Human fingernails have been studied for many years for potential use for dosimetry, based on the EPR signals induced by ionizing radiation, but a fully validated protocol to measure doses retrospectively has not yet been developed. The major problem is that the EPR spectrum of irradiated fingernails is complex and its radiation-induced signals (RIS) overlap with an endogenous signal called the background signal (BKS). RIS and BKS have similar spectral parameters. Therefore, detailed characterization of the BKS is required to develop a method for measuring the amount of RIS by removing the signal due to BKS from the total spectrum of irradiated fingernails. Effects of reducing and oxidizing treatments of fingernail samples on the BKS were studied. Numerical simulations of the observed BKSs were performed. Common features of the EPR spectra in fingernails are discussed. We also found that BKS can be generated in the fingernail clippings by oxidation in ambient air with dioxygen. Results support the hypothesis that BKS is an o-semiquinone radical anion. Comparison of the chemical and spectral properties of the BKS and with the RIS 5 (the stable signal suitable for dose assessment) suggest that both sets of radicals underlying these signals are o-semiquinone radicals. Given the common chemical properties of the BKS and RIS 5 it is unlikely that chemical treatment methods will provide a way to differentiate these two signals in irradiated nail spectra. Instead, other methods (i.e. dose additive methods, population-derived BKS means) may be necessary to selectively estimate the content of BKS and RIS 5 in irradiated nail spectra.