This part of ISO6242 defines how the air purity re-quirements of building users can be identified, ex-pressed and quantified. It describes users, objectives and the parameters used to express them. For each parameter, it specifies units of measurement, preferred increments for values and means of evaluation. It also lists the environmental and human factors affecting the choice of a value (criterion) for each parameter. It is intended for use a) In preparing briefs for building projects; b) In formulating building laws and regulations; c) In drafting standards and other normative docu- ments; and d) More generally when specifying the required performance of buildings in terms of users,re- quirements. Some of the parameters given in this part of ISO 6242 apply only to certain types of building. Compliance with this part of ISO 6242 does not therefore depend on implementing the whole of its contents in every case.