As with any purchase, buyers need to choose their offsets carefully, particularly as the voluntary offset market is largely unregulated. There are many issues to consider such as (a) the offset project type, (b) the validation and verification of the project by reputable third-parties, and (c) systems in place to control 'leakage', where the creation of a GHG reduction in one region causes an unintended increase in GHG emissions somewhere else (for example, protecting a forest in one location could simply shift logging to a forested area in a new location). But perhaps the most important issue to consider is additionality.

An offset project is considered additional if it isn't business as usual. Typically this means that the project wouldn't have happened without the extra funding from the sale of offsets. Additionality is extremely important, as the entire concept of offsetting - i.e. purchasing greenhouse gas reduction credits from a project elsewhere to neutralize one's own emissions - is based on the premise that those reductions wouldn't have happened otherwise. Only by buying offsets that have met additionality criteria can you be assured that your purchase is resulting in a net benefit for the climate.