To support the development of effective and state of the art policies on air quality, a quick update of the WHO air quality guidelines is necessary. The current version stems from 2005 and there is sufficient evidence that a tightening of the values would be justified. It would be recommendableto invest in a thorough study of the actual effects of green in cities and how it can be used in the most efficient way. A first step would be to inform, regional and national authorities, asthere appears to be a big difference in actual knowledge from the government side. It is important to find communication strategies that will reach as many stakeholders as possible. In order to guarantee reliable and harmonized information, there is an important role for international organisations such as the EU, UNECE, WHO and UNEP.