Article 2.5.2.3.7. In zones and agglomerations where the ozone levels meet the long-term objectives, the necessary measures shall be taken to keep these levels below the long-term objectives, provided factors such as the cross-border nature of ozone pollution and meteorological conditions so permit, and the best possible air quality that is compatible with sustainable development and a high level of protection of human health and the environment shall be maintained by taking proportionate measures.