What is a shelter's protection factor?

Short answer. A shelter's protection factor (PF) is the dose a person would receive in the open divided by the dose received inside ¹. A PF of 40 means receiving about one-fortieth (2.5 %) of the outdoor dose ⁵. A Lawrence Livermore review of about 400 building analyses found a typical PF of 10 in a house basement and about 600 in the basement of a heavy brick or concrete building ¹.

What exactly does "protection factor" measure?

Protection factor describes how much a location reduces the gamma radiation coming from fallout deposited outside, on the ground and on the roof. Lawrence Livermore National Laboratory (LLNL) defines it as the ratio of the "open field" dose, or dose rate, to the dose experienced inside the building ¹. The open-field reference is the dose measured about 1 m (3 ft) above a flat surface uniformly covered with fallout ¹.

Some documents express the same idea the other way round. A "reduction factor" or "transmission factor" is simply the inverse of the PF ¹. The U.S. federal planning guidance for a nuclear detonation uses "dose reduction factor" in the same sense as PF: a value of 10 means a person receives 1/10th of the dose of someone in the open, and a value of 200 means 1/200th ².

Three points help when reading PF figures:

• It is a ratio, not a dose. A PF says nothing about how much radiation is outside. The federal planning guidance notes that a shelter far from the heaviest fallout may be adequate even with little shielding, while the same shelter closer in may not be ².
• It varies inside one building. The same structure can offer very different protection from room to room and floor to floor ¹.
• It covers external gamma radiation from fallout. LLNL's figures concern radiation from particles deposited on the roof and surrounding ground. Breathing in contaminated air or eating contaminated food are separate pathways ¹.

How much protection do typical buildings give?

The LLNL review screened 26 published studies covering about 400 building analyses and reported a typical (median) value and the range for each building type and location ¹. Values are rounded in the source.

The ranges are wide. In house basements, the lower values came from basements with several walls above ground (for example on a slope) and from early times after a detonation. The higher values came from fully below-ground basements without windows and from later times ¹.

The same review compared its results with older summaries. For a frame house, Glasstone and Dolan's The Effects of Nuclear Weapons gives 10 to 20 in the basement and 1.7 to 3.3 above ground. For a multi-story apartment building, it gives 10 on lower stories and 100 on upper stories ¹.

The federal planning guidance sums it up in one sentence: houses with basements, large multi-story structures, parking garages and tunnels can generally reduce doses from fallout by a factor of 10 or more ². It adds that single-story wood frame houses without basements, and vehicles, provide only minimal shelter and should not be considered adequate in the area of dangerous fallout ².

What are the two levers: mass and distance?

Protection comes from two things working together: dense material between you and the fallout, and distance from where the fallout lands.

Mass. The federal planning guidance lists concrete, brick, stone and earth as good shielding materials, and wood, drywall and thin sheet metal as minimal shielding ². Cars are not adequate shelters because they lack good shielding material ². The 1966 Office of Civil Defense booklet for homeowners put the principle plainly: the heavier the material, the more the protection ⁵. Germany's civil protection advice from Berlin says the same: thick walls and earth increase protection against radiation ⁷.

Distance. Fallout settles on roofs and on the ground around the building. The same guidance notes that large buildings give the benefit of increased distance from deposited fallout when people gather away from exterior walls, and that this distance from exterior walls and roofs can substantially reduce dose ². The CDC tells people to stay as far away from the walls and roof of the building as they can ³.

The data show how much each lever matters. In heavy-construction buildings, moving away from windows and exterior doors on the same floor doubled protection, and moving to the best-protected floor increased it about twenty-fold ¹. In houses, moving away from windows gave only a modest gain (about 1.2 times), while going down to the basement increased protection about five-fold ¹.

Why do basements and building centers help?

A basement uses both levers at once. The surrounding earth acts as mass, and the occupants sit below the ground surface where the fallout lies, so direct radiation from outside has to pass through soil and foundation walls. The 1966 civil defense booklet explains that basements give the best home shelter because they are mostly below ground, which gives them a natural shield ⁵.

The center of a large building works mainly through distance and the many walls and floors in the way. Ready.gov advises putting as many walls as possible between you and the outside ⁴. The roof matters too: LLNL observed that protection factors above 1,000 were not found on the top floor, just below the contaminated roof ¹. In an empty supermarket with a light roof, radiation from the roof made up 66 to 89 % of the indoor exposure ¹.

One nuance is worth knowing. Inside a home basement, the best spot is not necessarily the middle. The 1966 civil defense booklet states that in nearly all basements the highest protection factor is in a corner and the lowest is in the center ⁵. Measurements in a basement under an empty metal shed showed the same pattern: lower protection near the basement center, higher near the edge ¹. "Go to the center" is advice for the above-ground floors of large buildings. Below ground, the corner furthest from any exposed wall tends to do better.

What official agencies recommend

• United States, CDC: go to the basement or the middle of the building, stay as far as possible from walls and roof, and close and lock windows and doors. A brick or concrete multi-story building or a basement is best, but any building is safer than being outside. Stay inside for at least 24 hours ³.
• United States, Ready.gov (FEMA): if a multi-story building or basement can be reached safely within a few minutes, go there immediately. The safest buildings have brick or concrete walls; underground parking garages and subways can also give good shelter. Remain in the most protective location (basement or center of a large building) for the first 24 hours unless threatened by an immediate hazard ⁴.
• United States, federal planning guidance (2010, maintained by FEMA): basements and large concrete structures are good examples of adequate shelter, generally reducing fallout dose by a factor of 10 or more ².
• Germany (Berlin district civil protection): go as deep as possible into interior rooms; cellars, underground car parks and subway stations offer the best protection ⁷.

Agencies differ on the numbers they attach to this advice. The federal planning guidance speaks of "a factor of 10 or more" for basements and large buildings ². A CDC video says the middle floors of a multi-story building give a protection factor of 100 ⁶. LLNL's review reports a typical 40 on the ground floor and 80 on upper floors of heavy-construction buildings, with very wide ranges ¹. The historical U.S. benchmark for marked public fallout shelters was a PF of at least 40 ⁵. These figures are compatible: they describe different buildings and different spots inside them.

None of these documents gives a method for calculating the PF of a specific building. That requires a site survey. Any change to load-bearing walls, floors or a basement ceiling to add shielding mass should be designed by a qualified structural engineer. Even the 1966 booklet warned that overhead shielding "requires very careful calculation of materials and fasteners to safely hold the heavy materials to be placed overhead" ⁵.

Frequently asked questions

What protection factor is considered "good enough"?

There is no single number for every situation, because adequacy depends on how much fallout is outside ². In the United States, public fallout shelters marked with the black and yellow sign historically had to offer a PF of at least 40 ⁵. The 2010 federal planning guidance says houses with basements, large multi-story structures, parking garages and tunnels generally reduce dose by a factor of 10 or more, and calls basements and large concrete structures good examples of adequate shelter ².

Is a car or a wood-frame house any protection?

Very little. The federal planning guidance says vehicles and single-story wood frame houses without basements provide only minimal shelter ². LLNL found a typical PF of about 2.2 above ground in houses with lightweight walls ¹.

Is the top floor of a tall building a good place to shelter?

Usually not the best one. LLNL found that protection factors above 1,000 did not occur on the top floor, directly under the contaminated roof ¹. CDC and Ready.gov point to the basement or the middle of the building ³⁴.

Does a higher PF mean zero radiation?

No. A PF of 40 still lets through about 2.5 % of the outdoor dose ⁵, and a PF of 10 lets through one-tenth ². That is why official guidance pairs good shelter with time: staying inside for at least 24 hours ³⁴.

Can I measure my own basement's protection factor?

Not reliably without instruments and a professional survey. Reported basement values in houses ranged from 6 to 67, depending on how much of the basement sits above ground and on timing ¹. A qualified engineer or radiation protection specialist can assess a specific building.

Get a shelter assessed for your site

Protection factor depends on the building, the soil and how much of the space sits below ground, so figures from studies are only a starting point. Have any structural work designed by a qualified engineer. Build your Phaenyx plan to see the risks around your home first.

Sources

1. Lawrence Livermore National Laboratory (Dillon, Kane, Nasstrom, Homann, Pobanz) — Summary of Building Protection Factor Studies for External Exposure to Ionizing Radiation, LLNL-TR-684121 (17 Feb 2016) — https://www.osti.gov/biblio/1256433 (accessed 2026-10-20)
2. U.S. Federal interagency committee led by the Executive Office of the President (maintained by FEMA) — Planning Guidance for Response to a Nuclear Detonation, Second Edition (2010) — https://19january2021snapshot.epa.gov/sites/static/files/2015-05/documents/planning-guidance-for-response-to-nuclear-detonation-2-edition-final.pdf (accessed 2026-10-20)
3. CDC — What to Do: Get Inside (updated 15 April 2024) — https://www.cdc.gov/radiation-emergencies/response/get-inside.html (accessed 2026-10-20)
4. Ready.gov (FEMA) — Radiation Emergencies — https://www.ready.gov/radiation (accessed 2026-10-20)
5. U.S. Department of Defense, Office of Civil Defense — Fallout Protection for Homes with Basements, H-12 (July 1966) — https://dahp.wa.gov/sites/default/files/Fallout%20Protection%20Homes%20with%20Basements.pdf (accessed 2026-10-20)
6. CDC (archive) — Transcript – Myth6 — https://archive.cdc.gov/www_cdc_gov/nceh/radiation/emergencies/videos/transcripts/transcript_myth6.htm (accessed 2026-10-20)
7. Bezirksamt Tempelhof-Schöneberg von Berlin — Schutz suchen in Krisenfällen — https://www.berlin.de/ba-tempelhof-schoeneberg/service/bevoelkerungsschutz/artikel.1656638.php (accessed 2026-10-20)