Emission factor

An emission factor is the amount of greenhouse gas emitted per unit of an activity, such as per kilogram of cotton or per kilowatt-hour of electricity.

A footprint multiplies each activity (how much material, energy or transport a product uses) by its emission factor, then adds the results. Factors come from published sources: government databases, industry studies and life cycle inventory databases.

The choice of factor matters. A good footprint names the source, year and geography of each one, so a reader can check it.