No, ionizing radiation is only harmful to an organism as a whole when its amount gets too high. We are constantly bombarded with very small amounts of ionizing radiation that occur naturally, and we get along just fine with our lives without being seriously harmed by this radiation. There are trace amounts of naturally-occurring radioactive atoms in the air, in the rocks, in our food, and inside our bodies. When these atoms radioactively decay, they emit ionizing radiation. By its nature of being ionizing, such radiation can damage individual molecules, even at low intensity. But if the amount of ionizing radiation exposure is very low, our bodies can handle a few damaged molecules without any problem, so that there is no net harm done to our bodies. Ionizing radiation is radiation that has enough energy per particle to rip electrons off of atoms and therefore break chemical bonds. In contrast, radiation types such as microwaves, radio waves, and visible light are non-ionizing, meaning that they do not have enough energy to permanently damage molecules beyond simple heating effects. Ionizing radiation includes X-rays, gamma rays, neutron radiation, proton radiation, and high-speed electrons. Natural sources of ionizing radiation include the radioactive decay of unstable atoms that exist everywhere and cosmic rays from space. Man-made sources include medical scans such as X-ray images as well as nuclear power plants, nuclear weapons testing, and any industrial or scientific process that involves nuclear reactions or high energies. Advocating that humans cease all nuclear activity in order that our exposure to ionizing radiation will be reduced to zero makes no sense since we will always be exposed to some amount of ionizing radiation from natural sources. The more logical approach is to allow nuclear research and technology to proceed, but put strong regulations and safety procedures in place so that humans are never exposed to ionizing radiation amounts that are above the safety threshold. The amount of total harm that ionizing radiation can cause a human depends on the total amount of radiation received, which is a function of the intensity of the radiation and the length of time that the person is exposed to the radiation. The total amount of ionizing radiation received by a body is termed the "dose". Since different tissues react differently to ionizing radiation, of more importance is the "effective dose", which is the total amount of ionizing radiation received that is able to do biological damage. A person that is exposed to higher-than-normal levels of radiation, but only for limited amounts of time, will not receive a significantly higher effective dose and thus may still be in the safe zone. For instance, employees can safely work in nuclear reactor facilities as long as they monitor their radiation exposure and limit their time in the facilities so that their dose does not exceed safe levels. It is hard to set one standard threshold above which radiation exposure becomes seriously harmful since the definition of "seriously harmful" is subjective. Medium-low doses of ionizing radiation can still cause nausea and may still cause a miniscule increase in the chance of getting cancer, although this increase may be too small to be considered significant. Despite the complexity of this field, general safety thresholds can still be set. Experimentally, cancer risk has only been found to increase for doses above 100 mSv per year according to the World Nuclear Association. A good safety threshold should therefore be set at a value that is well below 100 mSv per year. The U.S. Nuclear Regulatory Commission sets the occupational safety limit for ionizing radiation exposure to be 50 mSv per year. For comparison, natural background radiation provides a dose of 3 mSv per year, a single typical banana provides 0.0001 mSv, a single set of dental X-rays provides about 0.005 mSv, a single set of barium X-rays provides about 5 mSv, and a single full-body CT scan provides about 20 mSv. As you can see, a single medical scan is too weak to cause harm even though it may involve ionizing radiation. On the other hand, undergoing several full-body CT scans in a short period of time can make the radiation add up to a total dose that is at a harmful level. For this reason, medical doctors are trained to avoid ordering too many radiation-intensive scans for a single patient in a short amount of time (unless the benefits outweigh the risks, e.g. if the scan helps save a patient from imminent death, it is worth the increased cancer risk). When the dose is high enough, ionizing radiation causes two types of harm to humans: direct tissue damage and cancer. Direct tissue damage happens when enough molecules are broken apart that the cells simply can no longer function. This can lead to radiation burns, radiation sickness, organ failure, and even death. In contrast, cancer results when the cells receive a small enough amount of damage that they can still live and function, but damage to the genes causes the cells to pursue aggressive and uncontrolled growth.