Nowadays, Far-field Wireless Power Transfer (FWPT) has attracted many research efforts to conveniently power the Internet of Things (IoT) devices. Electromagnetic Radiation (EMR) safety in FWPT has brought much attention from the public. Existing works on FWPT mainly focus on improving the remote charging efficiency but overlooking the effects of EMR. A few works consider EMR safety but do not present accurate EMR quantization analysis because there lacks an accurate EMR computing model in IoT wireless charging scenario. In this paper, in order to evaluate and avoid the EMR's harmful impact, we first propose an accurate theoretical calculation equation for EMR and the concept of Charging Restricted Area (CRA). In the wireless charging area on a 2-dimensional plane, according to the EMR computing model, we further maximize the overall charging power by adjusting the power of chargers and ensure that the EMR in this area is lower than the EMR safety threshold. The wireless charging EMR safety problem is formulated as a linear programming problem with infinite constraints. To re-express the wireless charging EMR safety problem as a typical linear programming problem with finite constraints, the Sampling Safety Charging (SSC) algorithm is proposed. We have conducted extensive experiments to validate our proposed algorithm; the simulation results show that the performance achieved by our algorithm outperforms that achieved by the distributed RObustlySafE (ROSE) algorithm.