The Welsh Ministers must classify each zone according to whether or not the upper or lower assessment thresholds specified in Section A of Annex II to Directive 2008/50/EC are exceeded in relation to sulphur dioxide, nitrogen dioxide and oxides of nitrogen, PM₁₀, PM_2·5, lead, benzene and carbon monoxide.

The Welsh Ministers must review the classification of zones in paragraph (1) at least every five years, and must do so more frequently than every five years if there are significant changes in the activities which may affect levels of the pollutants referred to in paragraph (1) in ambient air.

When reviewing the classification of zones in accordance with assessment thresholds, the Welsh Ministers must comply with Section B of Annex II to Directive 2008/50/EC.