In radiation exposure situations, knowledge of the equivalent or effective dose is required to determine whether the individuals are working within safe limits. However, in most situations, doses cannot be measured directly, and tissue or organ absorbed doses and effective doses are determined computationally to help assess stochastic risks. This study incorporates voxel-based whole body models of ICRP (adult reference male and female phantoms) into the Monte Carlo code MCNP 6.1 to simulate radiation interactions inside the human body due to the photons emitted from 1179 different radioisotopes. Absorbed doses in tissues or organs were computed from simulations where a radioactive point source was placed 1 m away at 1 m above the ground for eight different source orientations with respect to each phantom. The results were subsequently multiplied by the radiation weighting factors (WR = 1 for photons) and the tissue weighting factors in accordance with the methodology of ICRP 103 to obtain organ equivalent doses and effective doses. The calculated effective dose for each radioisotope and source orientation was later divided by unit source activity and unit exposure time to convert to effective dose rate conversion coefficient (EDCC). This novel quantity is proposed in this study for more realistic dose assessments. The results show that the anterior-posterior sources led to the highest EDCC values for all the radioisotopes. In addition, the right posterior orientations yield the lowest EDCCs for most of the radioisotopes. Under all source orientations, EDCC values were highest for Cf-254 isotope and lowest for Sm-151. EDCC to traditional air kerma rate constant (AKRC) ratios were also calculated to compare these two quantities. For all source orientations and most radioisotopes, this ratio was less than one and its largest value was observed for the anterior-posterior source. The results obtained in this study are based on effective doses from photon emitting radioisotopes developed for whole body models under various source orientations, thus should more accurately represent the stochastic risk to individuals in an external exposure situations.