A four-zone scheme, green, yellow, orange and red, turns survey results into rules about who may enter an area, for how long and with what protection. Without a calibrated instrument no area can be classed green, because the evidence for green is a measured reading, and the zones here are defined by evidence and permitted activity rather than by dose-rate numbers that cannot be sourced here.
Part of Radiation Safety
Zone classification sorts ground into categories of increasing restriction so that a whole settlement follows the same rules without having to understand the measurements.
Why Colours Instead Of Numbers
Professional zone schemes are written in dose rates, with unit, quantity type and averaging time. A settlement without a calibrated instrument cannot apply them, and this guide supplies no dose-rate boundaries because none are sourced in the corpus. A zone is therefore defined by the strength of the evidence and by what it allows. If a calibrated instrument is later salvaged and checked, the community can attach numbers from a reliable source to the same four categories. Do not invent boundaries from an electroscope; see Improvised Detectors.
The Four Zones
| Zone | Evidence required | What it permits |
|---|---|---|
| Green | A working, checked, calibrated instrument shows readings indistinguishable from the reference area | Living, farming, normal use |
| Yellow | No indication found, but the evidence is indicators only, or old | Transit and short tasks; no sleeping, no growing or gathering of food, no drinking water |
| Orange | Indication of elevated radiation or contamination, or a known source | Entry for a specific task only, in protective clothing, with a time limit, a log and decontamination on exit |
| Red | Strong indication, a known unshielded source, or a damaged facility | No entry; distance and marking only |
With only a home-made electroscope, the best available class is yellow. An area never surveyed is unclassified and is treated as the most restrictive zone consistent with the task, usually orange for a short purposeful visit, red near any facility.
Zones Can Only Move One Way Without Evidence
Moving an area toward green needs better evidence, not the passage of time alone. Moving it toward red needs only a worse reading or a new report. Time does help, since the fallout rule of thumb in Fallout Patterns shows rates falling quickly in the first days, but long-lived isotopes in soil do not fade on a human scale; see Half-Life.
Worked Example: A Settlement With One Electroscope
A river road shows no change from baseline over four trips, and a drainage ditch beside it shows a fast fall twice. The road is yellow: usable for transit, not for camping. The ditch is orange or red depending on the result: marked and kept clear. The fields north of the road were never visited: unclassified and treated as orange for any task there. Nothing is green.
Common Mistakes
- Treating yellow as green. Yellow means “not found”, which is not “not there”.
- Setting zone edges at convenient lines. Edges follow data, with margin.
- Not telling newcomers. A scheme works only if everyone knows what the colours mean; see Signage & Markings.
Safety
The colour is a rule, not a guarantee
Green means checked with a real instrument at a given date, and yellow means unconfirmed. Never allow sleeping, food growing or water collection in a zone that has not been checked with a calibrated instrument. See Exclusion Zones for enforcement.
What’s Next
- Learn next: Exclusion Zones — marking and enforcing red and orange
- Combine with: Survey Procedures — the evidence behind each class