Radioactive materials do not make you awesome. They make you die.
Of course, most of us already knew this. But how does radiation really impact the human body? The answer to this question lies in how radioactive materials work. Of course, not all radiation is going to kill you. The kind that we have to worry about is known as "ionizing radiation" (which is produced by nuclear power plants, stars, black holes, and a number of other sources). As the Environmental Protection Agency (EPA) notes, these ionizing materials are given this name because they have enough energy to break chemical bonds and strip away electrons from atoms.
To put it simply, radiation impacts how cells function, which can cause them to mutate in undesirable ways. The EPA explains precisely why radiation is so harmful to our cells:
Ordinarily, natural processes control the rate at which cells grow and replace themselves. They also control the body's processes for repairing or replacing damaged tissue. Damage occurring at the cellular or molecular level, can disrupt the control processes, permitting the uncontrolled growth of cells cancer This is why ionizing radiation's ability to break chemical bonds in atoms and molecules makes it such a potent carcinogen.
The level of impact will depend on both the length of the exposure and the level of radioactivity. One way of measuring this absorbed dose is in units of Grays (Gy). For comparison, an X-ray is typically around 0.0014 Gy. So what level of exposure will you need to shy away from?
If your geiger counter starts reading around 1-4 Gy, pick another direction and move quickly. At this level, your blood cells begin to die. In order to ensure that you will survive and not have any negative side-effects from the exposure, you will need blood transfusions. When you go beyond 2Gy, you start to look like you have sunburn, and your skin drys and dies. It will slowly peel off your body. Without treatment, anything between 4 and 8 Gy is fatal. Even with treatment, these levels of exposure usually lead to cancer. When you get to 10Gy, you are usually dead in a few days. See the infographic below for more on the length of exposure and how it impacts us.