A chronic exposure study was designed simulating conditions of modern tunneling construction in which a hyperbaric atmosphere was combined with shotscrete lining methods producing a silicate aerosol at high relative humidity. Monkeys were exposed to either a DQ12 quartz dust concentration of 10 mg/m3 or a hyperbaric atmosphere at 2500 hPa or a combination of both over 26 months. All quartz-exposed monkeys developed silicosis. In this part of the study the retained mass of quartz dust was determined in the lungs using an O2 plasma ashing technique. Quartz concentrations of 7.05 ± 2.78 mg/g and 11.14 ± 3.29 mg/g dry lung tissue was found in the quartz-exposed group and in the quartz-exposed group at hyperbaria, respectively. Although the quartz concentration was rather homogeneous in a given lung, differences in the quartz concentration between individual animals indicated a sustained variation in deposition in the lungs and/or clearance from the lungs.