Background: This study introduces Shield Reduction Factor (SRF) for breast dose decline using two new Bismuth-polymer composites shields in chest CT. Method and Materials: SRF of Bismuth -Silicon (Bi-Si) and -Polyurethane (BiPU) were measured after chest CT by a multi-detector CT (MDCT, 16 detectors) and a single-detector CT (SDCT) for breast at spiral and axial modes using female chest phantom. Result: Higher effectiveness was observed in the use of the Bi-Si shield with an SRF of 1.25 and 1.66, and in the use of the Bi-PU shield with an SRF of 2.31 and 2.06 in SDCT and MDCT (100 mAs), axial mode, respectively. Experiments by 80 mAs at spiral/axial mode induced lower SRF by Bi-Si and Bi-PU shields. Conclusion: This study showed that Bi-Si and Bi-PU shields are new composites, light and economical, carrying promising potentials for breast dos reduction during chest CT in spiral and axial modes at 80 and 100 mAs, single and 16 MDCT.