SafekipediaSievert
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The sievert (symbol: Sv) is a special measuring unit used to understand the health risks of certain types of energy called ionizing radiation. This kind of energy can come from sources like X-rays, nuclear power, or radioactive materials. The sievert helps scientists and doctors figure out how likely it is that this energy could cause health problems.
This unit is named after Rolf Maximilian Sievert, a Swedish scientist who did important work in measuring radiation and studying its effects on living things. The sievert is especially useful for measuring doses of radiation that come from outside the body, like from a medical scan, or from inside the body after someone breathes or swallows radioactive substances.
The sievert is part of the international system of units used all around the world for accurate measurements.
Definition
The sievert is a unit that measures how radiation affects the body. It helps us know how much harm radiation can do to our tissues.
The sievert is related to another unit called the gray, which measures how much radiation energy is taken in. One sievert means one joule of radiation energy has been absorbed in one kilogram of tissue, showing the possible biological effect.
External dose quantities
When we talk about the sievert for measuring the effects of outside radiation on our bodies, we use special tools called dosimeters to find out how much radiation someone has received. These tools help scientists understand possible health problems by using studies and models.
There are different ways to measure and talk about radiation doses. Some measures look only at the physical amount of radiation, while others try to guess how the body might react. These guesses are based on computer models that mimic human bodies and how they absorb radiation. Tools and devices help us keep track of radiation levels and make sure they stay safe.
Calculating protection dose quantities
The sievert helps us learn how much harm radiation can do to the body. It is used to measure two important things: how radiation affects the whole body and how it affects specific parts of the body.
To find these effects, scientists use special numbers called weighting factors. These numbers help them understand how much damage different types of radiation can cause. Some types of radiation can hurt the body more than others, even if they put in the same amount of energy. The sievert tells us about the possible health risks from radiation, not just the energy that hits the body.
| Radiation | Energy (E) | WR (formerly Q) |
|---|---|---|
| x-rays, gamma rays, beta particles, muons | 1 | |
| neutrons | 2.5 + 18.2e−[ln(E)]2/6 | |
| 1 – 50 MeV | 5.0 + 17.0e−[ln(2E)]2/6 | |
| > 50 MeV | 2.5 + 3.25e−[ln(0.04E)]2/6 | |
| protons, charged pions | 2 | |
| alpha particles, nuclear fission products, heavy nuclei | 20 | |
| Organs | Tissue weighting factors | ||
|---|---|---|---|
| ICRP26 1977 | ICRP60 1990 | ICRP103 2007 | |
| Gonads | 0.25 | 0.20 | 0.08 |
| Red bone marrow | 0.12 | 0.12 | 0.12 |
| Colon | — | 0.12 | 0.12 |
| Lung | 0.12 | 0.12 | 0.12 |
| Stomach | — | 0.12 | 0.12 |
| Breasts | 0.15 | 0.05 | 0.12 |
| Bladder | — | 0.05 | 0.04 |
| Liver | — | 0.05 | 0.04 |
| Oesophagus | — | 0.05 | 0.04 |
| Thyroid | 0.03 | 0.05 | 0.04 |
| Skin | — | 0.01 | 0.01 |
| Bone surface | 0.03 | 0.01 | 0.01 |
| Salivary glands | — | — | 0.01 |
| Brain | — | — | 0.01 |
| Remainder of body | 0.30 | 0.05 | 0.12 |
| Total | 1.00 | 1.00 | 1.00 |
Operational quantities
These quantities help us measure and understand radiation exposure. They connect what devices read to the doses inside the body.
- Ambient dose equivalent: This measures radiation that can go deep into the body, like gamma rays. It is shown as H*(10), meaning the radiation level at 10 mm inside a special sphere model.
- Directional dose equivalent: This is for radiation that doesn’t go very far, such as small particles. It is shown as H'(0.07), meaning the radiation level at 0.07 mm inside the sphere model. It helps protect parts like skin and the eyes.
- Personal dose equivalent: This is used when someone wears a device to track their own radiation exposure. It is shown as Hp(10), meaning the radiation level at 10 mm inside the body.
Proposals for changing the definition of protection dose quantities
In 2010, groups called ICRP Committee 2 and ICRU Report Committee 26 started looking for simpler ways to understand radiation safety. They wanted to use numbers that show the overall effect of radiation or the energy it absorbs.
They suggested two main changes. First, for measuring radiation over an area, they proposed a new method that would not need a special sphere used before. Second, for monitoring body parts like the eye lens and skin, they suggested a different way to measure radiation effects. These changes would update older reports from these groups. A final draft of these ideas was shared for feedback in July 2017.
Internal dose quantities
Main article: Committed dose
The sievert helps us learn about how much harm radiation inside the body can cause. This can happen when tiny bits of radioactive material get inside us by eating or breathing them in. These bits stay in our body and give off radiation for some time.
Experts calculate how much this inside radiation might hurt us. They look at how much of the material got inside and use special math to see what it might do over many years. They want to make sure this inside radiation is as safe as if the same amount of radiation came from outside the body.
Health effects
Further information: Radiobiology
Radiation can affect health in different ways. Sometimes it can hurt tissues right away. Another way is that it might cause problems later, like cancer. Not everyone who is exposed to radiation will get sick, but some might.
The sievert unit helps us talk about these later effects, such as cancer. Experts think that the risk of cancer increases a little with more radiation. This idea is called the linear no-threshold model. Some people believe there might be a safe level of radiation that the body can fix. Babies and older people might be more sensitive to radiation than adults.
Dose examples
Main article: Orders of magnitude (radiation)
We don’t usually meet with big amounts of radiation in our everyday lives. The examples here show how different levels of radiation compare. These are just examples to help you understand, not a full list of all possible radiation doses. An "acute dose" happens quickly over a short time, while a "chronic dose" is a smaller amount that continues over a longer period.
Dose rate examples
All changes between hours and years here assume you’re always in the same steady area, without counting changes or times when you’re not exposed and without considering how radioactive materials change over time. Values changed to years are shown in parentheses. "/a" means "per year", and "/h" means "per hour".
| mSv/a | nSv/h | Steady dose rates below 100 nSv/h are difficult to measure. | ||
| 1 | mSv/a | (100 | nSv/h avg) | ICRP recommended maximum for external irradiation of the human body, excluding medical and occupational exposures. |
| 2.4 | mSv/a | (270 | nSv/h avg) | Human exposure to natural background radiation, global average |
| (8 | mSv/a) | 810 | nSv/h avg | Next to the Chernobyl New Safe Confinement (May 2019) |
| ~8 | mSv/a | (~900 | nSv/h avg) | Average natural background radiation in Finland |
| 24 | mSv/a | (2.7 | μSv/h avg) | Natural background radiation at airline cruise altitude |
| (46 | mSv/a) | 5.19 | μSv/h avg | Next to Chernobyl Nuclear Power Plant, before installing the New Sarcophagus in November 2016 |
| 130 | mSv/a | (15 | μSv/h avg) | Ambient field inside most radioactive house in Ramsar, Iran |
| (350 | mSv/a) | 39.8 | μSv/h avg | Inside "The Claw" of Chernobyl |
| (800 | mSv/a) | 90 | μSv/h | Natural radiation on a monazite beach near Guarapari, Brazil. |
| (9 | Sv/a) | 1 | mSv/h | NRC definition of a high radiation area in a nuclear power plant, warranting a chain-link fence |
| (17–173 | Sv/a) | 2–20 | mSv/h | Typical dose rate for activated reactor wall in possible future fusion reactors after 100 years. After approximately 300 years of decay the fusion waste would produce the same dose rate as exposure to coal ash, with the volume of fusion waste naturally being orders of magnitude less than from coal ash. Immediate predicted activation is 90 MGy/a. |
| (1.7 | kSv/a) | 190 | mSv/h | Highest reading from fallout of the Trinity bomb, 20 mi (32 km) away, 3 hours after detonation. |
| (2.3 | MSv/a) | 270 | Sv/h | Typical PWR spent fuel waste, after 10-year cooldown, no shielding and no distance. |
| (4.6–5.6 | MSv/a) | 530–650 | Sv/h | The radiation level inside the primary containment vessel of the second BWR-reactor of the Fukushima power station, in February 2017, six years after a suspected meltdown. In this environment, it takes between 22 and 34 seconds to accumulate a median lethal dose (LD50/30). |
History
The sievert began as something called the röntgen equivalent man. It came from older ways of measuring. In the 1970s, the International Commission on Radiation Units and Measurements suggested using a new system. In 1977, another group introduced the sievert.
In 1980, an international committee made the sievert an official unit. They explained more about how to use it in 1984 and updated these ideas in 2002.
Common SI usage
The sievert is named after Rolf Maximilian Sievert. Like other SI units named after people, its symbol starts with an upper case letter (Sv).
We often use smaller versions of this unit, called millisieverts (mSv) and microsieverts (μSv). These help us talk about very small amounts of radiation. For example, 1 millisievert is a small amount of sieverts, and 1 microsievert is an even smaller amount. Sometimes we need to talk about how much radiation is being given over time, like millisieverts per hour (mSv/h) or microsieverts per hour (μSv/h).
Ionizing radiation quantities
The following table shows radiation quantities in SI and non-SI units:
Although the United States Nuclear Regulatory Commission allows the use of the units curie, rad, and rem alongside SI units, the European Union European units of measurement directives said these units should no longer be used for public health reasons after December 31, 1985.
Rem equivalence
An older unit for measuring radiation dose is the rem, still used in the United States. One sievert equals 100 rem:
| 100.0000 rem | = | 100,000.0 mrem | = | 1 Sv | = | 1.000000 Sv | = | 1000.000 mSv | = | 1,000,000 μSv |
|---|---|---|---|---|---|---|---|---|---|---|
| 1.0000 rem | = | 1000.0 mrem | = | 1 rem | = | 0.010000 Sv | = | 10.000 mSv | = | 10000 μSv |
| 0.1000 rem | = | 100.0 mrem | = | 1 mSv | = | 0.001000 Sv | = | 1.000 mSv | = | 1000 μSv |
| 0.0010 rem | = | 1.0 mrem | = | 1 mrem | = | 0.000010 Sv | = | 0.010 mSv | = | 10 μSv |
| 0.0001 rem | = | 0.1 mrem | = | 1 μSv | = | 0.000001 Sv | = | 0.001 mSv | = | 1 μSv |
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