 Radiation doses are typically measured in grays, a unit that accounts for the total energy deposited by the radiation. However, to best understand the effect on living tissue, the unit of sievets is defined to also take into account the type of radiation, giving greater weight to alpha particles compared to gammas or electrons. A sievet is a very large dose, with typical exposures occurring at the micro sievet level. I stated earlier that all life is exposed to a sea of natural background radiation. Let's take a look at the typical annual levels of radiation that someone might be exposed to. All of the doses in this table are given in units of micro sievets. Natural doses typically make up 85% of someone's annual exposure. A major component is cosmic radiation, which is due to particles hitting our atmosphere and sending a rain of radiation down on us from above. While the average annual dose from cosmic rays is about 260 micro sievets, the dose can be very variable and depends strongly on altitude and the amount of shielding atmosphere. In fact, at cruising altitude in an airplane, the dose rates can be as high as 5-7 micro sievets per hour, a level that is well and truly above the recommended public limit for radiation exposure. This seems like madness. Why then are you allowed to fly in a plane? Isn't it dangerous? Well, this level is low enough to pose no immediate risk of radiation sickness, but there is indeed an increased risk of radiation-induced cancer. However, the risk is extremely small. Remember that the public limit for radiation exposure is based upon 24-7 exposure each and every day, whereas a few hours in a plane and brief exposure to radiation is offset by all the possible benefits that you might get from travelling. There is also radiation in the foods and liquids that we eat and drink. The average dose of 300 micro sievets received each year is subject to considerable variation from person to person, since certain foods can be naturally high in particular radiation. For example, Brazil nuts and bananas are high in potassium, and all potassium contains the naturally occurring radioactive isotope, potassium-40. The biggest source of natural radiation exposure tends to be due to the radon gas that is produced in the decay of naturally occurring uranium and thorium throughout the Earth. Note that radon gas is an alpha emitter, and the gas can be breathed directly into your lungs, the worst possible scenario for an alpha emitting isotope. Depending on the concentration of uranium and thorium in the underlying rock, as well as the ventilation properties of your house, the radiation dose received from radon can vary by many orders of magnitude. The last source of natural radiation listed is the gamma rays from various naturally occurring radioisotopes. It is noteworthy that about half of the gamma ray dose received from potassium-40 is actually the potassium-40 that is resident in your own body, mostly in your bones. Moving now to artificial sources of radiation, many medical procedures are now nuclear medicine based. While the average person receives 360 micro sievets of radiation from medical sources each year, the values across the population vary considerably. From zero for some people to 60 micro sievets for someone who has an x-ray, up to a thousand micro sievets 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 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 2,600 micro sievets, 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 micro sievets 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 sievets is very likely to be fatal. Note that there is a very very large difference between a fast and fatal dose of 7 sievets compared to the average background radiation dose received of only 2600 micro sievets spread across the entire year.