Your personal radiation dose budget

The unit

Ionising radiation dose is measured in sieverts (Sv). One sievert is a very large dose - far more than anyone gets in normal life - so the practical unit is the millisievert (mSv, one thousandth of a sievert). The published global-average annual dose per person, summed across all sources, is roughly 3 mSv per year. The figure varies by country and individual, but the structure of the breakdown is broadly stable across the surveyed world^[1].

The canonical reference is UNSCEAR, the UN Scientific Committee on the Effects of Atomic Radiation. UNSCEAR's periodic Sources, Effects and Risks of Ionizing Radiation reports publish the breakdown by component, with national variation tables in the annexes^[2].

The actual breakdown

The slices (rounded; exact figures vary by source and country):

• Indoor radon (and thoron) - roughly 1.3 mSv, ~42% of the global average. The single largest slice, and the only one with an action threshold most readers can actually act on. See the radon primer.
• Medical procedures (mostly CT scans) - roughly 0.6 mSv, ~20%. A heavy distribution: most readers in any given year get zero medical radiation; a few get a lot. CT scans are the dominant source within the medical category. Image Gently and Choosing Wisely have published patient-facing scripts for asking whether MRI or ultrasound could substitute, and whether the dose can be lowered^[3].
• Terrestrial gamma radiation - roughly 0.5 mSv, ~16%. Naturally occurring gamma rays from uranium, thorium, and potassium-40 in soil and building materials. Granite countertops, brick houses, and concrete made from alum shale contribute a measurable but not problematic amount. There is no meaningful individual action.
• Cosmic radiation - roughly 0.4 mSv, ~13%. From space, screened out by the atmosphere. The dose roughly doubles for every 2000 metres of altitude gain. Frequent flyers and people who live in high-altitude cities (La Paz, Denver, Lhasa) accumulate a few tenths of an mSv per year more than people at sea level. Airline crew are tracked as occupationally exposed in some countries.
• Internal (ingested potassium-40, polonium-210, etc.) - roughly 0.3 mSv, ~9%. Naturally occurring radioactive elements in food and water. The largest single contributor is potassium-40 in your body's own tissue, mostly muscle. There is no meaningful individual action.

That accounts for the global-average annual dose. The cinematic sources - civilian nuclear-power-plant releases, weapons-test fallout, the 1986 Chernobyl accident, the 2011 Fukushima accident - contribute on average less than 0.01 mSv per year per person globally, less than one third of one percent. They dominate the news; they do not dominate the actual dose budget for almost anyone reading this. (The exception, obviously: people who live within tens of kilometres of an active release, or who lived in specific contaminated zones in the 1986-1995 window; that is a different conversation from typical-person background.)

What you can actually move

Of the five major slices, only two are meaningfully controllable by an individual:

• Indoor radon is controllable. You can test your home (a $20 kit), and if it tests above the WHO reference level you can mitigate (typically $500-1,500 for a certified sub-slab depressurisation system in affected countries). That single decision can knock a measurable fraction off your annual dose, and a much bigger fraction off your lung-cancer risk if you also smoke. The project card walks through the steps.
• Medical imaging is partially controllable. Many CT scans are clinically necessary and you should get them. Some are not, and the conversation about whether MRI or ultrasound could replace CT for a given indication, or whether the dose can be lowered, is a published, peer-reviewed conversation. Image Gently is paediatric-focused; Choosing Wisely is multi-specialty^[3].

The other three slices - cosmic, terrestrial, internal - are not meaningfully controllable. You can move to a lower altitude or stop eating bananas, but neither has any health effect proportionate to the lifestyle cost. The right response to background is to know it exists, know its scale, and ignore it.

The big-picture point

The single biggest mistake in popular radiation coverage is confusing the most-cinematic source with the most-dose-contributing source. The most-cinematic sources (reactors, weapons, X-ray machines on TV) are responsible for a tiny fraction of the average person's dose. The largest source is the colourless gas seeping out of the ground under your house, which most people have never tested for. The second-largest source for many readers is the CT scan they consented to without asking whether there was an alternative.

Both of those are actionable, with $20 kits and one-sentence patient-facing scripts. Most of the rest of the radiation conversation is theatre - not in the sense that it is unimportant, but in the sense that it has no bearing on the dose that any normal person actually accumulates from year to year.

Sources

1. World Health Organization. Ionizing radiation: health effects and protective measures. https://www.who.int/news-room/fact-sheets/detail/ionizing-radiation-health-effects-and-protective-measures
2. UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation). Sources, Effects and Risks of Ionizing Radiation - 2017 Report, Annex A: Methodology for estimating public exposures, and earlier UNSCEAR reports. https://www.unscear.org/unscear/en/publications.html
3. IAEA Radiation Protection of Patients (RPOP); Image Gently Alliance; Choosing Wisely (US/Canada/UK). https://www.iaea.org/resources/rpop ; https://www.imagegently.org/ ; https://www.choosingwisely.org/

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