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Lesson 4: Background radiation

Introduction

In this short lesson we’ll see that we’re bombarded with radiation all the time quite naturally – this is called ‘background radiation’.

Try Why Do Astronauts Float by Julian Hamm

Radon gas comes from rocks and soil

Radon gas is an alpha emitter that comes from the radioactive decay of naturally occurring minerals in rocks and soil.  The gas seeps out of the ground and is quickly diluted in the air.

Radon in your home depends on where you live and what your house is like

But radon gas can collect in your home and become much more concentrated.  It's in your house that breathing in radon gas poses a health risk.

How much radon there is in your house depends on how easily radon can come up through cracks in the floor and how easily it can escape.

Some rocks produce more radon than others.  Granite produces lots of radon.  So some parts of the country have naturally higher radon levels than others.  But neighbouring houses can have very different radon levels depending on how easily radon collects in them.

Radon emits alpha particles.  Alpha radiation can’t penetrate very deep into the body.  But radon is a gas so we can breathe it in.  The alpha radiation from radon greatly increases the risk of lung cancer.

The government sets maximum radon levels before you ought to do something

If the radon level in your house is too high then the government advises you to reduce it.  You can do this by sealing the floor to stop radon entering or by installing a fan system to help it to leave.

Radon is much more dangerous if you smoke

Without any exposure to radon about one person in 200 would be expected to develop lung cancer by the age of 75.  If you smoke the risk is TWENTY TIMES greater.

At just above the maximum safe level set by the government about one extra non-smoker in 200 would be expected to get lung cancer.  But seven extra smokers would get it.

Smoking makes radon exposure much more dangerous.

At six times the government 'action level' only one more non-smoker would get cancer but an additional THIRTY smokers would get it.

If you don’t smoke, radon exposure increases your risk of lung cancer from very low to a bit higher.  If you smoke your risk of lung cancer goes from high to very high.

Cosmic rays from space

Cosmic rays consist of many different particles as well as gamma rays.  Even though lots rain down on the Earth the whole time many of them can pass straight through people without any effect.

We only worry about particles that cause ionization in a person’s body.

The higher you are the more more cosmic radiation you get.  For example by flying or living at high altitude.

Gamma radiation from rocks and soil

Almost all rocks contain natural radioactive compounds.  These compounds emit alpha and beta radiation.  But most of this radiation gets absorbed by the rocks themselves and never makes it into the air.

Gamma radiation is often produced at the same time as alpha and beta radiation.  Gamma radiation can pass through the rocks and so this is the type that we detect most of.

Bricks, cement and slate are made from stuff that comes from the ground so these also contain radioactive compounds.  Even though there is lots of natural gamma radiation about most of it passes straight through people.

Radiation emitted by the body

Naturally radioactive compounds are found in the air, soil and water.  So the food we eat is slightly radioactive.  Our bodies are made from the food we eat so we are also a little bit radioactive.

Most of the radioactivity comes from potassium-40 and carbon-14.

Most of this radiation is beta with a tiny amount of alpha so is absorbed in our body.  Sometimes gamma rays are also emitted.  These pass out of our body.

Nuclear power, weapons testing and Chernobyl

Chernobyl was the site of a nuclear power station in Ukraine where there was a major accident in 1986.  Even though it was the worst nuclear accident in history its contributions to background radiation are extremely small.  The same is true of all the nuclear weapons tests.

Nuclear power stations if they are properly run add almost nothing to background radiation.

Medical, mainly X-rays

When we go to hospital or the dentist we may have an X-ray.  X-rays do not come from radioactive sources.  They are made using a machine.  But they can cause ionization which is why they are included with background radiation.  If you don’t have any X-rays or radiation exposure at a hospital then you don’t receive any background radiation because of them.

Any X-rays that do escape through the hospital walls are so spread out that they are not an important contributor to background radiation.

Jobs using radiation

X-rays and radioactive materials are used a lot in medicine.  People who work with ionizing radiation receive more radiation than most people.  This is carefully controlled by shielding and keeping track of how much radiation they are exposed to.

A common way to do this is to use a badge with a strip of film.  Radiation exposes the film.  The film is developed at the end of each month.  The darker the film the greater the overall exposure to ionizing radiation.

The government sets limits to the maximum exposure each year.  This is typically about ten times background.

If people who work with ionizing radiation receive so much more radiation why is the slice of pie very thin?  It’s because the pie shows an average across the whole population and only a small proportion of people work with ionizing radiation.

It’s important to realise that background radiation in some parts of the country might be ten times higher than in other parts.  So workers exposed to ionizing radiation don’t really receive an unnaturally large amount.

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