deduction of liabilities), calculated by discounting current and future streams of income by the appropriate interest rate.

Net price:

Valuation used in natural-resource economics to estimate the economic value of a natural resource and its depletion. It is defined as the actual market price of a natural resource output minus all marginal exploitation costs, including a normal return to capital.


See National Income and Product Accounts.


Economic activity that produces goods and services not distributed by markets.

Nonrenewable natural resources:

Exhaustible natural resources, such as mineral resources, that cannot be regenerated after exploitation.


See net present value.



Organization for Economic Cooperation and Development.

Opportunity cost:

Value of the next best use (or opportunity) for an economic good, or value of the sacrificed alternative.

Ozone depletion:

Destruction of ozone in the stratosphere, where it shields the earth from harmful ultraviolet radiation. Its destruction is caused by chemical reactions in which oxides of hydrogen, nitrogen, chlorine, and bromine act as catalysts.


Particulate loadings:

Mass of particles per unit volume of air or water.

Particulate removal:

Removal of particulate air pollutants from their gaseous media using gravitational, centrifugal, electrostatic and magnetic forces, thermal diffusion, or other techniques.


Fine liquid or solid particles, such as dust, smoke, mist, fumes, or smog, found in air or emissions.

Photochemical air pollution:

Pollution caused by the reaction of unsaturated and saturated hydrocarbons, aromatics, and aldehydes (emitted owing to the incomplete combustion of fuels) with light. It causes eye irritation.

Physical accounting:

Natural-resource and environmental accounting of stocks and changes in stocks in physical (nonmonetary) units, for example, weight, area, or number. Qualitative measures, expressed in terms of quality classes, types of uses, or ecosystem characteristics, may supplement quantitative measures. The combined changes in asset quality and quantity are called "volume changes."

Pigouvian tax:

Tax levied on an agent causing an environmental externality (environmental damage) as an incentive to avert or mitigate such damage.


Substance present in concentrations that may harm organisms (humans, plants, and animals) or exceed an environmental quality standard. The term is frequently used synonymously with contaminant .

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