Units of radiation and radioactivity

In order to quantify how much radiation we are exposed to in our daily lives and assess potential health impacts as a result, it is necessary to establish a unit of measurement. The basic unit of radiation dose absorbed in tissue is the gray (Gy), where one gray represents the deposition of one joule of energy per kilogram of tissue.
However, since neutrons and alpha particles cause more damage per gray than gamma or beta radiation, another unit, the sievert (Sv) is used in setting radiological protection standards. This unit of measurement takes into account biological effects of different types of radiation. One gray of beta or gamma radiation has one sievert of biological effect, one gray of alpha particles has 20 Sv effect and one gray of neutrons is equivalent to around 10 Sv (depending on their energy). Since the sievert is a relatively large value, dose to humans is normally measured in millisieverts (mSv), one-thousandth of a sievert.
The becquerel (Bq) is a unit or measure of actual radioactivity in material (as distinct from the radiation it emits, or the human dose from that), with reference to the number of nuclear disintegrations per second (1 Bq = 1 disintegration/sec). Quantities of radioactive material are commonly estimated by measuring the amount of intrinsic radioactivity in becquerels – one Bq of radioactive material is that amount which has an average of one disintegration per second, i.e. an activity of 1 Bq.
Older units of radiation measurement continue in use in some literature:
1 gray = 100 rads
1 sievert = 100 rem
1 becquerel = 27 picocuries or 2.7 x 10-11 curies
One curie was originally the activity of one gram of radium-226, and represents 3.7 x 1010 disintegrations per second (Bq).
The Working Level Month (WLM) has been used as a measure of dose for exposure to radon and in particular, radon decay productsb.
Sources of radiation
Radiation can arise from human activities or from natural sources. Most radiation exposure is from natural sources. These include: radioactivity in rocks and soil of the Earth's crust; radon, a radioactive gas given out by many volcanic rocks and uranium ore; and cosmic radiation. The human environment has always been radioactive and accounts for up to 85% of the annual human radiation dose.
Radiation arising from human activities typically accounts for up to 15% of the public's exposure every year. This radiation is no different from natural radiation except that it can be controlled. X-rays and other medical procedures account for most exposure from this quarter. Less than 1% of exposure is due to the fallout from past testing of nuclear weapons or the generation of electricity in nuclear, as well as coal and geothermal, power plants.
Backscatter X-ray scanners being introduced for airport security will gives exposure of up to 5 microsieverts (μSv), compared with 5 μSv on a short flight and 30 μSv on a long intercontinental flight across the equator, or more at higher latitudes – by a factor of 2 or 3. Aircrew can receive up to about 5 mSv/yr from their hours in the air, while frequent flyers can score a similar incrementc. In the UK, the National Radiation Protection Board's 1999 survey showed that on average, nuclear power workers received a lower annual radiation dose than flight crew, and frequent flyers in 250 hours would receive 1 mSv.
The maximum annual dose allowed for radiation workers is 20 mSv/yr, though in practice, doses are usually kept well below this level. In comparison, the average dose received by the public from nuclear power is 0.0002 mSv/yr, which is of the order of 10,000 times smaller than the total yearly dose received by the public from background radiation.
Natural background radiation
Naturally occurring background radiation is the main source of exposure for most people, and provides some perspective on radiation exposure from nuclear energy. The average dose received by all of us from background radiation is around 2.4 mSv/yr, which can vary depending on the geology and altitude where people live – ranging between 1 and 10 mSv/yr, but can be more than 50 mSv/yr. The highest known level of background radiation affecting a substantial population is in Kerala and Madras states in India where some 140,000 people receive doses which average over 15 millisievert per year from gamma radiation, in addition to a similar dose from radon. Comparable levels occur in Brazil and Sudan, with average exposures up to about 40 mSv/yr to many people.
Several places are known in Iran, India and Europe where natural background radiation gives an annual dose of more than 50 mSv and up to 260 mSv (at Ramsar in Iran). Lifetime doses from natural radiation range up to several thousand millisievert. However, there is no evidence of increased cancers or other health problems arising from these high natural levels.
Radon gas has decay products that are alpha emitters. People everywhere are typically exposed to around 0.2 mSv/yr, and often up to 3 mSv/yr, from inhaled radon without apparent ill-effectd. However, in industrial situations its control is a high priority.
Public exposure to natural radiatione
Source of exposure Annual effective dose (mSv) Average Typical range Cosmic radiation Directly ionizing and photon component 0.28 Neutron component 0.10 Cosmogenic radionuclides 0.01 Total cosmic and cosmogenic 0.39 0.3–1.0e External terrestrial radiation Outdoors 0.07 Indoors 0.41 Total external terrestrial radiation 0.48 0.3-1.0e Inhalation Uranium and thorium series 0.006 Radon (Rn-222) 1.15 Thoron (Rn-220) 0.1 Total inhalation exposure 1.26 0.2-10e Ingestion K-40 0.17 Uranium and thorium series 0.12 Total ingestion exposure 0.29 0.2-1.0e Total 2.4 1.0-13