 Moving now to artificial sources of radiation, many medical procedures are now nuclear medicine based. While the average person receives 360 microsevets of radiation from medical sources each year, the values across the population vary considerably. From zero for some people to 60 microsevets for someone who has an x-ray, up to a thousand microsevets for a CAT scan, and far, far beyond if someone has the misfortune to require radiation treatment for something like a cancer. Environmental discharges from radiation vary considerably. Note though that the largest discharges of radioactivity arise from the burning of coal in power plants, releasing large amounts of uranium, thorium and their radioactive decay products into the air. The entirety of these large discharges outweigh even the emissions from the two major nuclear power accidents that have occurred at Chernobyl and Fukushima in the past. Although only a small contribution to the overall figure, another source of artificial radioactivity is the fallout from atmospheric testing of nuclear weapons in the 1960s and 1970s. It is interesting that the relatively uniform fallout of radioactive isotopes, such as cesium-137 or isotopes of plutonium, have since been put to use as a radioactive tracer monitoring sural soil erosion. This was certainly not intentional and in no way excuses the stupidity of detonating nuclear weapons in the atmosphere. On average, the total annual dose for radiation is of the order of 2600 microcevates, but with a huge amount of natural variability. It is worth noting that this continual low-level background dose is something that has been around for the entire time of life on Earth, and life as we know it evolved in this sea of background radiation. Small amounts of radiation can cause damage to cells that is completely repairable, other than the small but finite chance of inducing mutation and perhaps the later cancer. Note however that there are places on Earth where people live that have background radiation levels of upwards of 100 microcevates per hour. This is a thousand times the nominal public limit, yet the people who live there suffer no increased risks of cancer. So far we have talked about chronic radiation exposure. Chronic is a medical term referring to consistent low levels of exposure. Acute doses of radiation are those received over a short period of time, and an acute dose of greater than around 7 cvates is very likely to be fatal. Note that there is a very, very large difference between a fast and fatal dose of 7 cvates compared to the average background radiation dose received of only 2600 microcevates spread across the entire year.