Friday, April 22, 2016

Radiation Doses for Dummies

Not actually Erin.
& is used with permission.
PREVIOUSLY: Radiation Terminology for Dummies

Welcome to the final installment of "Radioactivity for Dummies" series. Over the past two months I've talked about why ionizing radiation is dangerous, how it works, how to shield yourself from it and how to keep it off you if you are accidentally exposed.

But sometimes precautions aren't enough, and through sheer bad luck you may one day find yourself exposed to ionizing radiation. If that happens, do not panic; you are not necessarily doomed -- and if you are doomed, then panic will only reduce the amount of things you can get done with whatever time you have left.

Radiation Exposure is a Function
I mean this in the mathematical sense; how much of a dose you absorb is dependent upon the input of no fewer than three variables.

A Function of Time
As I explained in my first blog post on the subject, radioactivity is simply the measure of how quickly an unstable substance decays into a more stable form. The faster it decays, the shorter its half-life is and the more dangerous it is to be around. Therefore, if a radioactive substance is emitting 4 rems per hour (rems being the measurement most common in the USA) , that works out to 40 millisieverts an hour. This is nearly the maximum yearly dose permitted to radiation workers in the US, but only if you hang around for the entire hour. If you quickly leave the area (see distance, below) your actual exposure will be much less.

However, if you stay in proximity to it for a long period of time, the danger to your body increases because radiation exposure is cumulative. 8 hours of exposure (320 mSv) will likely cause radiation sickness, and a full day's exposure gives a dose of 960 mSv, nearly a full sievert -- not enough to kill a healthy adult, but definitely enough to make one very sick, including nausea, hair loss, and bleeding.

Another time factor is that the human body can manage the same radiation dose if it is spread out rather than all at once. A 1 sievert dose all at once can overwhelm the body's ability to repair or replace damaged cells, but 1 Sv spread out over days, weeks or longer gives the body a chance to heal. This is known as fractionation of dose

A Function of Distance
In the same way that the warmth of a fire (thermal radiation) diminishes the further away from it you are, exposure to ionizing radiation diminishes by distance. This is known as the Inverse Square Law, and it essentially states "The further away you are, even if it's just a foot, the better it is for you."

Running away from sources of ionizing radiation is almost magically effective, because doing so not only increases your distance (which decreases your dose) but it also reduces the amount of time you are exposed to it (which also decreases your dose). This means that so long as the source of ionizing radiation isn't all around you, the very best thing you can do is run away -- which is likely what your fear instincts are going to have you doing anyway.

A Function of Shielding
I have explained several times that radiation can be blocked by materials that are also very common. Several feet of water, dirt or concrete will stop most forms of ionizing radiation, so if you can put an industrial building between you and the radiation source, the better you are. Going underground (such as into a subway tunnel) would be ideal -- although be warned that others may have the same idea and you could find yourself in danger of being crushed by a crowd -- but if you can't do that, just put a city block between you and the danger. The combination of time, distance, and the thickness of a city block full of concrete and metal and water ought to protect you splendidly.

Different Types of Doses

An acute dose is when a person receives a radiation dose over a short period of time (a day or less). It results in acute radiation syndrome, aka "radiation poisoning", which has several nasty effects.

Some of these are Non-stochastic effects, which means that they are related directly to the dose received.  Non-stochastic results have a threshold below which they will not occur, and whose effects are more severe as intensity of dosage increases. An example of this are radiation burns; at low level they act like sunburns, but higher levels result in second-degree burns with blistering.

Stochastic effects are randomly-occurring and whose results are independent of the size of dose. The effect typically has no threshold and is based on probability, with the chance of seeing the effect increasing with dose. Cancer is a stochastic effect.

Other examples of acute radiation syndrome can be found at this Wikipedia article, which I converted to this printable image below.

Keep in mind that these effects are based up Grays (absorbed dose) and not Sieverts (equivalent dose), the difference between which I explained last week. However, since 1 Gy equals 1 Sv when dealing with gamma and x-rays, this gives a good idea of how damaging exposure to such radiation can be. 

As you can see, more than 1 Sv has a high chance of fatality. If skin burns, damage to internal organs, or succumbing to disease due to destruction of white blood cells aren't enough to kill, then some form of immediate cancer likely will.

Also note how prompt, proper medical care is a game-changer for doses of 2-8, turning what could be near-certain death into 50% or greater odds for survival. While many preppers have an inherent distrust of hospitals after emergencies, it is in your best interest to seek medical care after a radiation exposure incident as it can literally mean the difference between life and death.

A chronic dose is when a person receives a large dose of radiation stretched out over a long period of time (days or longer). It does not take effect immediately; however, the greater the absorbed dose the faster the onset of symptoms will be.

Symptoms of chronic radiation syndrome include cataracts, atrophy of or fibrous formations of the skin following radiation burns, and a high chance of cancer. As mentioned above, cancer is stochastic; the higher your dosage, the greater your chance of contracting it, but dosage has no impact on how severe that cancer is. You might get a little melanoma; you might develop leukemia.

Cancer as a result of chronic radiation exposure can appear at any time during your life. According to this article written by Doctors of nuclear medicine, the latent period for leukemia is 7-10 years and for other ("solid") forms of cancer is 20-30 years. Fortunately, if you receive no more than a 0.1 Gy dosage, there are no indications for future appearance of cancer.

Chronic radiation syndrome is the best kind the get (for certain values of "best") because the body has a chance to repair itself. If you're old enough, odds are good that you'll die of other causes before the cancer kills you.

Is it my sincere hope that this series has taken the"fear of the unknown" which surrounds radioactivity and replaced it with knowledge. Yes, the effects of ionizing radiation are frightening; but knowing how it works, and more importantly knowing how to protect yourself from it, ought to reduce it from an "implacable boogey man" to a force of nature which can be guarded against, like fire or a tornado.

EPILOGUE: Devices that measure Ionizing Radiation (by Chaplain Tim)

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