Fallout is carried by wind and deposited unevenly, with rain and low ground concentrating it into hot spots, and its dose rate falls fast at first: by the 7-10 rule it drops by a factor of 10 for every sevenfold increase in time since the event. The rule is a rough guide for a single detonation, so most of the dose an unsheltered person would collect arrives in the first two days.
Part of Radiation Safety
Fallout is radioactive material lifted into the air by a nuclear detonation or a reactor release and carried by wind before it settles on the ground.
Wind Carries It And Rain Brings It Down
Fallout moves with the wind, settles downwind and arrives later at greater distance. Wind is named for where it comes from, so a west wind carries material east; see Wind Direction Reading. Rain can bring airborne material down in one place, so a district under a shower after an event may be far hotter than a dry district near it. The parent article, Nuclear Safety, warns that rain collected after a contamination event may carry fallout, while rain collected before it is safe. How the wind itself is monitored is in Weather Monitoring.
Terrain And Surfaces Redistribute It
After settling, material is moved by water and wind. It collects in ditches, hollows, the foot of slopes and drains, and under roof outlets; smooth hard surfaces shed it and rough, vegetated or soft ones hold it. This is general radiation-protection teaching, recalled and not from the corpus. Mapping these places is the work of Contamination Zones.
Source Matters: Weapon Versus Reactor
A detonation near the ground draws up soil, which carries more material down close to the site, while an air burst lofts material higher and produces much less local fallout. A reactor accident releases the more volatile products, such as iodine and caesium, in proportion to what escapes. Both statements are recalled general knowledge, not from the corpus, and should be checked. The practical consequence is that the 7-10 rule below suits fission products from a detonation and does not describe a reactor release or a spill from a single source.
The 7-10 Rule
The parent article gives it: radiation falls by a factor of 10 for every 7-fold increase in time. At 7 hours it is 1/10 of the 1-hour level, at 49 hours 1/100 and at 2 weeks 1/1,000. The check on the last step: 49 x 7 = 343 hours = 14.3 days.
Treated as a power law, the rule’s exponent is 1 / log10(7) = 1 / 0.845 = 1.18, near 1.2. That lets us estimate how the dose accumulates, as a derived illustration only. The dose between times a and b hours is proportional to (a^-0.2 - b^-0.2) / 0.2, in units of the rate at 1 hour multiplied by hours:
- 1 to 48 hours: 5 x (1 - 48^-0.2) = 5 x (1 - 0.461) = 2.69.
- 48 to 336 hours (2 weeks): 5 x (0.461 - 0.312) = 0.74.
- Share of the two-week total received in the first 48 hours: 2.69 / (2.69 + 0.74) = 78 percent.
So for a person kept outdoors from hour 1, about three-quarters of the two-week dose comes in the first two days. Shelter early and stay: the parent says at least 48 hours, ideally 2 weeks. Planned short excursions after the first days cost far less than the same excursions earlier; see Shelter Design.
Common Mistakes
- Applying the rule to everything. It is for a single detonation; the parent article calls it the 7-10 rule for fallout.
- Assuming the ground is clean when the sky is clear. Material is on the ground, not in the sky, after it falls.
- Believing decay solves long-lived isotopes. Caesium-137 persists for generations.
- Treating the 78 percent as a protection figure. It describes unsheltered accumulation under an idealised model.
Safety
The rule gives shape, not a safe time to go outside
Without a dose-rate reading you do not know the starting level, so the rule cannot tell you when the rate has become tolerable. Stay in until at least the parent’s 48 hours if you can, limit outings, and treat the ground as contaminated; see Personal Protection.
What’s Next
- Learn next: Weather Monitoring — watching the wind that carries it
- Combine with: Shelter Design — spending the dangerous days well