|Fig. 1: Cell undergoing radiation therapy. (Source: "Wikimedia Commons)|
Radiation from nuclear energy has been affecting humans for a long time, starting with the first scientists who discovered it in 1895 to the horror of Fukushima when it goes awry.  However, radiation has also been known to be used for healing purposes, like with chemotherapy. Regardless of whether the effects are positive or negative, it is incredibly important for everyone to understand how nuclear energy can affect you.
There are two main ways in which nuclear radiation causes harm; apoptosis and blockage of mitosis. The first, apoptosis, is inducing death in cells. When your entire body is exposed to this it can be quite obviously harmful, but in small targeted doses, this particular effect can help kill tumors that exist because they are growing uncontrollably without any apoptosis occurring. Using radiation to stimulate apoptosis in these specific cells is the general mechanism behind cancer treatments such as chemotherapy. The other way radiation can harm cells is by blocking reproduction, or mitosis. This is actually the way that radiation kills the most cells, particularly in areas of your body that are constantly making new cells and killing old ones, such as in your bones or gastrointestinal system. This is why some areas of your body are affected by nuclear radiation more quickly than others that do not reproduce cells as often, such as your brain. Radiation can also cause mutations in germ cells in humans, which can then be passed to offspring, meaning that the effects of radiation can be seen for generations.  Damage done specifically to DNA from radiation, particularly double strand breaks, can be extremely harmful and lead directly to cancers. Aside from the main mechanisms through which radiation directly harms cells, it can also harm cells that were not directly exposed to radiation. This is primarily done through a disruption in cell to cell communication starting from the cells exposed to radiation leading to a disruption in the functioning of neighboring cells. 
Nuclear radiation occurs naturally and artificially. This means that natural sources, such as rocks or volcanic areas can naturally release nuclear radiation, or it can be artificially made by humans either for medical purposes or through things like nuclear power plants.  Radiation has three types of particles that are emitted; α, β and γ.  Both α and β rays can be easily shielded but γ rays can penetrate through most substances, including human bodies and therefore tend to cause the most trouble. Our daily exposure to the more harmful types, β and γ, is 2.4 mSv/year. Low levels are generally benign, but to explain the units; mSv stands for millisievert, which is one standard dose of radiation to the body. One standard dose is essentially 1 J of radiation per 1 kg of bodily tissue. Also, the Sv unit is focused on amount of γ radiation and how much of it is absorbed by cells, as it is the most harmful type. Nuclear radiation starts to have negative effects at 100 mSv, causes radiation sickness at 1,000 mSv and can cause death at 4,000 mSv.  This is in regards to radiation being experienced by the whole body and these dosages were discovered . If radiation is experienced at high levels onto a contained area, it will mainly affect that part, such as with cancer radiation therapy which is used at levels as high as 60 Sv. Radiation sickness is characterized by nausea and a decrease in red blood cell count, meaning a weakened immune system that leaves you at a much higher risk of being sick or contracting an illness.  Lower levels of radiation are also predicted to be causing cancers when exposure is prolonged but many experiments to determine the validity of this relationship have been inconclusive.  Additionally, even a small amount of radiation over time will lead to a decrease in lymphocyte, granulocyte and platelet levels putting you at a much greater risk of infection.  To summarize, any amount of radiation can be detrimental to health, starting at a cellular level. However, if you stop being exposed to radiation, full recovery from radiation sickness is possible.
© Alyssa Hobson. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 J. B. Little, "Principal Cellular and Tissue Effects of Radiation," in Holland-Frei Cancer Medicine, 6th Ed., ed. by D. W. Kufe et al. (B. C. Decker, 2003).
 O. Desouky, N. Ding, G. Zhou "Targeted and Non-Targeted Effects of Ionizing Radiation," J. Radiat. Res. Appl. Sci. 8, 247 (2015).
 L. Yang, "Low Level Radiation on Human Health," Physics 241, Stanford University, Winter 2016.
 B. Milic, "Symptomatic Development of Acute Radiation Syndrome," Physics 241, Stanford University, Winter 2015.