The report assesses the levels and effects of exposure to ionizing radiation. The scientific annexes provide the supporting scientific rationale for UNSCEAR’s 2019 report. Annex A focuses on evaluation of selected health effects and inference of risk due to radiation exposure described in five scenarios for risk evaluation: 1) leukemia after medical computed tomography scans during childhood; 2) leukemia after occupational radiation exposure; 3) solid cancer after occupational exposure; 4) thyroid cancer after exposure during childhood or adolescence; and 5) circulatory diseases after acute radiation exposure. Quantitative risk estimates were presented based on major recent epidemiological studies and on the effects found among survivors of atomic bombings in Japan. Annex B focuses on the evaluation of lung cancer from exposure to radon provides information to assess recent developments in risk estimates of lung cancer and radon exposure, and to convey an up-to-date picture of radon dosimetry and of dose conversion factors applied for dose assessment of populations.