The atmosphere of mountain areas may be contaminated by pollutants originating mainly from road traffic, as well as tourist and community activities within such areas. This study mainly aimed to assess the concentrations of airborne potentially toxic elements (PTEs) in two mountain areas in Thailand using lichen biomonitoring. Thalli of the lichen Parmotrema tinctorum from the relatively unpolluted area in Khao Yai National Park (KYNP) were prepared and exposed at nine sites in the KYNP and nine sites in Doi Inthanon National Park (DINP) during the rainy and dry seasons. The lichen transplants were collected and analyzed for 15 PTEs, including Al, As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Ti, V, and Zn, using inductively coupled plasma-mass spectrometry. The result clearly showed that the atmosphere of many monitoring sites in both mountains were contaminated by the investigated PTEs. The contamination factors (CFs) revealed that several PTEs heavily contaminated the atmosphere at many monitoring sites. The pollution load indices (PLIs) clearly illustrated that the atmosphere of all sites had higher pollution loads in the dry season than in the rainy season, which was likely due to the higher numbers of motor vehicles and visitors. The highest pollution loads were observed at sites that had higher traffic density and human activities, including the park entrance site in the KYNP and the community site in the DINP. The lowest air pollution loads were discovered at the summit sites in both mountains. This study indicates that the atmosphere of mountain areas can be contaminated by some PTEs that are mainly produced by road traffic and local communities. It also confirms the ability of the transplanted lichen P. tinctorum to be an effective biomonitoring tool for airborne PTEs in natural environments. Keywords: air pollution; biomonitor; Doi Inthanon National Park; Khao Yai National Park; road traffic; tourism.