Article 6(4) of the Habitats Directive (which also applies to the Birds Directive) sets out an exception to the rule that a plan or project may not proceed where adverse effects on site integrity cannot be ruled out. The activity causing adverse effects may go ahead in the absence of alternative solutions for imperative reasons of overriding public interest, including those of a social or economic nature. The Member State must however take compensatory measures to ensure that the overall coherence of the Natura 2000 network is protected. In the case of priority habitat types or species (these are indicated in the Annexes to the Directives), plans or projects may only be permitted in these circumstances for reasons relating to human health or public safety, where there are beneficial consequences of primary importance for the environment, or if the Commission provides an opinion that other imperative reasons of overriding interest exist. In other cases not involving priority habitats or species, other imperative reasons such as social or economic factors may also be open to authorities.