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, such as illnesses that can be passed to future generations.
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.
According to international guidelines, being exposed to one sievert of radiation carries a small but measurable chance of developing serious health issues later in life. To find this value, scientists use another measurement called absorbed dose and then apply special factors that consider the type of radiation and where it affects the body. The sievert is part of the international system of units used all around the world for accurate measurements.
Definition
The sievert is a special unit used to measure the effects of radiation on the body. It helps us understand how much harm radiation can do to human tissue.
The sievert is linked to another unit called the gray, which measures how much radiation energy is absorbed. One sievert means that 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 and devices called dosimeters to find out how much radiation someone has received. These tools help scientists predict 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 we use every day help us keep track of radiation levels and make sure they stay safe.
Calculating protection dose quantities
The sievert helps us understand how much harm radiation can do to the body. It is used to measure two important things: the effect of radiation on the whole body and the effect on specific parts of the body.
To find these effects, scientists use special numbers called weighting factors. These numbers help them figure out how much damage different types of radiation can cause. For example, 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 are used to connect the readings from devices to the doses inside the body.
- Ambient dose equivalent: This is used to measure radiation that can go deep into the body, like gamma rays. It is usually 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 began looking for simpler ways to calculate and understand radiation safety measures. They wanted to use numbers related to overall radiation effect or the amount of energy absorbed.
They suggested two main changes. First, for measuring radiation over an area, they proposed using a new method that would not need a special sphere used before. Second, for monitoring specific body parts like the eye lens and skin, they suggested a different way to measure radiation effects. These changes would update some 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 understand how much harm radiation inside the body can cause. This happens 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.
Specialists figure out how much this inside radiation can hurt us by looking at how much of the material got inside and using special math to see what it might do over many years. They want to make sure this inside radiation is just as safe as if the same amount of radiation came from outside the body.
Health effects
Further information: Radiobiology
Ionizing radiation can affect health in two ways. One way is certain and happens every time a person gets a high dose, like serious tissue damage. The other way is random, such as getting cancer later in life. Not everyone exposed to radiation will get sick, but some might.
The sievert unit is used to talk about the random effects, like cancer. Experts believe that the chance of getting cancer goes up a little bit for every extra amount of radiation someone gets. This idea is called the linear no-threshold model. Some people think there might be a level of radiation that is safe because the body can fix small damage. It's also known that babies and older people might be more at risk from radiation than adults.
Dose examples
Main article: Orders of magnitude (radiation)
Big amounts of radiation aren’t something we usually come across in daily life. The examples here show how different levels of radiation compare. These are just examples to give you an idea, 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 started as something called the röntgen equivalent man, which came from older measuring systems. In the 1970s, a group called the International Commission on Radiation Units and Measurements suggested using a new measuring system. In 1977, another group introduced the sievert.
In 1980, an international committee officially added the sievert as a unit. They gave more details about how to use it in 1984 and updated those details in 2002 to match newer ideas about measuring radiation.
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). When we write the full word, it follows the normal rules for capitalization — it starts with a capital letter only at the beginning of a sentence or in titles.
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 0.001 sieverts, and 1 microsievert is 0.000001 sieverts. 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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