Abstract Internal dosimetry measurements of trans-uranic (TRU) radionuclides within the human body can be accomplished by γ -ray spectroscopy using high-purity germanium detectors. While measurements are typically conducted inside shielded chambers in order to minimize the background detector signal, the presence of radioactive 40K within the body cannot be removed, which produces a high-energy γ ray at 1461 keV. The 40K, in addition to airborne 222Rn, contributes to the radiation field within the chamber that increases to the minimum detectable activity that can be achieved in practice. The objective of the present work is to estimate the contributions from 40K internal to the human body and airborne 222Rn to the background signal in a lung counter germanium detector array using GEANT4 simulations, with a focus on TRU isotope energy region (17–59 keV). The investigations consisted of three main simulations: 1. Simulating the γ -ray flux at the geometric center of the CEMRC counting chamber arising from a 25 nCi source of 40K from (a) a point source, and (b) a source distributed throughout a BOMAB phantom. 2. Simulating the response of the CEMRC BEGe detectors arising from a 25 nCi source of 40K from (a) a point source, and (b) a source distributed throughout a BOMAB phantom. 3. Simulating the response of the CEMRC BEGe detectors to the environmental background in the counting chamber, consisting of airborne 222Rn (0–4 pCi/L) and 40K (0–160 nCi) internal to the human body. The results are used to investigate the 40K background contribution to a routine spectrum recorded by the lung detectors at the CEMRC Internal Dosimetry laboratory over a typical half hour counting interval used in the dosimetry of radiological workers. The minimum detectable activity was calculated for various 222Rn and 40K activities and compared to experimentally determined values. The contribution of cosmic rays was not included in this investigation. Highlights • Simulated response of BEGe detectors to a point source of 40K. • Simulated response of BEGe detectors to a distributed BOMAB source of 40K. • Simulated response of BEGe detectors to airborne 222Rn. • Comparison of simulated and measured MDAs for 238Pu and 241Am using BEGe detectors. [ABSTRACT FROM AUTHOR]