How Much Radiation Is in an X-Ray or CT Scan?

A chest X-ray delivers about 0.02 mSv, roughly three days of ordinary background radiation. A CT of the abdomen and pelvis is around 10 mSv, closer to four years' worth. The added risk from one scan is very small but not zero, and dose accumulates over a lifetime. MRI and ultrasound use no ionising radiation at all.

This is general information, not medical advice. If you have symptoms that worry you, speak to your GP or call 111.

What a millisievert is, briefly

The millisievert (mSv) is the unit used to describe effective dose: an estimate of the whole-body harm from a given exposure, weighted for how sensitive the tissues involved are. It is designed precisely so you can compare a chest X-ray with a CT with a transatlantic flight, which is why it is the number worth learning.

It is an estimate, not a measurement of what happened inside you. Actual dose varies with the scanner, the protocol, your body size and how many phases the scan involves.

The published UK figures

The table below uses typical effective doses from Public Health England's "Patient dose information: guidance", still the most widely cited UK reference for these numbers.

One caveat worth stating plainly, because most articles do not. That PHE table converts to background equivalents using a UK average of 2.2 mSv per year. UKHSA's more recent assessment, Ionising radiation exposure of the UK population published in June 2025, puts the average at about 2.6 mSv per year. On the newer figure the background equivalents above are slightly generous, so a CT abdomen and pelvis is nearer four years than four and a half. The difference does not change the conclusion.

Modern equipment also often does better than these typical figures. Iterative reconstruction, automatic tube current modulation and low-dose protocols have brought many CT doses down over the last decade. A low-dose lung CT, for instance, is deliberately run at a fraction of a standard chest CT dose.

What the UK background actually is

The UKHSA figure of roughly 2.6 mSv a year is mostly natural. Radon and thoron account for about 1.1 mSv of it, terrestrial gamma radiation about 0.4 mSv, cosmic radiation about 0.3 mSv, and naturally occurring radionuclides in food and the body about 0.3 mSv. Diagnostic medical procedures contribute about 0.4 mSv on average across the population, which makes medicine the only significant man-made source.

Where you live matters more than most people realise. Regional averages across the UK range from roughly 1.5 mSv to 7.5 mSv a year, largely driven by radon in granite areas such as parts of Cornwall and Devon. Somebody living in a high-radon home may accumulate more additional dose in a year than they would from a CT scan.

So how big is the risk?

Here is the honest version, without either reassurance or scaremongering.

The published estimates put the lifetime additional risk of fatal cancer from a single chest X-ray at around one in a million, and from a CT of the abdomen and pelvis at around one in 2,000, for adults aged roughly 16 to 69. The same source notes these risks are roughly double for children and around five times lower for older patients, because risk is driven partly by how many years of life remain for a cancer to develop.

Those numbers rest on the linear no-threshold model, which assumes risk scales down proportionally to zero with dose. At the low doses used in diagnostic imaging the model cannot be tested directly, because the effect is far smaller than the natural variation in cancer rates. It is used because it is cautious, not because it is proven at this level. Some scientists think it overstates low-dose risk. Nobody serious thinks it substantially understates it.

The practical translation: roughly one in two people in the UK will develop cancer at some point anyway. A CT of the abdomen adds something on the order of 0.05 percentage points to that. It is a real number, and it is very small next to a scan that finds what is causing your symptoms.

Dose is cumulative, and that is the part to remember

There is no annual radiation allowance for patients, and no counter that resets. But dose does add up over a lifetime, and the more scans you have, the more relevant that becomes.

This matters most for younger people, for anyone being monitored repeatedly for a long-term condition, and for anyone tempted by routine whole-body screening. If you have had recent imaging, tell the clinic. A previous CT might mean the new one can be limited to a smaller area, or skipped in favour of ultrasound or MRI, or simply compared against rather than repeated.

The Royal College of Radiologists' iRefer guidelines, which UK referrers use to decide which test to request, band investigations by dose for exactly this reason: no dose at all for ultrasound and MRI, Band 1 for under 1 mSv, Band 2 for 1 to 5 mSv, Band 3 for 5.1 to 10 mSv, and Band 4 for over 10 mSv, which is where CT of the chest, abdomen and pelvis together and PET-CT sit.

MRI and ultrasound use none of it

This is the single most useful fact on this page. MRI images the body with a strong magnetic field and radio waves. Ultrasound uses sound. Neither involves ionising radiation, and neither carries any radiation dose to add to your lifetime total.

That is why ultrasound is the first-line test for so many abdominal, pelvic, thyroid, testicular and soft-tissue questions, and why MRI is preferred for the spine, joints, brain and pelvis where it can answer the question. It is not always the right tool. CT is faster, better for lung tissue, bone detail, acute bleeding and stone disease, and often the only sensible choice in an emergency.

Cost pushes the same way, incidentally. On UK provider websites checked in September 2026, single-area ultrasound ran from about £69 to £540 and single-area CT from about £445 to £840, with private hospital pricing at the upper end and often excluding the consultant's fee. Our guide to MRI versus CT sets out which tool suits which question, and our breakdown of private CT scan costs in the UK covers the pricing in detail. Prices change, so treat these as market ranges rather than quotes.

Justification: why a scan should have a reason

In the UK, every medical exposure to ionising radiation must be justified in advance under the Ionising Radiation (Medical Exposure) Regulations 2017, usually shortened to IR(ME)R. The regulations apply to private clinics as much as to NHS departments, and the Care Quality Commission enforces them in England.

In practice this means a registered practitioner at the clinic has to agree that your scan is warranted before it goes ahead, taking into account what it will show, what has already been done, and whether a test without radiation would answer the question. Even if you have arranged and paid for the scan yourself, it is not automatic.

That is a protection, not an obstacle. If a provider offers you a CT with no clinical review at all, that is a reason to ask questions.

Where this argues against a scan

Being straight about this matters more than a booking.

Routine whole-body CT screening in people with no symptoms is not recommended by UK guidance. The dose is meaningful, and the bigger problem is incidental findings: harmless abnormalities that lead to more scans, more anxiety and sometimes more procedures without improving anything. Whole-body MRI screening avoids the radiation but not the incidental-findings problem.

And some symptoms need a doctor now rather than a scan you book yourself. Coughing up blood, a new breast lump, sudden severe headache, unexplained weight loss, or back pain with loss of bladder or bowel control all need urgent medical assessment, not a self-arranged appointment in three weeks' time.

Frequently asked questions

How much radiation is in a CT scan? It depends on the body part. Using Public Health England's published typical figures, a CT of the head is around 2 mSv, a CT of the chest around 8 mSv and a CT of the abdomen and pelvis around 10 mSv. Modern scanners and protocols often deliver less than these figures.

Is a CT scan dangerous? One CT scan carries a very small increase in lifetime cancer risk, not a large one. The published estimate for a CT of the abdomen and pelvis is in the region of one in 2,000 for an adult. That is small enough that a scan which answers a real clinical question is normally worth having.

How many CT scans are too many? There is no fixed number and no annual allowance for patients. Dose accumulates, so each scan adds a little, but the right question is always whether this particular scan will change what happens next. Tell any clinic what imaging you have already had so they can take it into account.

Do MRI and ultrasound use radiation? No. Neither uses ionising radiation. MRI uses a strong magnet and radio waves; ultrasound uses sound waves. That is one reason both are preferred for younger patients, in pregnancy and for repeated monitoring, when they can answer the clinical question as well as CT would.

Is it safe to arrange a private CT scan myself? Every medical exposure in the UK has to be justified in advance under the Ionising Radiation (Medical Exposure) Regulations 2017, whether it is NHS or private. A registered practitioner at the clinic must agree the scan is warranted, so a self-referred CT is still reviewed before it happens.

If a CT is the right test for your situation, you can compare CT scans and prices across UK clinics and ask each centre what dose-reduction protocols it uses.