in agricultural regions. Also omitted are possible reductions in damage to rubber and other products exposed to 03. Nevertheless, including such agricultural benefits would be unlikely to add more than $1 billion to the national total predicted here (15); South Coast benefits would increase minimally. Finally, the totals omit a dollar attribution for the improved visibility that should result from reduced ambient sulfate concentrations.
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It is important to find ways to predict the physical likelihood of the exclusions identified here and to ascertain individuals' willing- ness to pay for any such improvements. These omitted categories would have to have large benefits associated with them, however, to tip the apparently unfavorable balance between benefits and costs for either the national or the regional air pollution control plans that we have examined.