In general, the 2-σ (or 95% confidence level) uncertainty in the relative reactivities in most of ubiquitous VOCs (that have been studied extensively) is about 20-40%. The relative reactivity of a composite set of VOCs arising from a single source, such as motor vehicles, tends to be somewhat smaller (i.e., about 15-30%). Much of the uncertainty in this later case arises from potential errors in defining the speciation of the emissions as opposed to those associated with the chemistry of the species. For this reason, the use of relative reactivity to assess the ozone-forming potential of different sources is best suited to situations where the reactivity of the emissions is quite different. As will become apparent in later chapters, this tends to not be the case for emissions from motor vehicles using slightly different RFGs. That will, in turn, limit the ability to use reactivity to distinguish robustly between the air-quality benefits of various RFG blends.