Sv is the unit dose equivalent and the product of the absorbed dose in gray and quality factors.

Frequently used interchangeably with Gray (Gy).

The sievert (symbol: Sv is a unit in the International System of Units (SI) intended to represent the health risk of ionizing radiation, which is defined as the probability of causing radiation-induced cancer and genetic damage. 

The sievert is important in dosimetry and radiation protection. 

The sievert is used for radiation dose quantities such as equivalent dose and effective dose.

It represents the risk of external radiation from sources outside the body, and committed dose, which represents the risk of internal irradiation due to inhaled or ingested radioactive substances. 

One sievert results in a 5.5% probability of eventually developing fatal cancer based on the ionizing radiation exposure.

gray:absorbed dose

1 Gy = 1 joule/kilogram – a physical quantity. 

1 Gy is the deposit of a joule of radiation energy per kilogram of matter or tissue.

sievert:equivalent dose

1 Sv = 1 joule/kilogram – a biological effect. 

The sievert represents the equivalent biological effect of the deposit of a joule of radiation energy in a kilogram of human tissue.

Some tissues like bone marrow are particularly sensitive to radiation, so they are given a weighting factor that is disproportionally large relative to the fraction of body mass they represent. 

Hard bone surface are particularly insensitive to radiation and are assigned a disproportionally low weighting factor.

The sum of tissue-weighted doses to each irradiated organ or tissue of the body adds up to the effective dose for the body.

Ionizing radiation has deterministic and stochastic effects on human health. 

Deterministic are acute tissue effects that happen with certainty, with the resulting health conditions occurring in every individual who received the same high dose. 

Stochastic cancer induction and genetic events are inherently random, with most individuals in a group failing to ever exhibit any causal negative health effects after exposure, while an indeterministic random minority do, often with the resulting subtle negative health effects being observable only after large detailed epidemiology studies.

The use of the sievert implies that only stochastic effects are being considered.

 to avoid confusion deterministic effects are conventionally compared to values of absorbed dose expressed by the SI unit gray (Gy).

Planned exposure – 

For occupational exposure, the limit is 50 mSv in a single year with a maximum of 100 mSv in a consecutive five-year period, and for the public to an average of 1 mSv (0.001 Sv) of effective dose per year, not including medical and occupational exposures.

The natural radiation levels inside the United States Capitol are such that a human body would receive an additional dose rate of 0.85 mSv/a, close to the regulatory limit, because of the uranium content of the granite structure.

Someone who spent 20 years inside the capitol building would have an extra one in a thousand chance of getting cancer, over and above any other existing risk.  

The existing risk for an average American would have a 10% chance of getting cancer during this same 20-year period, even without any exposure to artificial radiation.

Dose examples

Gy = 1 Joule/kilogram = 100 rad. 

98 nSv: Banana equivalent dose, an illustrative unit of radiation dose representing the measure of radiation from a typical banana.

250 nSv: U.S. limit on effective dose from a single airport security screening

5–10 μSv: One set of dental radiographs

80 μSv: Average dose to people living within 10 mi (16 km) of the plant during the Three Mile Island accident

400–600 μSv: Two-view mammogram

1 mSv: dose limit for individual members of the public, total effective dose equivalent, per annum

1.5–1.7 mSv: Annual occupational dose for flight attendants 

2–7 mSv: Barium fluoroscopy-Barium meal, up to 2 minutes, 4–24 spot images

10–30 mSv: Single full-body CT scan

50 mSv: occupational dose limit, total effective dose equivalent, per annum

68 mSv: Estimated maximum dose to evacuees who lived closest to the Fukushima I nuclear accidents

80 mSv: 6-month stay on the International Space Station

160 mSv: Chronic dose to lungs over one year smoking 1.5 packs of cigarettes per day, mostly due to inhalation of Polonium-210 and Lead-210

250 mSv: 6-month trip to Mars—radiation due to cosmic rays, which are very difficult to shield against

400 mSv: Average accumulated exposure of residents over a period of 9–20 years, who suffered no ill effects, in apartments in Taiwan constructed with rebar containing Cobalt-60

500 mSv: The U.S. 10 C.F.R. occupational dose limit, shallow-dose equivalent to skin, per annum

670 mSv: Highest dose received by a worker responding to the Fukushima emergency

1 Sv: Maximum allowed radiation exposure for NASA astronauts over their career

4–5 Sv: Dose required to kill a human with a 50% risk within 30 days (LD50/30), if the dose is received over a very short duration.

The sievert has its origin in the röntgen equivalent man (rem) which was derived from CGS units.

Frequently used SI prefixes are the millisievert (1 mSv = 0.001 Sv) and microsievert (1 μSv = 0.000 001 Sv).

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