Caution is warranted in drawing too many conclusions from these estimates and comparisons. Certain assumptions might have had the effect of exaggerating the economic benefits, and there are major uncertainties about the health impacts, particularly because of weaknesses in human exposure data. Moreover, the study omits some of the major effects of acid deposition on forests, lakes, and buildings, and the impact of tropospheric ozone on ecosystems is not valued. The figures presented should therefore be viewed as order-of-magnitude estimates. Even with all these qualifications, however, it appears that the economic impacts of air quality on human health are highly significant.

Air Quality Benefits and Supplemental Accounts

The estimates of the benefits of pollution control just discussed reflect the value of changes in the level of air pollutants resulting from proposed regulations. They are relevant for regulatory or cost-benefit purposes, but they are not the appropriate values for economic accounts. Production accounts should measure the damages associated with remaining levels of pollution, in terms of both production accounts and change in asset values. This difference between abatement and residual damage can be quantitatively large. For example, ozone concentrations fell only 6 percent between 1986 and 1995. As a result, regardless of the benefits of preventing higher levels of ozone than those of 1986, the value of changes in ozone concentrations over this period would be relatively small. In contrast, lead and PM-10 concentrations fell 78 and 22 percent, respectively, over the same period, and consequently the damages from these chemicals would be much smaller in 1995 than in 1986. In other words, whereas comprehensive consumption would have a substantial negative entry due to lead and PM-10 in 1986, the negative values would be of much smaller magnitude in 1995. The result might be a substantial increase in the estimate of growth of comprehensive consumption over this period.

As discussed earlier, air pollution affects production activities, assets, and nonmarket activities. Most of the estimates from the EPA study refer to the production accounts: days of work lost, shortness of breath and acute bronchitis, loss of visibility, and crop losses are effects on production activities. Crop losses and the output losses from lost work-day are already included implicitly in the accounts because these relate to market activities. Supplemental accounts that would identify these losses separately would serve to connect them specifically to air pollution. The estimates for shortness of breath and acute bronchitis include both damages that may already be reflected in the production accounts (i.e., re-



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