The protection against eco- and human-toxicological impairments caused by atmospheric deposition of heavy metals requires legally defined assessment values. Since such values are missing for Luxembourg, the aim of this investigation was to evaluate different approaches to derive assessment values for the regulation of heavy metals that are in accordance with scientific and legal standards. To this end, assessment values for heavy metals were derived from the compilation of respective values implemented in European countries. In addition, (1) precipitation-related assessment values for the protection of soil for Cr, Zn, and Cu and (2) precautionary assessment values (critical loads for Cr, Zn, and Cu, as well as As, Cd, Ni, and Pb) for the protection of human health and ecosystems were calculated. The calculation of the regionally differentiated precipitation-related assessment values resulted in ranges of 17–272 g Cu ha−1 a−1, 167–2672 g Zn ha−1 a−1 and 17–272 g Crtotal ha−1 a−1. The critical loads for drinking water protection vary in the ranges from 1.23 to 2.14 g Cd ha−1 a−1, from 4.05 to 8.63 g Pb ha−1 a−1, from 2.6 to 5.9 g As ha−1 a−1, from 258 to 564 g Cu ha−1 a−1, from 1292 to 2944 g Zn ha−1 a−1, and from 12.9 to 29.9 g Crtotal ha−1 a−1. Ecosystems are significantly more sensitive to Pb, Cu, and Zn inputs than humans. For As and Cr, humans react much more sensitively than ecosystems. For Cd, the critical loads for drinking water, ecosystems, and wheat products are about the same.