Tuesday 10th May 12:08 pm
Meet the radioactive you
Tuesday 23rd June 2015 2:13 pm
Each of us is bathed in an invisible ocean of radioactive particles and energies — alpha and beta particles, gamma rays, X-rays, neutrinos, muons, etc. In the vast majority of cases, this radiation does us no harm. You can never get away from it, because all of us are personally emitting radioactivity — 24 hours a day.
First, let’s start with a unit used to measure radioactivity. It’s a little confusing. So let me give an example from the martial arts.
There’s a boxer throwing punches — in all directions. Most of them miss you. Only some of the punches land on you. Some might be hard and land like a sledgehammer, while others might be as soft as a feather. Moving on, think about where the punches land. If you suffer 50 little punches on your arms and legs, you would not incur any permanent damage. But if those 50 little punches were all to your left eyeball, you might well end up permanently blind.
In terms of radiation landing on your body, your bone marrow is much more sensitive to radiation than your big toe.
Finally, let’s consider the type of radiation. Matching energy for energy, alpha particles do 10 – 20 times as much damage as gamma rays.
Now there’s a unit of radioactivity called the REM (which stands for Roentgen equivalent in man). It incorporates all the factors I just mentioned — how much lands on you, where it lands and the type of radiation.
It’s a bit old-fashioned, but the Americans love to use it, so I’ll just give in and follow. One REM will give you a roughly 1/2000 chance of eventually growing a cancer. It’s a pretty big unit, so let’s talk mrem (millirems), or thousandths of a REM.
In the USA (for which we have good figures), the average annual dose per human is about 360 mrem. That’s equivalent to about 36 chest X-rays. Some 200 mrem of that dose comes from radioactive radon gas. This radioactive radon usually comes from granite. Granite naturally carries tiny amounts of uranium — which decay into radon.
When radon decays, it emits alpha and beta particles, and gamma rays. There are many locations in the USA where granite is near the surface. Furthermore, if you have granite kitchen benches, you pick up a little extra radon in your kitchen.
Then medical X-rays give the average American about 53 mrem/year. Don’t forget the ciggies (which most of us don’t smoke). Two packs of cigarettes give you as much radiation as one chest X-ray. (The radiation comes from polonium-210.)
The next major background radiation contributor is the world inside our skin. Atoms of radioactive potassium-40, carbon-14 and the like get breathed in, eaten or drunk, and then get incorporated into our bodies. Their radioactive decay gives us 40 mrem/year.
Furthermore, the natural world sprays us with 28 mrem/year. Clothes, paper, concrete and yes, food such as bananas and brazil nuts, are all slightly radioactive.
Those mysterious invaders from outer space, cosmic rays, give us 27 mrem/year. They smash into atoms of nitrogen and oxygen in our upper atmosphere, which then emit muons. You can think of muons as just very heavy electrons. But as they penetrate down towards the ground, muons get reabsorbed by the atmosphere (and don’t give off any more harmful radiation). So this particular radiation decreases as you get closer to sea level, but increases with altitude — an extra mrem/year for each 30 metres.
You receive the lowest cosmic radiation dose at sea level, but significantly more in the mountains — an extra 100 mrem/year if you live at 3000 metres. You get even more — 1 mrem/three hours — with a jet flight, because you are so much higher (up around 10,000 metres).
Battery-powered smoke detectors come next, at about 10 mrem/year. This is because of their radioactive americium-241. (Most mains-powered smoke detectors are not radioactive.)
Also giving the average American 1 mrem each year is the atmosphere radioactivity from past nuclear bomb blasts. Nuclear bombs were exploded in the atmosphere from 1945 until 1980. However, North Korea announced it had tested a ‘nuke’ in 2013.
Another 1 mrem/year comes from nuclear power plants. While the USA has about 100 commercial nuclear power plants, Australia has only one nuclear reactor — but it’s not a huge power reactor. Instead, it’s a tiny reactor that is used for research. It also supplies us with essential radioactive medical isotopes, such as technetium-99.
Finally, there’s your bestie and beloved partner, sleeping peacefully beside you for one third of each day. Bodies are a mini powerhouse of radioactivity, emitting some 40 mrem each year. We are all aglow, both with love and radioactivity. Most of that radiation is trapped by flesh. But a tiny amount escapes. How much? Just 1 mrem/year.
That particular risk I’m very happy to wear and share …
This blog first appeared on Dr Karl's Great Moments in Science
© 2016 Karl S. Kruszelnicki Pty Ltd