Dose · Radiation and safety
Radiation doses
Dose figures are only meaningful next to the source they were published in and the year that source was written. Every number on this page carries both.
Every radiology report, information leaflet and safety notice that mentions ionising radiation eventually produces a number. Reading that number correctly requires knowing which unit it uses, which scale it belongs to, and which framework produced it. This page sets out the three main ways doses are expressed in British diagnostic imaging, using figures published by the UK Health Security Agency and by legislation.gov.uk, and following the rule set out in our sources and method page: no figure appears here without its named source.
What is a millisievert and how big are these numbers?
The effective dose from a medical exposure is usually expressed in millisieverts, abbreviated mSv. The UK Health Security Agency comparison table published on 18 March 2011 gives a series of reference points, drawn from natural background radiation, everyday activities and medical procedures, that place imaging doses on a single scale. The UK average annual radiation dose is 2.7 mSv, and the same table records that, on average, people are exposed to about 2.7 millisieverts of radiation a year in the UK.
The table below reproduces the imaging, background and occupational rows of that UKHSA comparison. Nothing has been rounded, converted or combined: every value is copied exactly as published.
| Exposure or situation | Dose in mSv |
|---|---|
| Dental x-ray | 0.005 |
| 100 g of Brazil nuts | 0.01 |
| Chest x-ray | 0.014 |
| Transatlantic flight | 0.08 |
| Nuclear power station worker average annual occupational exposure (2010) | 0.18 |
| UK annual average radon dose | 1.3 |
| CT scan of the head | 1.4 |
| UK average annual radiation dose | 2.7 |
| CT scan of the chest | 6.6 |
| Average annual radon dose to people in Cornwall | 6.9 |
| CT scan of the whole spine | 10 |
| Annual exposure limit for nuclear industry employees | 20 |
Within this table, the computed tomography rows carry the highest values of any imaging procedure listed. This is consistent with the place of the CT scan in the UK imaging pathway, which is weighed against the duty to justify and optimise every exposure set by IR(ME)R 2017. That regulation, on legislation.gov.uk, requires every medical exposure to have been justified by the practitioner as showing a sufficient net benefit, and requires doses to be kept as low as reasonably practicable consistent with the intended purpose.
How does UKHSA express the risk attached to these doses?
A millisievert figure on its own says nothing about consequence. The UK Health Security Agency patient information page published on 22 September 2022 frames imaging doses in terms of lifetime cancer risk, and it does so without quoting any mSv value at all. It states that about 1 in 2, or 50%, of us will get cancer in our lifetime. It then states that the highest dose examinations would only increase this risk from around 50% to 51%, and that the most common examinations increase the risk by much less than 1%.
These three sentences are reported here exactly as UKHSA published them, attributed to their source and date, without further comment. The important structural point is that the two UKHSA publications serve different purposes: the 2011 table gives dose magnitudes in mSv, while the 2022 page gives a risk framing expressed in percentages of lifetime cancer incidence.
What are National Diagnostic Reference Levels?
A third family of numbers exists, and it is not expressed in millisieverts. National Diagnostic Reference Levels, or NDRLs, are published by UKHSA in the quantities actually displayed on imaging equipment: DAP in Gy cm², CTDIvol in mGy, DLP in mGy cm and CBDIw in mGy. The UKHSA NDRL page dated 11 December 2025 explains that these values are generally given as the third quartile value from a spread of values from a national survey. They are benchmark figures for typical practice, not measures of the dose received by any individual.
- Adult radiography, Chest PA: DAP 0.077 Gy cm², NDRL adopted 2025 (UKHSA, 11 December 2025).
- Adult radiography, Abdomen AP: DAP 1.7 Gy cm², 2025 (UKHSA, 11 December 2025).
- CT Head, indication acute stroke: CTDIvol 47 mGy, DLP 790 mGy cm, 2022, using a standard 16 cm CT dosimetry phantom (UKHSA, 11 December 2025).
- CT Chest, indication lung cancer: CTDIvol 8.5 mGy, DLP 290 mGy cm, 2022, using a 32 cm phantom (UKHSA, 11 December 2025).
- CT Pulmonary Angiography, indication pulmonary embolism: CTDIvol 9.1 mGy, DLP 310 mGy cm, 2022 (UKHSA, 11 December 2025).
- CT Abdomen and pelvis, indication abscess: CTDIvol 10 mGy, DLP 530 mGy cm, 2022 (UKHSA, 11 December 2025).
The same UKHSA page states that National Diagnostic Reference Levels for the most common nuclear medicine studies are given in the ARSAC Notes for Guidance, not in this table. The quantities also differ by modality: radiography NDRLs are expressed in DAP, while CT NDRLs are expressed in CTDIvol and DLP, with the phantom size noted alongside each CT value.
What dose limits does the law set, and for whom?
The Ionising Radiations Regulations 2017, Schedule 3, on legislation.gov.uk, set legal dose limits for two categories of person, and these are occupational and public limits. They are 20 mSv in any calendar year for any employee or trainee aged 18 years or over, 6 mSv in any calendar year for a trainee under 18 years of age, and 1 mSv in any calendar year for any other person. The same regulations also create an investigation threshold: IRR 2017 regulation 9 requires an investigation when an employee's effective dose exceeds 15 mSv in a calendar year, or a lower level fixed in writing by the employer.
No numerical dose limit applicable to patients is published by the sources used here, and none is given on this page. Medical exposures sit under a different regime: IR(ME)R 2017 governs them through justification and optimisation rather than through a numerical ceiling.
Checked against the source
- The UK average annual radiation dose is 2.7 mSv (UKHSA dose comparisons, 18 March 2011).
- A CT scan of the whole spine is listed at 10 mSv, and the annual exposure limit for nuclear industry employees at 20 mSv, in the same table (UKHSA, 18 March 2011).
- UKHSA states that about 1 in 2 of us will get cancer in our lifetime, and that the highest dose examinations would only increase this risk from around 50% to 51% (UKHSA, 22 September 2022).
- NDRLs are generally given as the third quartile value from a national survey, in DAP, CTDIvol, DLP or CBDIw (UKHSA, 11 December 2025).
- Nuclear medicine NDRLs are given in the ARSAC Notes for Guidance rather than in the UKHSA NDRL table (UKHSA, 11 December 2025).
One word, two quantities
An effective dose in millisieverts and a National Diagnostic Reference Level are different kinds of number. The mSv figures in the UKHSA comparison table of 18 March 2011 estimate the effective dose from an exposure, allowing comparison across situations such as flights, background radiation and scans. An NDRL, published by UKHSA on 11 December 2025 in Gy cm², mGy or mGy cm, is the third quartile of values from a national survey and serves as a benchmark for typical equipment performance, not as the dose received by any person.
Similarly, the IRR 2017 Schedule 3 limits of 20 mSv, 6 mSv and 1 mSv per calendar year are occupational and public limits for workers, trainees and other persons, and the 15 mSv figure is an investigation threshold for employee dose under regulation 9. None of these is a patient dose limit, and no numerical limit applicable to patients is published by these sources, whose exposures are governed by the justification and optimisation duties of IR(ME)R 2017.
Where each figure comes from
Three publications carry the numbers on this page: the UK Health Security Agency dose comparison table of 18 March 2011 for millisievert values, the UKHSA patient information page of 22 September 2022 for the risk framing, and the UKHSA NDRL page of 11 December 2025 for reference levels in DAP, CTDIvol and DLP. The legal limits and the investigation threshold come from the Ionising Radiations Regulations 2017 on legislation.gov.uk, and the justification and optimisation duties from IR(ME)R 2017. The unit indicates which framework a figure belongs to: millisieverts for effective dose, and Gy cm2, mGy or mGy cm for national reference levels.