The magnitude of cancer risk in humans exposed to low radiation doses (<100mSv) is uncertain and depends upon extrapolation of human population based estimates obtained at higher doses. The linear non-threshold extrapolation model currently used has some support from mechanistic/biophysical considerations but it is not based on understanding of the biological responses of the cells of origin for radiation cancer. Stem cells are widely recognised as the cells of origin for many cancers. Understanding the responses of stem cells to low radiation doses will be important therefore in providing mechanistic support for low dose cancer risk extrapolation. In this project, techniques emerging from stem cell biological and tissue kinetics research will be used to address several major areas of uncertainty in low dose (<100 mGy) cancer risk estimates:1. The identification and enumeration of stem/progenitor cells at risk2. Understanding low dose radio-sensitivity of stem cells and tissues.3. Improving understanding of mechanisms of age-dependant cancer risk4. Improving understanding of tissue specific differences in cancer risk.5. Identification of key events and individual susceptibility factors associated with cancer development.The primary output of the RISK-IR project will be peer reviewed scientific papers. Considerable efforts will be made to consolidate scientific evidence of relevance for radiation protection through reviews and dissemination of project results and implications to radiation protection specialists will be facilitated through a stakeholder meeting in the final year of the project.