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Although the use of moss as biomonitor of air pollution is relatively simple, the interpretation of the data needs reference values. Background values for Cd, Cu, Pb, and Zn accumulated in moss samples from Switzerland, collected every five years from 1995 to 2015 in the framework of the European Moss Survey, were statistically estimated. These background values can be used as reference for the assessment of spatial and temporal trends, to be expressed in terms of bioaccumulation ratios with actual values. The use of annual background values is of great importance to identify spatial trends, while period-wide background values identify temporal trends. The latter are consistent with those reported in other comprehensive similar biomonitoring studies in Europe and are required to be updated in time, possibly every five years. The use of cutoff values to be used as benchmark for bioaccumulation ratios is invaluable in having a scale for assessing ecological quality.