a thousand, and MM is used to denote a million cubic feet. One MCF of natural gas contains about a million Btu of thermal energy.
Activity-specific impacts result from particular energy use. For example, impacts from the emissions from an electric power plant or impacts from tailpipe emissions from a passenger car.
Activity-aggregate impacts are used to describe the impacts from energy use in a set of activities that include all impacts starting with the processing of primary energy, its conversions and its transportation to its end use point, its use to provide a set of energy services, and impacts associated with disposal of end use equipment. The aggregations are based on life-cycle assessment (LCA) methods and use a variety of data and models to estimate the impact. For example, electricity use to provide light in a building would include all the “upstream inputs” to produce feed energy for the power plant (mining, dams, etc.), the electricity production inputs to generate and distribute power to the site of the light bulb, and impacts associated with operation of the light bulb. Waste heat from the bulb and its disposal would be “downstream impacts.” Larger downstream impacts would be associated with the health and other consequences from emissions at the power plant.
In this report, life-cycle impact assessment (LCIA) is a goal that can only be achieved incompletely due to limitations in data availability and complexity of the detailed systems, but where important impacts are present their magnitudes are estimated to the extent possible.
Solid and liquid wastes are usually described using familiar units of volume or weight per unit time or quantity of energy produced. (cubic feet per minute [cfm]; tons per MWh; gallons per day; etc.). Where these waste streams contain contaminants, the concentration of the contaminant of concern is also important. (parts per million [ppm] by weight is the weight of contaminant in a million units of carrier weight; or pounds of contaminant per ton of carrier, or pounds of contaminant per gallon of liquid,)
Air emissions are usually described by emissions per unit of energy produced or used—such as lb per MWh of electricity, lb per MCF of natural gas, or grams per vehicle miles traveled (VMT) and sometimes in terms of concentration of pollutants in emissions stream—such as parts per million (by volume) or pounds per cubic foot. The choice of a VMT basis is a compromise, since the more meaningful metric of passenger miles