How Long Does Nuclear Fallout Last? The 7-10 Rule Explained

Most of the radiation in nuclear fallout is gone within two days. That is the short answer to “how long does nuclear fallout last,” and it is why official advice is to get inside and stay inside for at least 24 hours rather than to flee. The longer answer has a second part, because a small share of the contamination lasts for years, and people confuse the two.

This article explains the 7-10 rule, the math behind it, what it does and does not tell you, and why the cities hit in 1945 are inhabited today. If you want the full emergency timeline, start with how to survive a nuclear attack. To run the numbers yourself, use the fallout decay calculator.

The 7-10 rule: how long nuclear fallout lasts, in one table

Fallout is made mostly of fission products, atoms left over when uranium or plutonium splits. Hundreds of different isotopes form (REMM counts more than 300). Many of them are intensely radioactive for a short time. That is why the radiation fades quickly at first, then slowly.

The rule of thumb is this: for every sevenfold increase in time after the detonation, the radiation dose rate falls by a factor of ten.

Each step multiplies time by 7 and divides the dose rate by 10. The rule is a simplified form of the Way-Wigner decay law, which says dose rate falls as time to the power of about -1.2. Check the math: 7 to the power 1.2 is about 10.3. So a sevenfold time increase gives roughly a tenfold drop.

The formula works out to these values (our calculation, from the same approximation):

REMM, the HHS radiation medical resource, gives a similar example for a 10 kiloton detonation: about 20% of the initial exposure rate after 3 hours, 10% after 8 hours and 1% after 48 hours. The small differences from our table come from rounding and from how the curve is fitted. Both agree on the shape: the steepest fall is in the first day.

FEMA’s Planning Guidance for Response to a Nuclear Detonation states the same idea in plain words: fallout gives off over half of its energy in the first hour and then continues to decay rapidly.

Data: The standard source behind these rules is Glasstone and Dolan’s The Effects of Nuclear Weapons (1977). The t^-1.2 approximation is a good average for fresh fission-product fallout, but a fallout study published through DTIC reports that it is unreliable before about 30 minutes and after about 6 months.

Why the first 24 to 48 hours matter most

A percentage hides the real point. What hurts you is dose, the amount of radiation you absorb over time. Dose is the dose rate multiplied by how long you stand in it.

Here is our own arithmetic, assuming a made-up starting level of 100 roentgen per hour at one hour after detonation (FEMA’s own decay chart uses the same round number) and a person standing in the open the whole time. It integrates the 7-10 curve:

Look at the first row against the rest. Of the dose you would pick up outdoors in the first two days, about 87% arrives in the first 24 hours. That is why a day in a shelter does so much work. A shelter with a protection factor of 10 divides each number by 10.

Look at the last two rows too. The decay is fast, but not instant. A level that starts very high can still deliver a significant dose in the days after. This is why the official guidance says “at least” 24 hours and then “follow instructions,” not “go outside after a day.” Officials will use radiation readings to say where and when.

This is a model with an arbitrary starting point, not a prediction. Real dose rates at your location could be far higher or lower, or zero.

What official guidance says about shelter time

• Ready.gov: stay in the most protective spot for the first 24 hours unless there is an immediate hazard or authorities say otherwise. It also says sheltering usually lasts at least 24 hours. See Ready.gov: Nuclear Explosion.
• FEMA: “Get Inside, Stay Inside, Stay Tuned” for 12 to 24 hours as a baseline, unless provided additional guidance. It adds that optimal shelter times can range from a few hours to several days, depending on local dose rates and the dose picked up during evacuation.
• CDC: the dangerous radiation zone is defined as above 10 roentgen per hour and can lie 10 to 20 miles downwind.

FEMA’s description of the dangerous zone shows the rule in action: it reaches its maximum extent after the first few hours, then shrinks, perhaps from tens of miles to a mile or two in one day. The wider “hot zone” (0.01 to 10 R/h) can stretch hundreds of miles downwind and starts to shrink within 24 hours.

Practical consequences:

1. The first few hours are the dangerous ones. Be in the best shelter you can reach before fallout lands. See our apartment shelter guide if you do not have a basement.
2. Treat 24 hours as a minimum, not a countdown to freedom.
3. Leave only when officials tell you it is safe and where to go, or if fire or collapse forces you out.
4. If you have a dosimeter or a radiation meter that officials trust, readings beat calculations. Without one, follow the radio.

When does fallout arrive?

Not at once. Fallout is dirt and debris pulled into the fireball, made radioactive and carried upward in the cloud. It comes down as the cloud cools and drifts.

FEMA says the particles fall out of the cloud within minutes to hours after detonation, and that in many areas fallout will arrive an hour or more after the explosion. Ready.gov’s conservative figure is that you will have 10 minutes or more to find shelter before it arrives. Close to ground zero and in the heaviest zone, it comes sooner. Farther downwind, it arrives later and is lighter, because winds at different altitudes carry the particles different distances.

We do not have a trustworthy table of arrival time by distance, because it depends on yield, burst height and the wind at every altitude, and we will not make one up. The useful rule is the official one: shelter within the first 10 minutes, and do not assume a long delay.

FEMA also notes that fallout particles near the blast are large, like sand, so inhaling them is a smaller hazard than the radiation from particles lying on the ground. Most of the risk comes from standing near that dust, not from breathing it.

Fallout versus long-term contamination

Two different questions hide under “how long does radiation last.”

Dangerous levels: For a nuclear detonation, FEMA says most of the fission products are short-lived and more likely to produce local fallout, which makes them most hazardous in the first hours to days. This is the problem the 7-10 rule describes.

Residual contamination: A smaller part of fallout is long-lived. Cesium-137, for one, has a half-life of about 30 years, according to the EPA. Strontium-90 is also long-lived. These isotopes do not matter much for a person sheltering in the first week, because their share of the early dose is small, but they can limit land use, food and water for years. Officials, not individuals, decide those restrictions.

FEMA gives a modern example of how far low-level fallout can travel. It cites the 1953 Upshot-Knothole Simon test (43 kilotons on a 300-foot tower), where assessed dose rates were still above 1 milliroentgen per hour in parts of the country 36 hours after the detonation, largely because of rainout east of the Mississippi. That is far below dangerous levels. It shows that “detectable” and “dangerous” are different words.

Note: Rain can matter. Rainfall can wash fallout out of the air and concentrate it on the ground in patches. That is one reason officials rely on measurements rather than on a formula.

Why Hiroshima and Nagasaki are inhabited, and Chernobyl is not the same story

People often ask how Hiroshima and Nagasaki could be rebuilt if radiation lasts so long. There are two reasons.

First, both bombs were airbursts. The fireball did not touch the ground, so it did not draw large amounts of soil into the cloud. FEMA says both cities produced minimal fallout because of the height of the explosion, but still had a dangerous radiation zone at ground zero that lasted about one day and a hot zone that lasted about five days.

Second, the arithmetic of decay did the rest. The City of Hiroshima’s official explanation says about 80% of residual radiation was emitted within 24 hours, that at the hypocenter one day later the level was about one thousandth of the initial amount and that a week later it was about one millionth. It says today’s radiation in both cities matches natural background levels found anywhere on Earth. Those are the city’s figures. They are not a promise about a modern ground burst, which produces far more fallout.

Chernobyl is a different case. A reactor accident at a nuclear power plant releases a different mixture of isotopes than a weapon does. FEMA says that in contrast to an NPP incident, most of the fission products from a nuclear detonation are short-lived. Reactor fuel has been burning for months or years, so it contains a large stock of long-lived isotopes such as cesium-137 and strontium-90, and the accident spread them over land for decades. The 7-10 rule is for fresh weapon fallout. Do not apply it to a reactor accident.

What the 7-10 rule does not tell you

• It is a dose-rate rule, not a dose rule. Time spent matters as much as the fraction left.
• It assumes fresh fission products. Neutron-activated soil near ground zero and mixed isotopes behave differently.
• It ignores weather. Rain, wind shifts and washing change local levels. FEMA says weather and terrain will likely create an irregular shape with hot spots.
• It cannot tell you where fallout fell. You cannot see or smell radiation. FEMA says visible dust is not a reliable guide, because particles may not be noticeable on rough or dirty surfaces.
• It does not set your leave time. The leave time depends on your shelter’s protection factor, your route and what officials tell you. Use the decay calculator to understand the curve, then follow the radio.

Frequently asked questions

How long does nuclear fallout last?

Dangerous fallout radiation falls fast. Under the 7-10 rule, about 10% remains at 7 hours, 1% at 2 days and 0.1% at 2 weeks. Official shelter guidance is at least 12 to 24 hours. Small amounts of long-lived isotopes such as cesium-137 persist for years, but they are not the main hazard in the first days.

What is the 7-10 rule for fallout?

For every sevenfold increase in time after detonation, the dose rate falls tenfold. If the rate is 100 at 1 hour, it is about 10 at 7 hours, 1 at 49 hours and 0.1 at 343 hours. It comes from the Way-Wigner approximation, where dose rate falls as time to the power of -1.2.

How long should you stay in a shelter after a nuclear bomb?

At least 24 hours, per Ready.gov; FEMA says 12 to 24 hours as a minimum unless provided additional guidance. Stay longer if officials say so, and leave sooner only if there is fire, collapse or another direct threat. Do not go outside to check. You cannot sense radiation.

Is it safe to go outside after 48 hours?

Not automatically. After 48 hours, the dose rate is around 1% of its hour-1 value, but that can still be high if the starting level was very high. Wait for official guidance. Officials use measurements to tell you which areas are safe and which routes to use.

Can you still live near where a nuclear bomb exploded?

Often yes. Hiroshima and Nagasaki are inhabited, and the city of Hiroshima says current radiation there matches natural background. That reflects airbursts and 80 years of decay. A ground burst leaves more fallout and longer restrictions in the worst-hit areas, and officials decide what is safe.

Does rain make fallout worse?

It can change where fallout lands. Rain can wash particles out of the air and concentrate them on the ground in patches. FEMA notes that weather and terrain create irregular patterns with hot spots. That is why local radiation readings matter more than a general formula.

Lab verdict

The 7-10 rule is real physics and it is good news. Fallout is at its worst early and decays fast, which is why a basic action, getting inside a sturdy building and staying 24 hours, prevents most of the harm. The rule is also easy to misuse. It describes dose rate, not total dose, it fits fresh weapon fallout and not reactor accidents, and it cannot replace a measurement.

Use it to understand why officials say 24 hours, not to schedule your own exit. Run your own scenarios in the fallout decay calculator. Pair that with a good shelter spot (apartment guide) and the right supplies. If officials advise it, potassium iodide protects the thyroid from radioactive iodine, which is a separate issue from external dose.

Sources

• FEMA: Planning Guidance for Response to a Nuclear Detonation, Third Edition (2022)
• Ready.gov: Nuclear Explosion
• REMM (HHS): Nuclear Detonation overview
• CDC: Nuclear detonations, preparing and planning
• Glasstone and Dolan, The Effects of Nuclear Weapons (1977), via OSTI
• EPA: Radionuclide Basics, Cesium-137
• City of Hiroshima: Is there still radiation in Hiroshima and Nagasaki?
• DTIC: fallout decay and exposure rate study (t^-1.2 limits)