How does radiation cause harm?
Hi, Dr. Zoomie! You know, it’s pretty common knowledge that radiation can hurt us, make us ill, or even kill us. But how does it do that?
This is one of those relatively simple questions that, unfortunately, doesn’t have a very simple answer. But let me see what I can do.
To start, let’s agree on some terminology, especially on what we mean when we say “radiation.” To a physicist, radio waves, microwaves, radar, visible light, and heat (infrared) are all forms of radiation; physicists have even detected gravitational radiation. To a physicist, “radiation” simply means that energy is being transferred from one point to another; a beta particle is a form of radiation that transfers energy from an unstable atomic nucleus to whatever absorbs it, but a fastball is also a form of radiation that transfers energy from a pitcher’s arm to a catcher’s mitt. The type of radiation we’re interested in is called ionizing radiation – that’s radiation with enough energy to remove an electron from an atom, creating a pair of charged particles from an uncharged atom (removing an electron with a negative charge leaves behind an atom with a positive charge). It’s this ion pair that can go on to cause other changes that, ultimately, can harm our health.
The threshold energy to create an ion pair is about 3 electron volts (eV) in germanium and about 13.6 eV (it’s different for different materials) in oxygen. Don’t worry about exactly what an electron volt is – that doesn’t really matter much here – it’s the number of electron volts that’s important. Less-energetic radiation cannot remove an electron from an atom, can’t create an ion pair, and can’t start the sequence of events that might one day lead to radiation sickness, radiation burns, or cancer. In the electromagnetic spectrum, radio waves, infrared, and visible light are all have less energy than this and, because of this, none of these forms of radiation can cause cancer or radiation sickness. As an aside, we all know that thermal radiation and other forms of non-ionizing radiation can cause burns, just as can ionizing radiation – let’s come back to that shortly.
The key concept is the term “threshold energy.”
I live in a three-story building and the roof is about 40 feet above the sidewalk. It takes a very specific amount of energy to throw a baseball hard enough to end up on the roof – if I throw it with just the slightest bit less energy the ball will end up back on the ground and not on the roof – exactly the same way that an oxygen atom that absorbs a gamma ray with an energy of 13.5 eV will not become ionized. Unless the energy of a gamma ray, x-ray, beta particle, or some other form of radiation is higher than the ionization threshold, an ionization is not going to occur.
Ok – so now on to how ionizing radiation can cause health problems!
I’ve already mentioned that when ionizing radiation interacts with atoms inside a cell it will create ion pairs, but I never mentioned what happens after that. Sometimes these ion pairs will simply recombine – the negatively charged electron might be drawn to the positively charged ion – and then we’re back to where we started with a neutral, uncharged atom. But that doesn’t always happen – sometimes the electron and ion go on to interact with other atoms in the cell, causing changes that result in the formation of highly reactive chemical compounds called free radicals. These free radicals might then react with any of the other molecules within the cell, possibly damaging the cell walls, the organelles within the cell, or even damaging the DNA in the cell’s nucleus, causing point mutations or even breaking one of the strands of the DNA. Damage to the cell and its contents can impair the ability of the cell to function properly; it can even lead to the cell’s death. Damage to the DNA can lead to cancer years or decades later.
What was described above is indirect damage – there are intermediate steps between when radiation interacts with atoms and when the damage occurs to the cell or DNA. Radiation can interact directly as well – radiation that strikes the DNA molecule can add enough energy to the molecule to cause one of the strands to snap; some forms of radiation can knock out a chunk of DNA.
Here’s the thing – our cells have biochemical mechanisms to repair damage to the DNA and to other parts of the cell. These repairs aren’t perfect – but they’re very, very close – so if radiation levels are relatively low and the damage is occurring relatively slowly then the body can do a good job of repairing the damage as it occurs; in cases like this the most common health risks are the chance of getting cancer a few decades in the future due to DNA damage the repair mechanisms miss or that are repaired incorrectly. This is called a stochastic effect (stochastic is a technical way of saying “random” – stochastic effects are things that might or might not happen and they’re expressed as a probability. For example, if I’m exposed to a dose of 100 rem over my lifetime it will give me a 5% chance of developing a serious cancer and a 95% chance that there will be no health effects at all from the radiation exposure.
But what happens when a person is blasted with a lot of radiation in a short period of time?
Well…at some point the rate of new DNA damage exceeds the rate of DNA damage repair; this is the point at which new DNA damage starts to pile up; the point at which we start to worry about acute health effects (also called deterministic health effects) such as damage to the bone marrow (causing a reduction in the number of blood cells), skin burns, radiation sickness, and more. In a sense, this sort of radiation injury is similar to thermal injury (e.g. burns) – burns happen when heat builds up faster than our bodies can remove it and when the temperature gets to be too high the cells accumulate damage and, eventually, start to die.
And that’s pretty much it – the type of damage radiation inflicts on a small scale (i.e. molecules and cells) is pretty much the same whether the dose rate is high or low. But at low doses and low dose rates, when the damage is being repaired fairly effectively, we don’t expect to see any short-term effects; we’re primarily worried about the long-term stochastic risk of developing cancer from damage that’s not repaired or repaired improperly. But at high doses and dose rates, where the radiation damage outstrips the ability of the body to repair it, we’re starting to worry about prompt deterministic effects – exactly what those are will depend on the type of radiation and how deeply it penetrates into the body and the dose that the body (or the affected part of the body) receives, as well as the radiation sensitivity of the cells that are exposed. For example:
- If the radiation is beta radiation, it only penetrates about ½” into tissue, so the person exposed might develop skin burns, but nothing else – this happened to a few workers after the Fukushima meltdowns.
- If the radiation is penetrating gamma or neutron radiation then the most radiation-sensitive organs will be affected first – the blood-forming organs, the lining of the intestinal tract, and the skin cells.
- And if the dose is frighteningly high (more than 500 rem) then there’s a chance that damage will be so widespread that the person who was exposed will die of single or multiple organ failure.