How does radiation affect our organism at an atomic level?

I assume you mean ionizing radiation, like gamma rays. At an atomic level, it can knock out an electron from an atom and ionize the atom. At the molecular level, this can cause molecules to break up or atoms to combine into other molecules they otherwise wouldn't. This is particularly bad for DNA. read more

Since brain cells do not reproduce, they won't be damaged directly unless the exposure is 5,000 rems or greater. Like the heart, radiation kills nerve cells and small blood vessels, and can cause seizures and immediate death. (3) Thyroid. The certain body parts are more specifically affected by exposure to different types of radiation sources. read more

Radiation levels and their effects The following table gives an indication of the likely effects of a range of whole-body radiation doses and dose rates to individuals: 10,000 mSv (10 sieverts) as a short-term and whole-body dose would cause immediate illness, such as nausea and decreased white blood cell count, and subsequent death within a few weeks. read more