What is radiation? How harmful is it?

[This is a transcript of the video embedded below.]


Did you know that sometimes a higher exposure to radiation is better than a lower one? And that some researchers have claimed low levels of radioactivity are actually beneficial for your health? Does that make sense? Are air purifiers that ionize air dangerous? And what do we mean by radiation to begin with? That’s what we will talk about today.

First of all, what is radiation? Radiation generally refers to energy transferred by waves or particles. So, if I give you a fully charged battery pack, that’s an energy transfer, but it’s not radiation because the battery is neither a wave nor a particle. On the other hand, if I shout at you and it makes your hair wiggle, that sound was radiation. In this case the energy was transferred by sound waves, that are periodic density fluctuations in the air.

Sound is not something we usually think of as radiation, but technically, it is. Really all kind of things are technically radiation. If you drop a pebble into water, for example, then the waves this creates are also radiation.

But what people usually think of, when they talk about radiation, is radiation that’s transferred by either (a) elementary particles, that’s particles which have no substructure, for all we currently know, or that’s (b) transferred by small composite particles, such as protons, neutrons, or even small atomic nuclei and (c) electromagnetic waves. But electromagnetic waves are strictly speaking also made of particles, which are the photons. So, really all these types of radiation that we usually worry about are made of some kind of particle.

The only exception is gravitational radiation. That’s transferred in waves, and we believe that these gravitational waves are made of particles, that’s the gravitons, yet we have no evidence for the gravitons themselves. But of all the possible types of radiation, gravitational radiation is the one that is the least likely to leave any noticeable trace. Therefore, with apologies, I will in the following, not consider gravitational waves.

Having said that, if you want to know what radiation does, you need to know four things. First, what particle is it? Second, what’s the energy of the particle. Third, how many of these particles are there. And forth, what do they do to the human body. We will go through these one by one.

First, the type of particle tells you how likely the radiation is to interact with you. Some particles come in huge amounts, but they basically never interact. They just go through stuff and don’t do anything. For example, the sun produces an enormous number of particles called neutrinos. Neutrinos are electrically neutral, have a small mass, and they just pass through walls and you and indeed, the whole earth. There are about one hundred trillion neutrinos going through your body every second. And you don’t notice.

It’s the same with the particles that make up dark matter. They should be all around us and going through us as we speak, but they interact so rarely with anything, we don’t notice. Or maybe they don’t exist in the first place. In any case, neutrinos and dark matter are particles you clearly don’t need to worry about.

However, other particles interact more strongly, especially if they are electrically charged. That’s because the constituents of all types of matter are also electrically charged. Charged particles in radiation are mostly electrons, which you all know, or muons. Muons are very similar to electrons, just heavier and they are unstable. They decay into electrons and neutrinos again. You can also have charged radiation made of protons, that’s one of the constituents of the atomic nucleus and it’s positively charged, or you can have radiation made of small atomic nuclei. The best known of those are Helium nuclei, which are also called alpha particles.

Besides protons, the other constituent of the atomic nucleus are neutrons. As the name says, they are electrically neutral. They are a special case because they can do a lot of damage even though they do not have an electric charge. That’s because neutrons can enter the atomic nucleus and make the nucleus unstable.

However, neutrons, curiously enough, are actually unstable if they are on their own. If neutrons are not inside an atomic nucleus, they live only for about 10 minutes, then they decay to a proton, an electron and an electron-anti-neutrino. For this reason you don’t encounter single neutrons in nature. So, that too is something you don’t need to worry about.

Then there’s electromagnetic radiation, which we already talked about the other week. Electromagnetic radiation is made of photons, and they can interact with anything that is electrically charged. And since atoms have electrically charged constituents, this means, electromagnetic radiation can which interact with any atom. But whether they actually do that depends on the amount of energy per photon.

So, first you need to know what kind particle is in the radiation, because that tells you how likely it is to interact. And then, second, to understand what the radiation can do if it interacts, you need to know how much energy the individual particles in the radiation have. If the energy of the particles in the radiation is large enough to break bonds between molecules, then they are much more likely to be harmful.

The typical binding energy of molecules is similar to the energy you need to pull an electron off an atom. This is called ionization, and radiation that can do that is therefore called “ionizing radiation”. The reason ionizing radiation is harmful is not so much the ionization itself, it’s that if radiation can ionize, you know it can also break molecular bonds.

Ionized atoms or molecules like to undergo chemical reactions. That may be a problem if it happens inside the body. But ionized molecules in the air are actually common, because sunlight can do this ionization, and not something you need to worry about. If you have an air purifier, for example, that ionizes some air molecules, usually O two or N two.

The idea is that these ionized molecules will bind to dust and then the dust is charged, so it will stick to the floor or other grounded surfaces. But this ionization in air purifiers does not require ionizing radiation, so it’s not a health risk. Except that air purifiers may also produce ozone, which is not healthy.

Where does ionizing radiation come from? Well, for one, ultraviolet sunlight has enough energy to ionize. But even higher energies can be reached by ionizing radiation that comes from outer space, the so-called cosmic rays.

Most ultraviolet radiation from the sun gets stuck in the stratosphere thanks to ozone. Most cosmic rays are also blocked or at least dramatically slowed down in the upper atmosphere, but part of it still reaches the ground. This already tells you that your exposure to ionizing radiation increases with altitude. In fact, average people like you and I tend to get the highest doses of ionizing radiation on airplanes.

The particles in the primary cosmic radiation are mostly protons, some are small ionized nuclei, and then there’s a tiny fraction of other stuff. Primary here means, it’s the thing that actually comes from outer space. But almost all of these primary cosmic particles hit air molecules in the upper atmosphere, which converts them into a lot of particles of lower energy, usually called a cosmic ray shower. This shower, which rains down on earth, is almost exclusively made of photons, electrons, muons, and neutrinos, which we’ve already talked about.

Ionizing radiation is also emitted by radioactive atoms. The radiation that atoms can emit is of three types: alpha, that’s Helium nuclei, beta, that’s electrons and positrons, and gamma, that’s photons. Radioactive atoms which emit these types of radiation occur naturally in air, rock, soil, and even food. So there is a small amount of background radiation everywhere on earth, no matter where you go, and what you touch.

This then brings us to the third point. If you know what particle it is, and you know what energy it has, you need to know how many of them there are. We measure this in the total energy per time, that is known as power. The power of the radiation is the highest if you are close to the source of the radiation. That’s because the particles spread out into space, so the farther away you are, the fewer of them will hit you. The number of particles can drop very quickly if some of the radiation is absorbed. And the radiation that is the most likely to interact with matter, is the least likely to reach you. This is the case for example for alpha particles. You can block them just by a sheet of paper.

And then there’s the fourth point, which is the really difficult one. How much of that radiation is absorbed by the body and what can it do? There is no simple answer to this. Well, okay, one thing that’s simple is that high amounts of radiation, regardless of what type, can pump a lot of energy into the body, which is generally bad. Most countries therefore have radiation safety regulations that set strict limits on the amount of radiation that humans should maximally be exposed to. If you want to know details, I encourage you to check out these official guides, to which I leave links in the info below the video.

Interestingly enough, more radiation is not always worse. For example, you may remember that if there’s a nuclear accident, people rush to buy iodine pills. That’s because nuclear accidents can release a radioactive isotope of iodine, which may then enter the body through air or food. This iodine will accumulate in the thyroid gland and if it decays, that can damage cells and cause cancer. The pills of normal iodine basically fill up the storage space in your thyroid gland, which means the radioactive substance leaves the body faster.

But. Believe that or not, some people swallow large amounts of radioactive iodine as a medical treatment. This is actually rather common if someone has an overactive thyroid gland which causes a long list of health issues. It can be treated by medication, but then patients have to take pills throughout their lives, and these pills are not without side effects.

Now, if you give those patients radioactive iodine that kills off a significant fraction of the cells in the thyroid gland, and can solve their problem permanently. This method has been in use since the 1940s, is very effective, and no, it does not increase the risk of thyroid cancer. The thing is that if the radiation dose is high enough, the cells in the thyroid gland will not just be damaged, mutate, and possibly cause cancer. They’ll just die.

Now, this is not to say that more radiation is generally better, certainly not. But it demonstrates that it’s not easy to find out what the health effects of a certain radiation dose are. The physics is simple. But the biology isn’t.

Indeed, some scientists have argued that low doses of ionizing radiation may be beneficial because they encourage the body to use cell-repair mechanisms. This idea is called “radiation hormesis”. Does that make sense? Well. It kind of sounds plausible. But the plausible ideas are the one you should be most careful with. Several dozen studies have looked at radiation hormesis in the past 20 years. But so far, the evidence has been inconclusive and official radiation safety committees have not accepted it. This does not mean it’s wrong. It just means, at the moment it’s unclear whether or, if so, under which circumstances, low doses of ionizing radiation may have health benefits, or at least not do damage.

So, I hope you learned something new today!

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