Radiation dose units and measurement


Confusion is caused by the large number of units that can be used to describe radiation. This is in part due to different parts of the world using different units to describe the same thing in the way that metres and feet can be used to measure distance, but it is mainly due to the fact that different things are being measured.

The amount of radiation your body is exposed to is called your “radiation dose” and is measured in a unit called a millisievert (often written as mSv). The rate at which your radiation dose is increasing with time is called the “radiation dose rate” (sometimes “dose rate” for short) and is measured in millisievert per hour (mSv/hr or mSv.hr-1).

A nuclear accident could increase radiation rates for those nearby during the accident and, to a much lesser extent, to those near the scene of the accident in the following years.


The activity of a source of radiation is a measure of the rate of disintegrations within it. This is measured in becquerels (Bq).
1 Bq = 1 disintegration (radioactive decay) per second.
1 MBq = 1,000,000 disintegrations per second.
1 TBq = 1 x 1012 disintegrations per second (1,000,000,000,000)

The original unit of activity was the Curie (Ci). This is defined as the activity of 1g of radium-226. The conversion factor is 3.7×1010 Bq = 1 Ci and 1 Bq = 2.702 x 10-11 Ci. The Curie is named after Marie and Pierre Curie who discovered radium in 1898. The Curie is used in some historical records but rarely seen in modern texts.

Radiation dose, equivalent dose and effective dose

Radiation dose is a measure of the impact of radiation on the body. It is a complex concept because of the several different ways radiation can act on your body and the fact that different parts of the body have different levels of sensitivity.

To determine the level of harm caused by a radiation dose it is necessary to understand:

(1) How much energy the radiation deposits in the affected tissue (absorbed dose) (see Radiation Interactions for more detail)

Absorbed dose (Gy) is the energy (in Joules) absorbed in an amount of matter (in kg). Often seen as mGy = 1/1,000 Gy.

(2) How much harm that deposited energy causes to those affected tissues (equivalent dose) (see Equivalent dose for more detail)

Equivalent dose (Sv) is a measure of the ability to damage living tissue. It is obtained by multiplying the Absorbed dose (Gy) by a radiation weighting factor (WR) which accounts for the different effectiveness of different radiations. Often used as mSv = 1/1,000 Sv (milli-sievert) or µSv = 1/1,000,000 Sv (micro-sievert).

(3) The level of harm that is caused to the body by the harm done to those affected tissues (effective dose). (See Effective dose for more detail)
Effective dose (Sv) Is a measure of the ability to harm an individual. Obtained as the equivalent dose (Sv) multiplied by a tissue weighting factor WT representing the susceptibility and importance of the organ affected and summed over the whole body. In the response phase to a nuclear accident the main interest of decision makers is the likely effective dose (a surrogate for harm) to individuals which will be expressed in μSv (micro-sieverts) (hopefully), mSv (milli-sieverts) (possibly), or Sv (sieverts) (we really hope not!).

The limit on effective dose for any person other than an employee or trainee is 1 mSv in any calendar year, a CT scan of the head gives 1.4 mSv, 2.7 mSv/year is the average annual background dose in the UK, 6.9 mSv is the average annual radon dose to people in Cornwall, 20 mSv/yr is the annual dose limit in the UK nuclear industry, 100 mSv is the level at which changes in blood cells can be readily observed, acute radiation effects start at about 1000 mSv and 5000 mSv would be expected to kill half of those exposed.

Background radiation

Radiation is naturally occurring and unavoidable. The average annual radiation exposure in the UK is about 2.7 mSv. This is the radiation dose we get from the materials around us, radiations from space, and from medical procedures. Radiation levels in the UK are kept under review by UKHSA who issue occasional reports summarising the average radiation dose in the UK (example here).

A short film on background ionising radiation can be found on Youtube. A more detailed report can be found on the government website .

The World Nuclear Association has published an information leaflet on background radiation which states that “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 (Link here).

The PHE has published a paper showing average doses from medical diagnosis exposures, the comparison of these with background radiation and an estimate of the increased changes of cancer (Link here). This shows Myocardial perfusion (Tl-201) at 18 mSv as the highest dose medical treatment. This could be a useful paper to discuss if people were to receive these levels of exposure due to the radiation emergency.

RIFE - local dose levels

The Food Standards Agency produces an annual report summarising most of the environmental monitoring that takes place around the UK nuclear sites ( Link here)

A recent publication in this series (Link here) states that for Torness, taken as an example "in 2015, the total dose from all pathways and sources of radiation was 0.020 mSv or 2 per cent of the dose limit, and unchanged from the previous four years. Direct radiation was the dominant contributor to the dose and the representative person was an adult. [….] The estimated dose to a terrestrial food consumer was 0.006 mSv, which was approximately 0.5 per cent of the dose limit for members of the public of 1 mSv, and unchanged from recent years. The dose to a fish and shellfish consumer was less than 0.005 mSv."

To gather this data “a variety of foods, including milk, crops, fruit, and game as well as grass, soil and freshwater samples, were measured for a range of radionuclides. Air sampling at three locations was undertaken to investigate the inhalation pathway. […] Samples of seawater, sediment and Fucus vesiculosus, as useful environmental indicators, were collected in addition to seafood. Measurements were also made of gamma dose rates over intertidal areas, supported by analyses of sediment, and beta dose rates on fishing gear.”

The RIFE reports would be important in a post-accident situation as they provide a measurement of the radiological conditions around the site prior to any accident to use as a baseline.