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Lesson 5: How radiation harms


In this lesson we’ll look at how nuclear radiation can harm us.  We'll learn about radiation sickness and cancer risk and how the amount of radiation you're exposed to can be measured.

Try Why Do Astronauts Float by Julian Hamm

You only get large doses if something goes badly wrong

You only get large doses of radiation in an accident or nuclear explosion.  Huge doses kill so many cells that you die quickly.  If the radiation itself doesn’t kill you it still makes you very ill.  You can catch infections more easily and these can kill you.

Whole body and local doses

A large dose is most dangerous if it’s spread over your whole body because it damages so many cells.  The same dose concentrated in a small area may produce local burns but won’t kill you.  You wouldn’t even notice a small dose but it will increase your risk of getting cancer in later life.

Radiation damages cells that are dividing

Radiation is most damaging to cells that are dividing rapidly.  This is why it kills cancer cells but also makes your hair fall out.

Which type of nuclear radiation is most dangerous?

Alpha radiation is most dangerous inside the body because it causes so much ionization in a small volume.  Remember you can't breath in radiation but you can breath in a substance that emits radiation.  If you breath in an alpha emitter, the alpha radiation can damage your lungs.

Outside the body gamma radiation is most dangerous because it’s difficult to shield, even though it causes little ionization when it hits you.

How radiation damages our cells

Radiation harms us because it transfers its energy to our cells, damaging them.  The particles themselves are not ‘poisonous’.  Alpha and beta particles give up their energy and then stay in our body like harmless sub-atomic dust.

Gamma radiation disappears when we absorb it.  It’s energy remains and this can cause electrons in our cells to move quickly.

The amounts of energy involved are very small.  A teaspoon of boiling water will give you a nasty scald.

But the same amount of energy delivered by gamma radiation is enough to kill you.  Nuclear radiation is so dangerous because it can damage the most sensitive parts of cells deep in your body.

What does the energy of a radiation dose depend on?

The harm that radiation does depends on the type of radiation but the amount of energy absorbed doesn't.

You absorb more energy if you're exposed for longer or if the source is bigger so it emits more radiation per second.  You'll also absorb more energy the higher the energy of each particle.

A beta particle is much faster than an alpha of the same energy

An alpha particle has about 7000 times the mass of a beta particle.  This means it needs to be going much slower to have the same kinetic energy as a beta particle of the same energy.

A gamma ray is a type of high-energy invisible light so it travels at the speed of light.  The higher the energy of the gamma ray, the shorter its wavelength and the higher its frequency but its speed doesn't change.

But it's the energy of the particles that's important when working out your energy dose not how fast they're moving.

The energy of alpha and beta particles

Alpha particles from the same source all have the same energy.  In other words they all travel at the same speed.  Some sources produce alphas of higher energy than others.

Beta particles from a given source come with a continuous spectrum of energies.  In other words the speed is random, from very low up to a maximum speed that depends on the source.  Different beta sources have different peak energies.

This was a real puzzle for scientists at the start of the twentieth century but was solved by some brilliant creative thinking by one of the founders of quantum theory, Wolfgang Pauli.

DNA damage with alpha, beta and gamma

If you are exposed to the same energy dose by gamma or beta radiation then they cause roughly the same amount of harm.  But this amount of energy delivered by alpha particles can be up to 20 times more damaging.

Radiation is most harmful if it damages the DNA in our cells.  The radiation can damage the DNA directly or it can produce very reactive molecules called free radicals which can cause even more damage.

DNA damage causes long-term and short-term problems

DNA molecules hold the pattern to make proteins.  Proteins are the actors that DO everything we need to keep us alive.  Enzymes, hormones and antibodies are all examples of proteins.  If the DNA is damaged then we can’t make the proteins we need and in we can quickly get ill.

Different parts of your DNA tell cells when to divide, when not to divide or to commit suicide if the DNA is damaged too much.  If these parts of the DNA are themselves damaged then cell division can get out of control and a cancerous tumour is formed, normally many years later.

DNA is damaged all the time quite naturally but we have evolved ways of repairing the damage.  If lots of damage happens to the same cell then the DNA may not be repaired correctly.  This is why alpha radiation is much more dangerous than the same amount of beta or gamma radiation.

How close together ionizations happen is the key to the harm done to DNA

Alpha and beta both tear the same number of atoms to bits if they both have the same amount of energy to lose.  But the ionizations from the beta (and gamma) will be quite spread out whereas the ionizations from the alpha will be close together.

This means the alpha can damage the DNA of one cell a lot whereas gamma and beta will damage the DNA of lots of cells a bit.  When we say that alpha radiation is ‘highly ionizing’ we mean the ionizations are close together not that there are more of them.

Gamma radiation tends to cause ionization indirectly.

The sievert as the unit of absorbed dose

So the dose of radiation we receive depends on the amount of energy we absorb and the type of radiation.

We measure radiation dose using a unit called the ‘sievert’ (Sv).  It's named after the mid-20th century Swedish physicist Rolf Maximilian Sievert.

Background radiation gives us a dose of few thousandths of a sievert every year.  A thousandth of a sievert is called a millisievert (mSv).

How much does gamma radiation increase cancer risk?

Let's think about how much a group of 400 people's cancer risk will increase as they are exposed to different doses of gamma radiation.

Over a lifetime about 170 of them would be expected to get some sort of cancer.  These cancers could be caused by lots of different things: a poor diet, smoking, a virus, genetic make-up or just pure chance.

Cancer only kills some people who get it.  So we won't be thinking about actual deaths from cancer.

A dose of 20 millisieverts is the maximum dose that people who work with radiation are allowed to be exposed to each year.  About one extra person would get cancer when all 400 people were exposed to this dose.

Cancer from radiation looks just like cancer from other causes so you can't tell who that extra person was.

100 mSv is the dose of gamma rays received by anyone living about 2 km from the atomic bomb dropped on Nagasaki.  About an extra 4 people out of 400 would get cancer at this dose.

As you increase the dose above 100 millisieverts the proportion who contract cancer later in life increases.

Radiation sickness

Cancer is a long-term effect.  Above about 200 millisieverts there start being short-term effects.

First your red blood cell count begins to drop because your bone marrow is damaged.  You won’t notice a small drop but a bigger one will make you feel tired, dizzy and short of breath.

If you go to hospital you may have a CAT (Computed Axial Tomography) scan.  It uses a high dose of X-rays – about 10 times the gamma dose you would get from background in a year.  It’s about 0.01 sieverts (10 millisieverts).

Above about 0.5 sieverts (500 millisieverts) some people will start to feel sick.  The immune system is also affected so there is an increased risk of infection.

Above about 1 sievert a small proportion of people may die if not given medical attention.

Everyone exposed to 2.5 sieverts will feel sick and almost a third will die without treatment.

Above about 3 sieverts radiation causes internal bleeding which will help kill those already weakened by sickness and infection.

By 6 sieverts almost everyone is either dead or seriously ill from internal bleeding.

Above 7 sieverts everyone will die within two weeks and this time gets shorter as the dose increases further.

Radiation sickness is certain, cancer is completely random

Radiation sickness happens quickly, often within hours of exposure.  Cancer happens many months, years or decades after.

There is another important difference.  The effects of radiation sickness are certain.  Above 3 sieverts everyone feels ill.  The higher the dose, the worse you feel.

Cancer is not certain: it is random.  You can be exposed to a very high dose and not get cancer.  The higher the dose the more LIKELY you are to get cancer.  But a higher dose doesn’t give you ‘worse’ cancer.

There are some more controversial ways in which radiation can cause you harm.

Protecting yourself from nuclear radiation: distance, shielding, time

We’ve seen how radiation can harm us.  Now let’s look briefly at how we can protect ourselves from being harmed by radiation.

Unless you are near a serious nuclear accident or a nuclear explosion you will never get radiation sickness.  In the extremely unlikely case that you are near one of these events then there are some things you can do to reduce your risk.

The first thing to remember is that radiation does not travel far but radioactive dust does.  So keep indoors away from the dust.

The second is that even gamma rays are stopped to some extent by brick and earth.  So stay in the cellar if you have one.

The third thing is that the most radioactive substances are around for the shortest period of time.  Waiting several days before leaving your house will allow these to stop being radioactive.

Radioactive iodine and thyroid cancer

A nuclear accident or explosion may produce lots of radioactive iodine which can contaminate your food and so enter your body.  Iodine accumulates in your thyroid gland, which can give you thyroid cancer.

You can ‘fill up’ your thyroid gland with non-radioactive iodine by taking potassium iodide pills.  This will stop your thyroid taking up radioactive iodine because it has already absorbed as much as it can.

Radiation protection for lower doses

These precautions are for severe accidents only.  Most radiation protection is designed for people who work with it every day.  or example, doctors and patients need to be protected from radiation used in medicine.

The principles are similar:  keep your distance, use shielding and limit your time of exposure.

Everyone who might be exposed to radiation as part of their job is monitored carefully to check they don’t get too much.

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