This protocol and pilot study evaluated whether smartphone signal strength indicators can be converted into RF-EMF exposure estimates using derived formulas and regression models. The study reports a positive log-linear association between LTE RSSI and far-field (base station) exposure aggregated by location, while handset-related exposure at the ear and chest during data transmission showed negative log-linear trends with improving signal quality. The authors conclude the ETAIN 5G-Scientist app may support large-scale RF-EMF exposure assessment, but emphasize the need for more data to improve accuracy and address uncertainties in individual measurements.