A novel method for assessing postmortem interval using radon radioisotopic decay – an internal radon ‘time of death clock’

Reliably assessing the time-of-death of a corpse, known as a postmortem interval (PMI), has challenged forensic medicine for centuries. Current methods for estimating PMI are limited by an inherent lack of precision and accuracy, as they are influenced by factors including but not limited to humidity, temperature, disease, body position, and decomposition. Calculating PMI in a way that is less confounded by biological and environmental circumstances is important, as PMI is used to establish timelines necessary for the evaluation of alibis or potential suspects in criminal cases, as well as assessment of legal rights in civil cases. Here, we propose a theoretical framework for predicting PMI using the relative isotope abundance of radioactive radon decay products in dead biological tissue. Radon inhalation is a functionally universal event and, in the 222Rn decay chain, radioisotopes of 210Pb, 210Bi, 210Po are of useful due to their longer but varied half-lives. The theoretical approach that we propose leverages measurement of specific isotope abundance ratios and solving decay chain equations to determine PMI. In this approach, a unique solution to the differential equation curve is obtained, while reducing dependence on having to know parameters such as the long-term radon exposure of the individual. Measurements taken at two different post-mortem time points captures the derivative of the curve, which is predicted to enhance accuracy and minimize error. We use Monte Carlo simulations to evaluate the proposed model. If validated empirically, we speculate that this PMI estimation approach may prove beneficial to forensic science.