Atmospheric particle pollution is a serious environmental issue threating human health, especially in the megacities of China. As a key aerosol component with high risk implications, PM_2.5 associated trace metals pose significant risks to human health through inhalation. The bioaccessible fraction absorbed by human lung is a prerequisite for reliable assessment of the potential hazards of toxic trace metals in PM_2.5 via inhalation exposure. To accurately assess the inhalation risks of airborne metals, four in vitro physiologically based extraction tests (PBET) with synthetic agents comparable to human lung fluids were conducted to assess pulmonary bioaccessibility of trace metals in PM_2.5 from different function areas (urban, industrial, suburban, rural) of Nanjing city, China. Based on the comparisons between bioaccessibility and human health risks of airborne trace metals from varied sources using different extraction methods, the primary inhalable toxic trace metals and suitable extraction method were expected. The standardized and systematic methods for in vitro bioaccessibility tests of inhalable metals are needed. and for future risk assessments of atmospheric particulate metals, incorporating the bioaccessibility supported by suitable speciation methods are directions.