The aim of this study was to investigate the effectiveness of different shielding materials in protective clothing using dicentric frequency in human peripheral lymphocytes as a marker of radiation-induced damage. Blood samples from a healthy donor were exposed to 70 kV x-rays behind shielding materials lead (Pb), tin/antimony (Sn + Sb) and bismuth barrier/tin/tungsten (Bi + Sn + W) with the same nominal lead equivalent value of 0.35 mm lead. Irradiation was performed either in contact (exposure position A, containing secondary radiation) or at a distance of 19 cm behind the shielding materials (exposure position B, containing only the unaffected transmitted photons). Using shielding material Sn + Sb, a significantly higher dicentric yield was determined at exposure position A relative to position B, whereas no significant differences were found between the exposure positions using shielding materials Pb or Bi + Sn + W. For doses up to 434.4 mGy at exposure position A, the slopes of the linear dose-response curves for dicentrics obtained behind shielding materials Pb and Bi + Sn + W were not significantly different, whereas a significantly higher slope was determined behind Sn + Sb relative to Pb and Bi + Sn + W. Using moderately filtered 220 kV x-rays as a reference, maximum RBE values at low doses (RBE(M)) of 1.22 ± 0.10, 2.28 ± 0.19 and 1.03 ± 0.12 were estimated immediately behind shielding materials Pb, Sn + Sb and Bi + Sn + W, respectively. These findings indicate a significantly higher RBE(M) of 70 kV x-rays behind shielding material Sn + Sb with respect to Pb or Bi + Sn + W. Using previous dicentric data obtained for exposure of blood from the same donor to x-rays at energies lower than 70 kV, it can be assumed that the increased RBE(M) of the broad spectrum of 70 kV x-rays (mean energy of 44.1 keV) may be attributed predominately to secondary (mainly fluorescence) radiation generated in the shielding material Sn + Sb that is able to leave the shielding material. Even if it is uncertain whether the marked dependency of the RBE at low doses on photon energy for chromosome aberrations is also representative for late radiation effects in healthy subjects, it should be taken into account that several prospective cohort studies have shown positive associations between higher chromosome aberrations in lymphocytes of healthy subjects and increased cancer incidence. Thus, it can be concluded that any additional biological damage by radiation exposure of healthy subjects, e.g. by using certain non-lead based shielding materials of protective clothing, should be avoided.