EmissionFactor

An emission factor is a coefficient that relates the amount of a pollutant or greenhouse gas released to an activity associated with a defined source. It is used to estimate emissions without direct measurement, by multiplying the activity data by the emission factor. Emissions E = A × EF, where E is the emissions, A is the activity, and EF is the emission factor.

Emission factors are expressed in units such as kilograms or grams of pollutant per unit of activity (for example, kg CO2 per liter of fuel, g NOx per cubic meter of exhaust, or kg CO2 per kWh of electricity). They can apply to direct emissions from combustion or process operations and may be specific to technology, fuel type, region, and operating conditions.

Development and sources: EF values are derived from measurements, engineering calculations, or statistical modeling. International guidelines and databases, such as the IPCC Guidelines for National Greenhouse Gas Inventories, EPA AP-42, or the European EMEP/EEA Air Emissions Database (EFDB), provide widely used factors. Many projects also rely on country- or sector-specific factors.

Applications and limitations: Emission factors enable rapid screening and inventory work, including national inventories, corporate footprints, and life cycle assessment. However, factors vary with technology, fuel quality, age, maintenance, and load, introducing uncertainties. They should be used with region- and activity-specific data when possible, and updated as technologies evolve.

Related concepts: Emission factors are paired with activity data and are foundational to environmental accounting, inventory development, and life cycle assessment workflows.

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