Gray, Sievert, Rad… a Sci-Fi Writer’s Guide to Radiation

In recent years, there has been an explosion of post-apocalyptic fiction and one of most prominent aspects of many of these settings: nuclear fallout and other forms of radiation either as a primary antagonist or at least a constant challenge for characters that arises from these settings. Radiation is mysterious, it can be dangerous, and a convenient plot device for writers. So many characters that were created at the dawn of the “atomic age” following the second world war have “radiation” as a part of their origin story. Spiderman… bitten by a radioactive spider. The Incredible Hulk… the result of a gamma radiation experiment gone wrong. Godzilla… seems to use radiation to heal. But what is radiation really?


Even among physicists the term “radiation” can have slightly different meanings in different contexts. Generally it refers to the propagation of energy, usually as an electromagnetic field, through space. Optical light, microwaves, infrared, ultraviolet… etc. As a subset of that, and what I’ll be focusing on here, is ionizing radiation–where there is enough energy in packets of that radiation to knock an electron out of an atomic orbital. If you want to impress your friends at parties, the energy threshold where this starts to happen is about 4 eV (315 nm) and that’s in the ultraviolet portion of the electromagnetic spectrum. Everything to the left of that in the figure below is ionizing… x-rays, gamma rays, cosmic rays (not shown). This can also include fast moving sub-atomic particles… when that’s the nucleus of a Helium atom after a particular mode of decay in a radioactive isotope, you’ve got an alpha particle. If that’s an electron, we call it a beta particle. Sometimes in radiation medicine protons, helium or carbon ions for cancer treatments that take advantage of the specific charge and mass to target precise depths in the body.

The electromagnetic spectrum from gamma rays to radio waves, logarithmically increasing wavelength left to right, with the visible spectrum highlighted in colour.
Source: Philips Ronan Gringer,

Measuring Radiation

Over the years, radiation has been measured in different quantities. Currently the most common currency is called the Gray–defined as 1 Joule per kg of the medium exposed to the radiation. This quantifies radiation absorbed dose. In the past we used use the rad–which works out to about 0.01 Gy. The Gray was officially adopted in 1975. You might also see another (outdated) unit called the Roentgen (or R), which has (had) a more technical definition that refers to the about of charge produced in air in a free air ion chamber, but this was abandoned because as people became able to produce higher energy sources, the measurements were impractical.
A concept called effective dose also comes into play when we’re talking about human exposures and the risks associated with them. Not all physical doses are created equal. To a cell, for example, a heavy, charged alpha particle blasting it’s way though does a lot more damage per unit energy absorbed than a speeding electron. It’s kind of like the difference between a bullet and cannon ball. To account for the relative amount of damage created by the specific type of radiation absorbed dose is translated into equivalent dose. Then, we have a system that also normalizes risk depending on which tissues were irradiated. The risk of developing cancer after having your whole body irradiated is different if the radiation only goes into your hand as opposed to your whole body. Putting this all together, yields effective dose which is measured in Sieverts or Sv.

Background Radiation and Common Effective Doses

Radiation is all around us. Cosmic rays, radon gas, potassium 14 in bananas… every day we’ve got photons and other particles passing through us (and doing damage to our cells). This is referred to as background radiation. Cumulatively the dose from background radiation adds up to about 2.4 mSv of effective dose per year or 0.0024 Sv/yr. Of course this can vary. There populated parts of the world, such as Ramsar Iran, where the background doses have been measured up to about 260 mSv/yr, though I believe average doses are closer to about 6 mSv/yr. Over the years a considerable amount of scientific attention has been paid to the residents of Ramsar and for what it’s worth, numerous health metrics have demonstrated that there doesn’t appear to be a significant increase in cancer rates in such areas.

In most places the background dose is about 2-3 mSv. I use 3 mSv as a handy reference. You go about your daily life receiving this level exposure, you’re unaware that it’s even there and generally doesn’t cause too many problems. So it’s useful to use this as a normalizing standard. Below, I’ve added a table with some approximate effective doses.

EventEffective Dose (mSv)Fraction of Background
background (annual)2.4 1
dental x-ray0.0050.002
CT scans5 – 202 – 8
lethal dose (whole body)4,0001667
Effective doses are approximate, for the purposes of general public education and writing cool science fiction stories.

How Radiation Interacts with Humans

From the point of view of a human cell, radiation can be thought of like little bullets zipping through water. Inside that cell you have a coil of really important material… your DNA… it encodes all the relevant information for building the body and enables the cell to do what it does. That DNA is the sensitive component of the cell.

As radiation travels through the cell it interacts either directly or indirectly with the water-like medium inside. As atoms of the medium become ionized they generate highly reactive chemical species called free radicals and these can diffuse, interact with the DNA and damage it. To a cell, damaged DNA is a big deal.

In fact it’s such a big problem that cells have evolved mechanism to constantly scan for it. An analogy that I sometimes use is that the DNA is a like a library of information. It’s critical for the cell to operate, so critical that every book in the library has two copies. That library hires an army of editors who work day and night constantly comparing each book to its companion. You can have minor DNA damage where maybe a letter gets replaced or a sentence gets cut. Those are pretty easy for the editors to fix. Repair of the damage is often on option. But sometimes you get more severe damage where you have pages torn out of both copies of the book! That’s an analogy to something called a double strand break where both of the helixes in the DNA double helix are severed.

When that happens the cell has a few options. Often, if there’s enough damage it will trigger a process referred to as apoptosis-programmed cell death. In the analogy that’s like burning the library down if one book is out of whack.

Unfortunately sometimes the repair happens, but not… perfectly. This leads to a change in the code. When this happens, in most cases, it’s relatively inconsequential. But sometimes that damage can linger and cause problems much later on… manifesting as something like a cell that can’t control its reproductive cycle… cancer.

What Happens To Characters Who Survive Nuclear Blasts?

Assuming of course we’re talking about radiation exposure and not the percussive blast itself, for reference the increase in cancer risk as a result of radiation exposure is roughly 4%/Sv for adults. In radiation protection this is applied in a linear fashion: an exposure of 0.5 Sv results in a 2% excess risk, an exposure of 2 Sv results in an 8% excess risk. But of course this is a simplification. When you really look at it, there things such as the age of the person exposed come into play (a young person has more time to develop a cancer than an older person), and there is a lot of debate as to what happens when doses are relatively low, like in the tens of mSv range. But the 4%/Sv can give you a rough ballpark for determining downstream risks to any characters

And as per the table above, the lethal dose that would kill about half of the exposed people to it (without major medical intervention) is about 4 Sv.

From a medical point of view, it’s the rapidly dividing cells in the body that are effected first. The GI tract is often affected, so high dose exposures will result in horrible consequences like diarrhea and vomiting. Your red blood cells are also pretty sensitive, which can lead to an effect where circulating cells are wiped out leading to major fatigue, but then as “stored up” cells are released into circulation characters may temporarily recover for 24-48 hours or so. But if no new cells are being made, they crash pretty quickly afterward.

As the dose goes up, the symptoms get more severe. When you get into exposures upwards of dozens of Sv, you end up with neurological complications and even with medical intervention such exposures are very difficult to survive.

So in the end… unfortunately there’s no evidence of radiation leading to superpowers, at least not that I’m aware of. That said, it can be used for some pretty amazing things, like curing cancer. But that’s another blog post.