One of the most important difficulties is that the physical measurements used are often inaccurate indicators of actual human exposures. Average emissions of the precursor pollutant, average concentrations over the year, or concentration data for limited sites are generally not representative of concentrations to which the population is exposed and may be a misleading basis for developing damage estimates. For example, tropospheric ozone forms mainly in warm weather. Thus total annual hydrocarbon emissions, the precursor to tropospheric ozone, are a poor indicator of potential levels of tropospheric ozone. Tropospheric ozone levels also very significantly over the distance of a few city blocks. One of the major challenges both for better environmental policy and for the construction of environmental accounts is to obtain better measures of direct human exposure to the important harmful substances among a representative sample of people.

Accumulation of Stocks

Many environmental problems result from the accumulation of residuals. These substances include most radiatively active trace gases, which remain in the atmosphere for decades or centuries, and many radioactive materials, which have half-lives of decades or centuries. Similarly, recovery from stratospheric ozone depletion is a process requiring years or decades. And agricultural chemicals often migrate very slowly through soils, contaminating drinking water only after several years or decades.

Environmental accounting therefore needs to develop and include appropriate methods to account for those persistent pollutants, such as heavy metals that accumulate in the environment and last for many years. Each year's emissions or production of residuals adds to the stock in the environment, and it is necessary to understand the processes by which these stocks decay or dissipate. In some cases (as with radioactive substances), those processes are easily understood, while in other cases (such as subsoil toxins or the carbon cycle), understanding the processes poses enormous scientific challenges. In the economic accounts, the stock-flow dynamics are similar to those of gross investment and depreciation of capital. While there is a conceptual similarity, however, there is no readily observable market price for these stock changes. Hence, valuation of a change in stock requires estimating the value of the impact of additions over the lifetime of the stock, accounting for dissipation, and appropriately discounting future effects. It should also be recognized that, with a few exceptions, the stocks are extremely heterogeneous, so that measuring a simple "environmental capital stock" is likely to be extremely difficult.

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