Secondary cosmic ray particles are constantly present at the surface of the earth. On average, at sea level one muon particle hits per square centimeter per minute along the vertical direction. These particles impart a yearly dose of 26 millirems to a human. Most of the sea-level cosmic ray particles are muons in the energy range of 100 MeV up to several hundred GeV or more. Some of these muons at the lower end of the energy range stop and decay inside the bio-system. As they carry a considerable mass energy, they generate a large number of secondary electrons in biological tissues leading to trails of ionizations. They contribute to the fragmentation of small molecules and to single and double strand breaks in DNA. A Gant4-based simulation model has been employed to study sea-level muon interactions in the bio-system. In this study, a human body phantom that includes body materials such as tissue, muscle, bone and organs was irradiated with simulated cosmic ray particle fluxes. Concrete shieldings in different thicknesses were then placed above the phantom for studying radiation doses imparted by cosmic ray muons. The preliminary result for the annual dose from this simulation in the body was about 22 millirems, which is consistent with the average annual radiation dose from cosmic rays. This study found that, contrary to commonly available literature, the dose received by the human body when it was shielded by a thin (1-2 meters) concrete slab was higher than the dose received by an unshielded human on the street. The highest dose simulated was 33.7 millirems when a 1.5 meter concrete slab was present. When the thickness of the concrete slab gradually increased, the dose received by the phantom started decreasing after 2 meters and the muons were blocked by an 8-meter concrete slab. Detailed simulation results including dose distributions and track structures are presented.