 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 sieverts is defined to also take into account the type of radiation, giving greater weight to alpha particles compared to gammas or electrons. A sievert is a very large dose, with typical exposures occurring at the micro sievert 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 sieverts. 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 sieverts, 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 sieverts 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 sieverts 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.