Gamma Radiation Protection Factor

Exposure to too much gamma radiation (the rays given off by radioactive fallout from a nuclear explosion) will make you seriously ill or kill you. And as you cannot sense gamma radiation without special instruments, you won’t even know it is happening until it is too late.

Harm from radiation is strongly related to both total dose and dose rate. You can take a lot more radiation if it comes in small doses with time to recover from the cellular damage in between.

The ways to reduce your exposure are:

Get farther away
The further away a gamma radiation source is from you, the weaker the rays are and the less they will affect you.

Put some shielding between you and it
Shielding basically just means a lot of mass or weight. Air is a crummy shield unless we’re talking about miles of it; water, dirt, cement, steel, lead, lots of books – these are much better.

Wait for the radioactive material to decay
Eventually there will be less of it left; and the mix expected from a nuclear weapon decays quickly. For example, after a nuclear detonation, you probably won’t even think about leaving a radiation fallout shelter for the first two or three days.

There are two main kinds of shelters:

Expedient ones, that you discover or make in a big hurry with whatever is at hand.

Prepared ones, that you have designed and built in advance as a safe place.
Both kinds have the same goal as far as radiation is concerned: get as much mass as possible between you and the radiation source. Obviously a prepared shelter can be set up to do this really well, but it’s surprising how well even the simplest expedient shelters can do.

To evaluate or understand what makes a fallout shelter safe from gamma radiation, it is important to understand Protection Factor (PF) and the concept of halving thickness.

Halving Thickness:
A halving thickness is the amount of material that will block half of the gamma rays passing through it. Any mass will block them, whether lead or feathers, sand or chocolate bars, as long as you have enough mass. Here is a list of materials and their approximate halving thicknesses.

(References differ slightly when listing these figures.)

2.4” concrete
3.6” sand or packed dirt
7.2” water
11” wood
8″ books or magazines (depends on the density)
3.4” red bricks
6” broken anthracite coal

Now, one layer of any item above will block half the gamma rays. That is 1/2, which is called a protection factor (PF) of 2 (read only the denominator of the fraction). 1/2 of the rays are hitting you, 1/2 are blocked. By adding one more halving thickness, you block half of the remaining gamma rays, so now 1/4 are hitting you. So you have a protection factor (PF) of 4. Another layer blocks 1/2 of that remaining 1/2 of the radiation, so that means only 1/8 of the original total outside radiation is hitting you, and you have a PF of 8.

A fourth layer of anything listed above blocks half of that 1/8 radiation still entering, so now we only have 1/16 of the outside gamma rays hitting our body. ( PF 16)
5 layers= PF 32
6 layers=PF 64
7 layers=PF 128
8 layers=PF 256
9 layers=PF 512
10 layers=PF 1024


So how much protection do you need?

Obviously, the more protection the better.

An overall shield is characterized by its total “protection factor”. For example, a shield that only lets 1/40 of the gamma rays through, has a protection factor of 40.

Old FEMA civil defence guidelines specified a minimum protection factor of 40 for public fallout shelters.

In the Canadian government’s Fallout Protection Survey of Canada conducted in the 1960s, the minimum protection factor for a building to be listed as a shelter was PF 10.

However, if you were fairly near to a detonation or if there were a large number of detonations, you would probably try for a PF well above that.

But the bottom line is that you want to ensure that the cumulative dose of radiation you receive is below 25 Roentgen (R) in first week, or in worst case scenario, below 100 Roentgen (R) in the first week. (Our online training courses specifically detail tools and procedures that can help to ensure this.)

Here’s the old FEMA penalty chart that was included in one of their shelter manuals: