Total mercury air pollution was evaluated in the former mining area of Gelnica (Slovakia) using tree bark, mosses (Climacium sp., Pleurosium sp.), and lichen (Pseudevernia sp.). Samples were collected (tree bark) and exposed (moss and lichen bags) on the heaps and near the mines. Additionally, the internal parts of the mines were evaluated. The mercury content in the bio-monitors was evaluated using an AMA-254 analyzer. The results showed significant differences in tree bark mercury content between the mines and heaps. The Hg content in mosses and lichens was not influenced by the type of mining work. The lichen Pseudevernia sp. was found to be the best Hg accumulator compared with mosses. Keywords: tree bark; moss and lichen bag technique; environmental monitoring; open mining pits; heaps of waste material.