This part of ISO 10121 establishes a classification system for GPACDs supplying single pass outdoor air to general ventilation systems using outdoor air polluted by local urban sources and/or long-distance pollution. The classification system is intended to aid in assessing molecular pollution in addition to the particulate pollution dealt with by ISO 16890-1. The selected normative pollutants are ozone, sulphur dioxide, nitrogen dioxide and toluene due to their relevance to the intended application. Three duty levels are assigned for each pollutant reflecting the typical performance range of devices intended for the application. The duty levels are related to the performance needed, but it is not in the scope of this classification to relate a specific duty level need to a certain place or city size. The air flow type to be covered is outdoor air to buildings i.e. a one pass filtration from a vast outdoor recipient of known fairly stable concentrations even if the concentration may have daily and yearly variations and/or local increased concentrations due to main traffic routes. The testing is performed according to clause 5, clause 6 and ISO 10121-2. The classification is expressed as an integrated removal efficiency for the highest duty level that the device may be awarded. A separate class is given for each of the four normative pollutants. Since selection of normative pollutants and duty levels are specific and unique to the intended application, all other applications are excluded. In particular, GPACDs in recirculation applications and/or dealing with pollution from indoor sources as well as pharmaceutical, microelectronic, nuclear, homeland security and military applications are specifically excluded.