Radiation Damage: Biology is just Physics with lots of extra steps right?

You can’t see it, you can’t feel it, you can’t smell it, and you can only tell where it is either by its effect or by specialized detecting equipment. It’s easy to understand the fear and hesitancy when it comes to radiation exposure.

I can’t do much to try and fight that kind of fear. I could go on about all the detection methods we have available to us. I’ve talked about how we shield radiation and how good we are at it, and how each type of radiation works. But I’ve never really gone into what the effects of radiation on the body or on a persons health are, and it’s because of one simple reason. I don’t know a damned thing about biology. I stopped taking biology after grade 10 science in high school cause it wasn’t required to get into engineering, and I’m squeamish about blood so I didn’t want to do the dissections in grade 11 or 12.

You’re getting the artists rendition of this because the actual pictures made me shudder. So yeah, dibs out thank you very much.

So I might not be able to tell you how radiation exposure can elicit specific symptoms or illnesses, but what I can do is delve into the absolutely fundamental interactions between radiation and our own squishy, delicate, very important, and loved bodies.

So since we already know what radiation is, what are we? Well, according to an adorable anime I saw we are collections of approximately 37 trillion cells, each of which are composed of a shell kind of thing that somehow lets important stuff in and out, a central blob that holds all our DNA and tells the cell what job it has and how to do that job. And then it’s all enabled to go about it’s wiggly little water balloon like life by everyone’s favourite part of a cell, the mitochondria, also known as the powerhouse of the cell, also knows as the only thing anyone remembers about high school biology.

I mean, there’s also all this stuff too. I warned you that I’m useless when it comes to biology.

Now since I don’t have a degree in biology, or medicine, I couldn’t tell you what damage to any of these structures can cause, but in most cases I’m betting it behaves much like any other delicate machine in that it simply stops working. But I have also been told that there is a great deal of redundancy in our cells so a single breakdown is generally not even noticed.

So we likely need… Let’s be pessimistic here and say we only need a few dozen breakdowns in any one area before serious problems occur. But as the video said, cell breakdowns are generally normal and expected with tens of billions of them happening per day normally. So we would need a staggering number of them to happen simultaneously in order to even match the natural rate of cell replacement.

But how does radiation cause these breakdowns of the cellular machinery? Basically the same way that Little Timmy from down the street broke his mothers mirror, by running into it really hard (Little Timmy isn’t that bright.) An alpha, beta, gamma particle or neutron hits the protein or molecule that makes up the machinery, or structure, or DNA, and breaks a bit of it off. That bit gums up the rest of the works before bounding off to collide with more things until it is out of energy or it leaves your body.

And that is the most important part; how much of the energy of a radioactive emission can be spent breaking molecular bonds inside you. If we go by this set of charts of chemical bond energies we can guess that the average common bond strength in the human body is around 350 kJ/mol which works out to 3.63 electron volts per bond on average. Compare that to the average energy of: alpha particles at 5 million electron volts, beta particles at about 1 million electron volts, or gamma particles at about 335 thousand eV on average (I averaged all emissions listed at that link. Not as accurate as a weighted average based on which isotopes are most common, but the best I can do for now unless someone else wants to do the leg work). This means that on average an alpha particle has enough energy to break approximately 1.4 million atomic bonds, a beta can break 275,000 and a gamma can break just shy of 100,000.

But just comparing energy doesn’t give us the whole story. We also need to know how likely a particle of radiation is to interact with a molecular bond in your body. It doesn’t matter if a particle has enough energy to break a billion bonds in your body if it will never interact with you (looking at you, neutrinos). I’ve already mentioned before how likely types of radiation are to interact with matter here, and here. But I am told a picture is worth a thousand words.

Thanks to CloundyLabs for this amazing imagery!

What you are seeing is radiation from a chunk of uranium ore in a cloud chamber. It neatly shows the difference in how much each kind of radiation interacts with the world around it. The short fat lines closest to the chunk are the alpha particles, they have a lot of energy and they bump into almost every atom in their path which is why they make such thick paths through the cloud chamber. The beta particles are a little harder to spot as they are much fainter than the alpha particles, but they have tell-tale tracks that kink and wind through the chamber. It shows how the electrons bounce from interaction to interaction and can even be turned around from their original path. Finally the hardest one to find is the gamma ray path. Can you guess what it is?

The gamma path are the very, very faint little puffs of disconnected clouds that point away from the source and that is due to how rarely a gamma ray actually interacts with anything. And if it passes through you without interacting with you then it has the same effect as if it had never gone through you at all. I know it sounds weird that something can go through you without actually touching you, especially when you remember all those times you accidentally kicked a coffee table in the dark, but you have to remember that from the perspective of a gamma ray you are an only very slightly less empty chunk of empty space.

So lets try to figure out how much radiation it takes in order to have a noticeable impact. There is a Huge, complicated, and just ridiculously arcane body of knowledge on this topic thanks to 60 years of operation under the LNT hypothesis of radiation safety. Your body regularly handles the death and removal of between 50 to 70 billion cells a day, so lets assume that we can handle double that much for a short time. Each cell has about 100 trillion atoms in it, and each atom has to have connections to at least 1 other atom to be part of a molecule in your cell and up to an average maximum of 4 since we are carbon based. And as any min/maxing Dungeons and Dragons player can tell you the average roll on a 4 sided dice is 2.5.

The bane and lifeline of all wizards until level 5

Therefore each cell in our body can be approximated as having 250 Trillion molecular bonds. Now we know that alpha particles, while on average the most energetic kind of nuclear radiation, are too interactive with matter to actually be a threat to our health. Unless they somehow get inside us, our skin is enough to block them so we can generally ignore that component of exposure. So we only really need to think about exposure to beta and gamma radiation in terms of health effects. On average they are capable of directly breaking 100 to 275 thousand molecular bonds per particle. I don’t know what the limits are for how many bonds need to get broken in order to cause a cell to die, or to develop a metabolic problem, or to become cancerous. It’s very unlikely to be 1 unless you are the unluckiest person alive, but it’s probably also less than 275,000 because cellular machinery is pretty fragile simply due to its size.

a table of thicknesses of materials required to absorb half of the energy of incident gamma rays at three energies

Since we can’t give a deterministic answer about how much it takes to cause a problem in your cells from radiation exposure we have to fall back on statistics. I mean it is the science of large numbers and I think 37 trillion cells per human times 250 Trillion molecular bonds per cell counts as one hell of a large number. We can assume that all beta radiation will completely transfer all of its energy into your body since the particle is so light and highly charged so it is easy to stop. But for Gamma radiation according to the table above, about 50% of the energy is deposited per 6 cm of water it has to travel through at our previously mentioned average of 335 keV. And since a human can be fairly accurately described as a slightly sturdy water balloon with more complex emotions, we can assume that this number is close enough for government work. Taking an average human being between 10 and 40 centimeters thick at the thickest means that we can assume an average absorption of 80% of the energy of a gamma ray.

(Yes I am aware that I’m being very general in my assumptions and that absorption would change determined by body part and you are more than welcome to read the article about sievert calculations that I linked earlier to determine more exacting dosages if you are hardcore like that.)

The measurement for determining the severity of radiation damage to a person is the Sievert and is equivalent to 1 Joule of radiation energy deposited in a person per kg of said person in a whole body exposure. We determined earlier that each beta particle will deposit approximately 1 MeV per particle and each gamma ray will deposit 270 keV. That means that a 70 kg person (154 lbs for our american readers) would have to be exposed to 44 trillion betas or 162 trillion gamma rays to get a 100 millisievert dose, which is the 1 year limit for radiation workers in north america. Heavier people would require larger exposures to get the same level dose and smaller people would require less.

It’s almost impossible to put those kinds of numbers into perspective as they are so far beyond everyday life. Going back to the idea of the daily replacement rate of ~60 billion cells per day, if we peg that at the 100 mSv/year dose we just calculated then that means that you would have to be exposed to an amount of radiation of between 1.4 to 5.14 Million Bequerels every second of every day assuming it was all beta and gamma as alpha is not really dangerous unless you get it inside yourself.

100 mSv limit is not even the limit where harm occurs, but is instead the limit where effects become statistically likely. To get to a level where harm is guaranteed is order of magnitude larger exposures, which is something that our current LNT model does not accurately educate people about. Right now we treat radiation as if one particle hitting you every second for 1 million seconds deals the same damage to you that 1 million particles hitting you in one second does. But again I mention that your body replaces 50 to 70 billion cells every day, which indicates that there is a rate at which we can sustain damage and have it be repaired without build up in the body. This is actually part of a major push in radiological fields to replace the LNT model of radiation safety with another, more accurate risk assessment model, either the Threshold model or the Hormesis Model.

I know radiation can be unsettling or scary, but it is not the automatically dangerous thing that some groups try to make us think it is either. Just try to remember that next time someone tries to scare you with large numbers about exposures that not all big numbers are created equal.

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