Right now, as you read this, tiny bullets of radiation are flying through your body. A few come from the banana you might have had at breakfast; some rain down from exploding stars far across the galaxy; most seep quietly up from the rocks and soil under your feet. You cannot feel them, see them or smell them — and almost all of the time they do you no harm at all.

The alpha, beta and gamma radiation given off by unstable nuclei is ionising: it can knock electrons off the atoms it passes through. That single ability is the whole story of this page. It is exactly what makes ionising radiation genuinely dangerous — it can damage the molecules inside living cells — and, in careful hands, exactly what makes it enormously useful, powering everything from smoke alarms to cancer treatment. Same physics; the difference is only how, and how much.

Background radiation: the gentle rain we live in

You do not need a nuclear power station nearby to be exposed to radiation. There is a low, steady level of it everywhere, all the time, called background radiation. It has always been here — life on Earth evolved bathed in it — and a Geiger–Müller tube left ticking on an empty table will still click away, several times a second, on background alone.

Most of that background is entirely natural, and only a small slice is artificial (made by people):

• Radon gas — by far the biggest single source. Radon is a radioactive gas that seeps out of the uranium naturally present in rocks and soil, then collects in the air of houses and mines. Because you breathe it in, its alpha radiation acts from inside your lungs.
• Rocks, ground and buildings — gamma rays from the natural radioactive elements in granite, brick and concrete.
• Food and drink — living things take in small amounts of radioactive isotopes such as carbon-14 and potassium-40, so your own body is very slightly radioactive.
• Cosmic rays — high-energy particles that stream in from the Sun and from distant stars, and slam into the top of the atmosphere.
• Medical — mostly X-rays and scans. This is easily the largest artificial source, though still far smaller than radon.
• Everything else — nuclear power, weapons-test fallout and industry together add up to well under one percent. (This surprises people: nuclear power is a tiny sliver, not the main source.)

The exact mix varies from place to place. Fly in an aeroplane and your cosmic-ray dose shoots up because there is less air above you to soak it up. Live in a granite region such as Cornwall and your radon dose can be several times the national average. Explore the breakdown below — pick each source to see its share and why it's there.

Choosing the right radiation for the job

Here is the key idea that ties every use together: engineers and doctors don't just grab "some radiation." They pick the type — alpha, beta or gamma — and the half-life that match the task, using how far each type penetrates and how long the source stays active. Get the match right and the same rays that could hurt you become a precise tool.

• Smoke alarms — alpha (α). A speck of americium-241 ionises the air in a tiny chamber, letting a small current flow; smoke absorbs the alphas, the current drops, the alarm sounds. Alpha is perfect because it is intensely ionising (so a minuscule source does the job) yet stopped by a centimetre of air and the plastic case (so it can never reach you).
• Thickness gauges — beta (β). As paper, foil or plastic rolls off a machine, a beta source sits on one side and a detector on the other. The amount of beta getting through depends on the thickness, so the reading controls the rollers automatically. Alpha would be stopped by even the thinnest sheet; gamma would sail straight through whatever the thickness, so the reading wouldn't change — only beta is sensitive in the right range.
• Medical tracers & PET scans — gamma (γ), short half-life. A gamma-emitting isotope (such as technetium-99m) is swallowed or injected; because gamma passes out through the body, a camera outside can watch it flow and spot a blockage or a tumour. A short half-life matters: it must decay away quickly so the patient isn't left radioactive.
• Sterilising & treating — gamma (γ). Penetrating gamma from a cobalt-60 source kills the bacteria on surgical instruments and inside sealed food packets, and carefully aimed beams destroy cancer cells in radiotherapy. Here we want radiation that reaches deep and does damage — to the microbes or the tumour.
• Dating the past — half-life. The steady, clock-like decay of carbon-14 dates ancient wood and bone, while uranium isotopes locked in rock date the Earth itself. Nothing is "used up" here — the tool is the reliability of the half-life.

Notice the logic each time: how deeply it penetrates decides whether you reach the target or are safely stopped, and how long the half-life is decides whether the source lasts for years (a smoke alarm) or fades in hours (a tracer inside a person).

The danger: ionising radiation and living cells

The very thing that makes radiation useful — its ability to ionise, to knock electrons off atoms — is what makes it a hazard. When ionising radiation passes through living tissue, it can smash apart the delicate molecules a cell is built from, and the most important casualty is DNA, the instruction manual inside every cell.

• Low doses can mutate DNA. A cell usually repairs the damage or dies quietly, but occasionally a mutation slips through and, years later, can seed a cancer. Low doses raise a risk; they don't guarantee harm.
• High doses kill so many cells so fast that whole tissues fail — this is radiation sickness: burns, hair loss, sickness, and at extreme doses, death.

Because the harm depends on both the type of radiation and the tissue it hits, we don't just count particles — we measure the biological dose in sieverts (Sv). A sievert is a big unit, so everyday doses are quoted in millisieverts (mSv): a typical person receives a couple of mSv a year from background, and a single chest X-ray is a small fraction of that. Higher dose means higher risk — which is exactly why the people who work with radiation are so careful.

Staying safe: shielding, distance, time — and keeping clean

You cannot switch radiation off, but you can cut how much reaches you. The people who work with sources rely on four simple ideas:

• Shielding — put matter in the way. Alpha needs only paper or gloves; beta a few millimetres of aluminium; penetrating gamma needs thick lead or concrete.
• Distance — step back. Radiation spreads out, so its intensity falls off fast with distance; handling a source with long tongs keeps it well away from you.
• Time — limit how long you're exposed. Half the time means half the dose.
• Protection & monitoring — protective clothing and gloves, and a dosimeter or film badge worn to track exactly how much dose has built up.