Saturday, December 26, 2020

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!

You can join the chat on this video today (Saturday) at 6pm CET/noon Eastern Time here.


  1. Radiation units make me crazy: rads, curies, rems, sieverts. If you get 8 Sv, you don't have to worry about it.

    1. That is the part of radiation physics I know very little about. These are calibrated according to how many coulombs of charge a radioactive material generates by emitting radiation.

    2. ...because you will be most probably dead soon.

  2. Sabine,

    Thanks for the interesting video. Two questions:

    • You note that radiation is energy transferred in many forms. What constrains energy to a particular form?

    • When you transfer energy to a guitar string by plucking it, it must be under tension to exhibit wavelike behavior. By what means is spacetime tensioned to exhibit wavelike behavior?


  3. Hi Don,

    I don't understand the first question, sorry. What do you mean by "constrains" and what is a "form"? I certainly did not say that energy is "constrained" to any "form".

    As to the second question, the tension is loosely speaking proportional to the square of the speed of light which appears in in the wave-equation. I say loosely speaking because you'd need other constants besides the tension to make that analogy work, and also for space-time you do of course not have a microscopic theory, so speaking of a "tension" makes no sense.

    1. Sabine,

      “I certainly did not say that energy is "constrained" to any "form".”

      Right, you did not.
      For me, the notion of forms of energy was likely introduced in 8th grade science; the two broad categories of kinetic and potential with each further distinguished by terms like thermal, electrical, mechanical, etc. In general, form would be the set of observable properties that serve to distinguish one thing from another. Granted, this is something that cannot be most explicitly done with words. Yet, I read that, even in its first micro-seconds, the universe had already distinguished between the forms of quarks and gluons and that protons and neutrons followed within the first second. As to these latter forms, I read that, rather than being static, the enduring form of nucleons is akin to a dance pattern with swiftly changing partners. So, the energetic dance is ruled by a form that persists through cosmological epochs.

      And, ‘constrained’ was my word, not something in your post. Constrained as in the speed of light being reduced by a third in passing through water. I take it to covey meanings of ‘to hold,’ containment, resistance and restraint. So, my first question translates to:

      If we set physics proper aside at one remove and look at the universe without all the details, do we observe a general pattern of energy being dynamically constrained to particular forms (with form defined as above)?

      Not sure this question passes as sufficiently well formed, but it is best effort.


    2. Sabine,

      “Radiation generally refers to energy transferred by waves or particles.”

      I have questions, but first a homegrown preface:

      The understanding of energy as a universal principle and fundamental agency of change in all its forms took generations of exacting laboratory work and must have been an astonishing revelation when all the pieces fit together. There must have been some tingling of grey neck hairs among those few who first apprehended. It was not widely appreciated at the time, likely because there was no Symmetry Magazine or BackReaction online. All the world’s verbs were discovered to be derivative of a single, quantifiable property that is exactingly conserved in all translations. That is still a stunning revelation.

      Energy may be transformed between different forms at various efficiencies. Items that transform between these forms are called transducers. Examples of transducers include a battery, from chemical energy to electric energy; a dam: gravitational potential energy to kinetic energy of moving water (and the blades of a turbine) and ultimately to electric energy through an electric generator…” (Wiki)

      The word transducer and ductwork have the same root in the Proto-Indo-European -- *deuk- "to lead". In former occupation I was basically a user of tools and still have a shop full of transducers, the devices that turn energy to particular purpose. Often that is a cutting edge, but one finds a burdensome range of necessary purposes and hence shelves full of various devices. Given that experience, I saw a similar pattern in observing biological forms. They are leavened and sustained by the flux of energy and one can see that any particular organism is a most efficiently compacted and coordinated construct of highly complex molecular transducers.

      So, one may realize that it is not energy in general, but the particulars of its path that is most significant to biology and likely physics. What makes the ductwork? What constrains energy to particular paths? Is it a myriad of ad hoc mechanisms, or is there a general principle at work?

      Is it useful to consider the possibility that energy has an inherent counterpoise in some companion principle of constraint? Granted, that may not be a simple question.


  4. Radiation of the wave and particle variety are generally due to the electromagnetic, nuclear and weak interactions. These have distinguishing features respectively of emitting photons in the γ-ray spectrum, helium He4 nuclei or α-particles and electrons or positrons. In the case of the weak interaction, if an antielectron or positron is emitted that annihilates with an electron in the environment and produces γ-ray photons. The quantum gravitational field in principle is there too, but to generate strongly interacting quantized gravitons requires near Planck energy. These do not generally occur.

    The nuclear interaction at its fundamental level is the exchange of a gauge particle, called a gluon, between quarks. A quark has a type of charge, called color and labelled (r, y, b). The gluon carries a pair of colors, of which there are 8 possible combinations. This corresponds to the dimension of the space for the symmetry group SU(3) of Quantum ChromoDynamics (QCD) for the nuclear interaction. An instance of how this works with an elementary 3 vertex interaction is a quark plus gluon transitioning to a quark plus gluon and exchanging a color charge, such as q^b + g^{ry} → 2-quark + antiquark excited state → q^y + g^{rb}. If instead of the red color this gluon had anti-blue the gluon would be absorbed and there would only be the q^y quark. This latter interaction is more probable as it does not involve the virtual formation of quarks and anti-quarks, Hence, virtual quarks play a role as well as seen in the first example. The quantum physics of this is fascinating and it is most strongly interacting at low energy.

    There is then the emergence of the nuclear QCD interaction, what might be called classical nuclear physics. If we have a proton with quarks (u,u,d) and a neutron (u,d,d) the proton can quantum transition into (u,d,d) + (d-bar, u), which is a neutron plus pion and this is then absorbed by the neutron (d-bar, u) + (u, d, d) → (u,u, d). So the proton and the neutron has in a sense swapped. Therefore, there is a deuteron p+n and no such thing as a di-proton, at least none that is at all stable. This gives rise to the MeV scale nuclear interaction and how nucleons are bound, fuse or fission. Quite often in nuclear interactions a helium-4 nuclei is emitted. This is the α-emitter. This does not penetrate even skin.

    The next interaction based on strength is Quantum ElectroDynamics. This is the quantization of the electromagnetic field. A charge with a radial electric field that is suddenly accelerated will have a wave in that electric field radiate out to re-adjust that field with a cycling magnetic field. This is the electromagnetic wave. In QED there is the procedure of quantization which is interpreted as the photon. Infrared is radiation and generally if at low intensity so as not to burn is harmless. Same with light at higher energy. When you get to ultraviolet light at even higher energy there is now the prospect for matter-radiation interactions. This is further on up with X-rays and so forth. Most radioactive materials that emit γ-ray photons are dengerous as these are very penetrating and damaging.

  5. continued Next is the weak interaction, or Quantum Flavor Dynamics (QFD). This is an interaction of the form u + μ^+ → W^+ → d-bar + ν_μ. This is a charged weak current that exchanges the quark flavor from u to d-bar and converts a muon into its corresponding neutrino. A common example is u → u + Z → d + e + ν_e. This is an interaction of an up quark with a virtual neutral current Z or W^0, leading to a down quark plus an electron (β-radiation particle) and neutrino. Of course, a lot more can be said. This last case is what causes the decay of a neutron into a proton. Generally, the weak interaction transforms a neutron into a proton, and if energetically possible in some nuclei the other way around. What is damaging here is that electron. This is mildly penetrating, though a small amount of shielding can stop these.

    This leads to the next point. There is a difference between radiation and radioactivity. Radiation is the wave or particle emitted. Radioactivity refers to material that emit such radiation. In a sense we could say a hot stove is radioactive emitting IR photons. However, we usually do think of higher energy radiation. These usually are nuclei that are unstable and transition by one of these three interactions.

    When it comes to radiation and health physics, a subject I am not that acquainted with, the issue is often with radioactive materials. The biggest issue is ingesting or inhaling them. Even though an α-emitting element is of no danger outside of us, a dust particle that emits α radiation is very dangerous. The same for the weak interaction and its β-particle. Then γ-rays are the most damaging, and being in the presence of a γ emitter is dangerous. Lots of lead shielding is required.

  6. I propose a word "causiation" for radiation propagating at speed c as light in vacuum.

    In fact, there are scienfifically argumented reasons to consider that light is not particles, photons, in flight between matter particles but photonic phenomenon occurs only at excitations of matter structure. Really, something correlates when energy transfers as wave packets but it still needs studying.

  7. ... 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... is almost exclusively made of photons, electrons, muons, and neutrinos, which we’ve already talked about."

    An important component of these showers consists of neutrons, which (being chargeless) are the most penetrating, which is why the neutron flux at high altitude has the biggest effect on things like the memory chips in the avionics computers in airplanes. These devices have to be designed with provisions to accommodate "single event upset" (SEU) due to an energetic neutron flipping a bit in a RAM chip or processor. (This refers to upsets that are physically mild enough that they doesn't actually damage the device, they just flip the logic state of some memory location.)

    I suppose it's possible the neurons of a human brain are susceptible to similar "single event upsets" caused by the neutron flux at high altitude, but maybe our brains have some built-in error tolerance that self-corrects against momentary upsets. There have been many studies mapping out the SEU rates for microprocessor devices at various altitudes, but I've never heard of any studies on brain upsets. Those would be would be difficult to measure, since our brains don't do formal parity checks (as far as I know).

  8. I have read about a third of your book “Lost in Math” and I empathize with your disillusionment. I have found joy again in the fundamentals after forgetting the advanced physics. I don’t think I ever really understood those parts conceptually.

    I have read the intensity of a spherical radiating point source diminishes by an inverse square law described by the intensity equation. The surface area of a sphere is 4pi R^2. If the total flux through the sphere is conserved then the flux per area drops by the same 4piR^2 factor as the radius increases to 1/4piR^2.

    This idea is present in both Newton’s law of gravity, Coulomb’s law and Gauss’ law. Does this suggest electromagnetism and gravity are mediated by types of radiation?

    As light reaches us from a distant star the intensity per area also follows the same inverse square law. Does the energy of the light that reaches us have less energy per photon? Or should we find the wavelength of light is the same as when it is emitted but the number of photons and energy received per area is lower?

    I have forgotten a lot but I remember that light travels at a constant speed in a vacuum and is defined as an electromagnetic wave. But the vacuum does have electric permittivity and magnetic permeability. Do those constants dampen the energy of the electromagnetic wave by resisting the fields? I know light cannot slowdown so how could I measure this resistance sapping the energy away from light over a large distance?

    1. Without Doppler shift the energy of each photon E = ħω is the same. The photons just diverge from each other. Antennas on Earth are currently receiving less than 50 photons per bit of information transmitted by the Voyager spacecrafts. They are getting far enough out and their power limited so before long even if they do not technically fail the fidelity of information received will degrade beyond recognition.

    2. Continued as I hit pub too soon. The impedance of space is given by Z =|E|/|B|. The electric field has units of volts/m and the magnetic field is Wb = amps/m. The above then has units volts/amps, which is what we expect from Ohm's law V = IR. This impedance of vacuum is 377Ohms. This is the resistance in a circuit that one finds if you are pumping out an electromagnetic wave. The energy you are sending into space is a measure of this impedance. From the perspective of the circuit this is a voltage drop or energy loss due to the impedance.

  9. Another example

    Human skin can produce vitamin D with the help of UV radiation. Too much UV can damage (burn) the skin and some UV radiation can cause cancer (melanoma). Too little UV could cause vitamin D deficiency and all kinds of problems.

    Luckily, some changes in diet or pills can help out nowadays.

  10. One of my fondest memories is building a homemade cloud chamber at about age 13 with the help of my father, in the basement of our Teaneck, New Jersey home. We followed the standard construction method, a clear container, dark cloth on the bottom, with dry ice underneath and illuminated with an incandescent bulb. I clearly remember the sight of the first streaks of ionizing radiation formed as droplets condensed from the mist within the chamber. Such a fascinating experiment, illustrating how we are constantly subjected to high energy particles from space that make it through our thick atmosphere.

  11. Sabine you say:
    “ … 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.”

    Where can I find these “several dozen studies”?

    I know several investigations about this topic which all show that hormesis does work for a low dose of radiation.

    Just two examples:
    1) In the 1980s a housing complex was constructed in Taipei in Taiwan. 20 years later it was detected that contaminated, radiating steel was used for the buildings. After this detection two teams of the medical university of Taipei started to investigate the degree of the expected increase of cancer cases and cancer deaths. The surprising result was that there was no increase but a strong decrease.
    2) It is a known fact that persons who survived the nuclear bombing of Hiroshima and who were at least 6 kilometres away from the explosion point later had a lower probability of getting cancer and an increased lifetime compared to the average population of Japan.

    I friend of mine is a professor for biophysics, and his job was to find causes of cancer in our environment. And we was successful to find such causes. He told me that on the other hand there is no doubt that a radiation of a low dose is a protection against cancer.

    I find it really scandalous that these results are generally ignored by the governments and that there are no intentions to investigate this situation thoroughly. Many of the political decisions for instance in Fukushima look very irrational and were in the view of these facts bad for the people who had to move away without a need.

    1. antooneo,

      Yes, I came across these examples. I do not think they are being ignored. This is a good starting point.

    2. You might also find this useful, esp for what the discussion of the supposed evidence in favor of low-dose radiation benefits is concerned.

  12. Steve,

    I can't edit comments. Thanks for letting me know.



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