Ionising radiation is energy released when unstable atoms break down, and it injures people by knocking electrons off the atoms in their cells; no human sense can detect it. Without instruments a settlement controls exposure through three levers only: less time near a source, more distance from it, and more mass between it and people.
Part of Radiation Safety
Radioactivity is the spontaneous break-up of unstable atomic nuclei, and the radiation it releases is the form of energy that damages living tissue. This page is the orientation for the rest of the section.
Radiation Is Energy Leaving Unstable Atoms, Not A Substance
The parent article Nuclear Safety defines radiation as energy released by unstable atoms as they break down into more stable forms. Two words get confused and the confusion causes real mistakes. A radioactive material is the stuff: dust, a metal capsule, a fuel rod, a patch of soil. Radiation is what the stuff emits. You can be struck by radiation from a distance and carry nothing away with you; or you can carry the material itself, on your boots or in your lungs, and go on being irradiated by it wherever you walk. The difference decides almost every practical rule in this section; it has its own page, Contamination vs Irradiation.
Some radiation is harmless in the ordinary sense (visible light, radio waves). The kind that matters here carries enough energy per particle or photon to strip electrons from atoms. That is ionising radiation, and it damages DNA, kills cells and causes cancer, as the parent article puts it. See Ionizing vs Non-Ionizing.
No Human Sense Detects It
Ionising radiation has no smell, taste, sound or visible glow that a person can rely on. A person standing in a lethal field feels nothing at the time. The symptoms arrive later, which is why Biological Indicators can only tell you what has already happened. Detecting radiation takes either an instrument or a crude device such as an electroscope, and the limits of those devices matter as much as their use; see Detection Methods.
Time, Distance And Shielding Are The Only Levers
The parent article sets out three rules, and everything else in this section elaborates them:
- Time. Half the time in a field means half the dose, other things equal.
- Distance. For a small source in open air, intensity falls with the inverse square of distance: double the distance and the dose rate falls to one quarter.
- Shielding. Dense material between a person and a source reduces gamma radiation, measured in half-value layers; see Half-Value Layers.
Without an instrument none of the three can be tuned to a measured number. What a settlement can do is manage them by assumption: treat unknown areas as hazardous, keep visits short, stay far from anything suspect and put mass around where people sleep. That is an argument for conservatism, not for inventing a number from a home-made gadget.
The Hazard Does Not End With The Collapse
The parent article states that there are over 440 nuclear reactors worldwide, plus thousands of medical, industrial and research radiation sources. Without trained operators, cooling fails, containment degrades and material escapes. None of this fades by itself on a human timescale for the longest-lived isotopes; see Half-Life.
Where To Go Next In This Section
- Physics: Types of Radiation, Units of Measurement.
- The body: Health Effects.
- Finding and mapping it: Detection Methods, Contamination Zones.
- Protection: Shielding & Protection.
- Facilities and food: Nuclear Facility Hazards, Food & Water Safety.
Safety
You cannot feel a dangerous dose
A person cannot tell a safe area from a lethal one by sensation. If you have no calibrated instrument, treat any area you cannot rule out by distance, time since the event or knowledge of the site as hazardous, and keep exposures short and rare. Never decide an area is safe because nothing felt wrong.
What’s Next
- Learn next: Types of Radiation — the four kinds and what each does
- Combine with: Nuclear Safety — the parent overview this section expands