The Green Key has three gradations: bronze, silver and gold. Each gradation requires an organisation to comply with a minimum of nineteen mandatory standards. In addition to these nineteen mandatory standards there are still another twenty optional standards.
The measure in which these twenty optional standards are complied with, determines what gradation of the Environmental Barometer an organisation holds. Since 2004 Burgers' Zoo has been the proud owner of the golden gradation of The Green Key, because Burgers' Zoo also complies with no less than sixteen optional standards.
For more information about The Green Key you can visit The Green Key