How fast does nuclear fallout decay? The 7-10 rule

Short answer. Fallout from a nuclear detonation weakens fast. By the "7-10 rule", every sevenfold increase in time after the explosion cuts the dose rate tenfold: about one-tenth of the 1-hour level at 7 hours, one-hundredth at 49 hours (about 2 days) and one-thousandth at about 2 weeks ¹. U.S. and Canadian agencies say to get inside and stay sheltered for at least 24 hours ³⁴⁵.

What is the 7-10 rule?

The 7-10 rule is a rule of thumb from The Effects of Nuclear Weapons by Glasstone and Dolan, a reference work published by the U.S. Department of Defense and the Energy Research and Development Administration ¹. It states that "for every sevenfold increase in time after the explosion, the dose rate decreases by a factor of ten" ¹.

The clock starts at the moment of the explosion, not when fallout lands where you are ¹³. The rule describes the dose rate, meaning how intense the radiation is at a given moment. It does not describe the total dose a person collects over time. Those are two different numbers, and both matter.

The U.S. Department of Health and Human Services' radiation emergency site (REMM) gives the same rule in plain terms: fallout "decays quickly, roughly by a factor of 10 for every 7-fold increase in time" ².

Where does t^-1.2 come from?

Fallout is a mix of many fission products. Each one decays at its own rate, and the short-lived ones fade first. Added together, the mixture follows a curve close to a simple power law: the dose rate falls roughly in proportion to time raised to the power -1.2 (written t^-1.2) ¹.

The 7-10 rule is that curve in a form you can do in your head. Raising 7 to the power 1.2 gives about 10. So each time the elapsed time is multiplied by 7, the dose rate is divided by about 10.

Glasstone and Dolan state the limits clearly. The 7-10 rule is "accurate to within about 25 percent up to 2 weeks or so" and within a factor of two up to about 6 months ¹. They also call it useful "only for illustration and some planning purposes", because real fallout varies with how particles form and what materials are activated ¹. The rule assumes one detonation at a known time. It does not replace a radiation meter or official instructions.

How fast does the dose rate fall?

The table uses the 1-hour dose rate as 100%.

Glasstone and Dolan add a second reading of the rule: at the end of 1 week, the dose rate is about one-tenth of its value after 1 day ¹.

REMM's example for a hypothetical 10-kiloton explosion gives similar figures: initial exposure rates down to 20% after 3 hours, 10% after 8 hours and 1% after 48 hours ². REMM does not state the reference time for these percentages, so they do not line up exactly with the table. Both show the same pattern: most of the drop happens in the first two days.

Why do the first 24 to 48 hours matter most?

Because the radiation is at its strongest when it is youngest. FEMA's planning guidance says fallout "gives off over half of its energy in the first hour" and then keeps decaying rapidly ³. It adds that the most dangerous concentrations of fallout particles usually land within the first few hours, often tens of miles downwind ³. REMM puts it simply: exposure to fallout "is the most dangerous in the first few hours" ².

The total dose tells the same story. Glasstone and Dolan give an example where fallout arrives 6 hours after the explosion with a dose rate of R per hour. An unprotected person would receive about 9R in 1 day, 12R in 2 days and 16R in 5 days ¹. More than half of the 5-day dose comes in the first day. Staying behind good shielding during that first day avoids most of the exposure.

Two practical points follow:

• Fallout takes a little time to arrive. Ready.gov says you may have 10 minutes or more to reach adequate shelter before fallout arrives ⁴.
• Waiting pays off quickly. Moving through a contaminated area at hour 1 means about ten times more exposure than at hour 7, and about a hundred times more than at hour 49 ¹. Under the rule, the hours you stay sheltered at the start count far more than hours later on.

How much a building protects you depends on its mass and layout. FEMA considers shelters adequate if they cut the dose by a factor of 10 or more, and points to basements and the middle of large, dense buildings ³. Judging whether a specific building or structure gives that protection is a job for a qualified engineer, not a rule of thumb.

What official agencies recommend

Agencies agree on the first step: get inside and stay there. They differ slightly on how long.

FEMA's core public message is "Get Inside, Stay Inside, Stay Tuned" ³. Get inside a basement or the middle of a large building. Stay inside unless there is an immediate threat such as fire, building collapse or a medical emergency. Stay tuned to radio, television or phone alerts for instructions ³. Because fallout direction is hard to predict at first, FEMA's guidance tells planners to give this message to everyone within 50 miles until hazard areas are identified ³.

Note the wording: 12 to 24 hours, or 24 hours, is a minimum, not a release time. Authorities may tell people in the most affected areas to stay longer, or to move to better shelter once radiation levels have dropped ³. The decision to leave should come from official instructions, not from the clock alone.

How does this affect how much water and food to store?

The 7-10 rule sets the timeline that your supplies have to cover. Official advice asks you to be ready for at least 24 hours in place ³⁴⁵, and that minimum can be extended. Power, water and shops may also be disrupted for longer: FEMA expects power and other infrastructure to be out close to the blast ³.

Ready.gov's general advice is to keep food, water and other supplies "to last for several days", including one gallon of water per person per day ⁶. Other countries set different amounts. See our guides on how much water to store per person and what goes in a 72-hour kit for the country-by-country figures.

Frequently asked questions

Is fallout safe after 48 hours?

Not automatically. At about 49 hours, the dose rate is roughly 1% of its 1-hour value ¹. But if the starting level was very high, 1% can still be dangerous. Only measurements and official instructions can say when an area is safe ³.

Does the 7-10 rule apply to a nuclear power plant accident?

It was written for fallout from nuclear explosions ¹. FEMA notes that, in contrast to a nuclear power plant incident, most fission products from a nuclear detonation are short-lived, which makes them most hazardous in the first few hours to days ³. Do not apply the rule to a reactor accident; follow the advice of local authorities for that kind of emergency.

Why do agencies say 12 to 24 hours and not 2 weeks?

The danger is highest in the first hours, when fallout gives off over half of its energy in the first hour ³. Sheltering through that window avoids most of the dose. After that, authorities can measure levels and decide whether people should stay, move to better shelter or leave ³.

Can I use the rule to decide when to go outside?

The 7-10 rule is a planning estimate, accurate to about 25% for the first 2 weeks ¹. It assumes a single explosion at a known time. Without a radiation meter and official guidance, it cannot tell you the actual level outside your door.

Plan your shelter time

The 7-10 rule shows that the first day decides most of the exposure. The Phaenyx personal plan brings the official guidance for your country together in one written plan: where you would shelter, for how long, and what supplies that time requires. Build your Phaenyx plan.

Sources

1. Glasstone, S. & Dolan, P. J. — The Effects of Nuclear Weapons (3rd ed., 1977), Chapter IX, §9.15, §9.25, §9.28 — https://www.atomicarchive.com/resources/documents/effects/glasstone-dolan/chapter9.html (accessed 2026-10-20)
2. REMM, U.S. Department of Health and Human Services — Nuclear Detonation: Weapons, Improvised Nuclear Devices — https://remm.hhs.gov/nuclearexplosion.htm (accessed 2026-10-20)
3. FEMA — Planning Guidance for Response to a Nuclear Detonation, Third Edition (2022), Chapters 1-3 (PDF pp. 38-76) — https://www.fema.gov/sites/default/files/documents/fema_nuc-detonation-planning-guide.pdf (accessed 2026-10-20)
4. Ready.gov — Radiation Emergencies — https://www.ready.gov/radiation (accessed 2026-10-20)
5. Government of Canada — Radiological and nuclear emergencies – Get prepared — https://www.canada.ca/en/services/policing/emergencies/preparedness/get-prepared/hazards-emergencies/radiological/how-prepare.html (accessed 2026-10-20)
6. Ready.gov — Build A Kit — https://www.ready.gov/kit (accessed 2026-10-20)