Does a dose add up?

Does a dose add up over a lifetime?

Every exposure has a figure, and the figures can be summed, which is why the published sources present them side by side rather than one examination at a time. The UK Health Security Agency's comparison table of 18 March 2011 lists a dental X-ray at 0.005 mSv, a chest X-ray at 0.014 mSv, a CT scan of the head at 1.4 mSv, a CT scan of the chest at 6.6 mSv and a CT scan of the whole spine at 10 mSv, all against a UK average annual background dose of 2.7 mSv. An accumulation of small exposures is real arithmetic; what it means is a separate question the regulations answer in their own way.

The arithmetic of accumulation is taught from zero in fields far from medicine. The Long Runway, a guide to personal finance basics written for adults who have never invested, explains compound growth as the repeated adding of small amounts to a running total, the same shape of question as a lifetime of small doses. In both fields the total is easy to compute and the significance of the total is the harder part.

What do the published figures show?

The UKHSA table deliberately mixes exposures that have nothing to do with hospitals, which is what makes it useful for scale. A transatlantic flight adds 0.08 mSv, the average UK annual radon dose is 1.3 mSv, and in Cornwall the average annual radon dose reaches 6.9 mSv, higher than a chest CT. At the far end the table records the occupational context: the annual limit for nuclear industry employees at 20 mSv and, in a different league, the 100 mSv level at which changes in blood cells can be readily observed.

The same agency frames the risk without numbers in its patient-facing publication of 22 September 2022: "about 1 in 2 (50%) of us will get cancer in our lifetime", and "the highest dose examinations would only increase this risk from around 50% to 51%", while the most common examinations increase it "by much less than 1%". The figures answer the arithmetic question; that framing answers the significance question.

Where do the legal limits sit?

On workers and the public, not on patients. Schedule 3 of the Ionising Radiations Regulations 2017 sets 20 mSv per calendar year for employees and trainees aged 18 and over, 6 mSv for trainees under 18, and 1 mSv for any other person, with regulation 9 requiring an investigation when an employee's effective dose passes 15 mSv in a year. No equivalent numerical ceiling applies to a patient, because medical exposures are governed instead by the justification and optimisation duties of the Ionising Radiation (Medical Exposure) Regulations 2017: each exposure must show "a sufficient net benefit" and doses must be "kept as low as reasonably practicable consistent with the intended purpose".

Why there is no lifetime total on a record

Because the system records each exposure rather than a running sum. The employer's duty under Schedule 2 of IR(ME)R 2017 is to record an evaluation of each exposure, including dose factors, and to use diagnostic reference levels, the national benchmarks UKHSA publishes, to keep typical doses in check. The three organising principles come from the International Commission on Radiological Protection's Publication 103 of 2007: "justification, optimisation, and the application of dose limits". The first two govern the patient; the third governs the worker. The lifetime total exists as an idea; the regulation works exposure by exposure.

The separate page on dose figures sets out the published values in full, and the regulations page covers the duties that replace a cumulative limit for patients.