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The term “radiation” is thrown around a lot.
Like, you might have heard that your cell phone
gives off radiation.
Or maybe you just want to understand whether
those x-rays your doc ordered are dangerous.
But we can’t have a really meaningful
conversation about the relative dangers of
radiation exposure without being clear about
how much radiation we’re talking about.
And unfortunately, thanks to some accidents
of history, the units we use to measure radiation
and radiation exposure are… kind of a mess.
But there’s only two you really need to understand:
the gray and the sievert.
To rewind a little, let’s start
with that term radiation.
In physics, it refers to the energy carried
by particles or waves, so technically,
everything that reflects or emits light
is “giving off radiation”.
But, generally, when people talk about
harmful radiation exposure, they mean
ionizing radiation.
That means particles with enough energy
to rip electrons off of atoms,
or, ionize them.
This is the radiation we care about
being exposed to because it carries
enough energy to break chemical bonds
and cause mutations to DNA that
increase your risk of cancer.
And to get this one out of the way real quick:
cell phones don’t send signals using
ionizing radiation—they use radio waves,
which don’t have enough energy to damage cells.
The ionizing radiation we worry most
about comes from nuclear decays—
that’s when an atom breaks apart
into smaller chunks, releasing other particles
in the process.
And how easily these particles can ionize is,
in part, determined by their energy,
so that’s where we get to our
first radiation units.
Particle energy is usually given in units
called electronvolts, or eVs,
and a typical radioactive particle may have
an energy of about one megaelectronvolt, or MeV.
That’s a million eVs.
And while that might sound like a lot,
it’s nothing compared to our everyday,
standard unit of energy – the joule —
the energy needed to lift 100 grams up
by one meter — which has 6 trillion MeVs in it.
So one particle has a tiny amount
of energy on a human scale.
Of course, radioactive sources don’t
generally give off one particle at a time.
So we also have units to describe
how many radioactive decays happen
in a source per unit of time.
One becquerel, the standard unit
for decay rates, means your source
has one nuclear decay going on in it
per second—basically, a tiny amount of activity.
Your body typically and safely emits
several thousand becquerels all the time.
And there are older units that
describe decay rates, too.
The curie, for example, was based on
the decay of radium-226,
but it’s now defined as 37 billion becquerels.
Somewhat related is the roentgen,
which was in fashion for awhile—
and recently popularized by the
Chernobyl miniseries on HBO.
It focuses on the air instead of
the radiation source.
Essentially, it quantifies how many electrons
are being knocked off per cubic centimeter of air.
And if you’re standing next to something
emitting one curie of radiation or in a room
where a whole lot of electrons are being
knocked off of air molecules,
that’s probably not great for you.
But from a medical perspective,
it’s not enough to know how many particles
of radiation are in a room
or are being emitted by something.
You need to know exactly how much
radiation your body is absorbing:
this is called the absorbed dose.
The standard unit for absorbed
radiation dose is the gray.
A dose of one gray means that one kilogram
of matter has absorbed one joule
of radiation energy.
For instance, a CT scan in a hospital
might expose you to seven milligrays,
or seven thousandths of a gray.
Some people still use the rad—
a historical unit now equivalent to 0.01 gray.
But whether you’re talking rads or grays,
there’s an additional complication:
some types of ionizing radiation do more
damage to the body than others.
So the same amount of grays can do
different amounts of damage,
depending on the type of ionizing radiation.
There are lots of types of ionizing radiation,
but the three main ones are called
alpha, beta, and gamma.
Each refers to a different type of
particle ejected during nuclear decay.
Alpha particles are the heaviest,
slowest, and most easily stopped,
while gamma particles are the
lightest, fastest, and hardest to contain.
And that’s what finally brings us to the sievert.
It modifies the gray to account for
the different health risks associated
with these particles.
To get sieverts, you multiply grays
by a number specific to each type of radiation.
Most of the time, like for beta and gamma particles,
the number you multiply by is just one.
But with some, like alpha particles,
you multiple by twenty because
alpha particles pack a real punch.
And that ‘twenty’ number isn’t arbitrary:
it’s chosen based on the latest,
constantly updated research into
the effects of radiation on human health.
In that way, the sievert is a unit
for human convenience:
it takes the gray, which measures something
exact and physical, and modifies it
to tell humans how dangerous a type of
radiation exposure might be when
it interacts with our bodies.
You also may have heard people talk about rems
when discussing exposure.
It’s the same idea, though an outdated version.
Rems is short for ‘roentgen equivalents in man’—
one rem is now defined to be 0.01 sieverts.
But really, the sievert and the gray are
the only units you need to remember
to understand the overall impact of
radiation exposure and make informed
decisions about how you travel,
receive healthcare, and…
generally live your life.
Because the truth is there are
lots of sources of radiation,
including natural ones, and everyone
is constantly exposed to some of it.
On average, people are exposed to
a few millisieverts per year
from their everyday lives.
The amount from medical scans varies
depending on the size of the scan
and the duration in the machine—
from less than 0.01 millisieverts for
a quick joint x-ray to about
ten millisieverts for a full abdomen CT.
These raise your lifetime risk of cancer
by one in a few million to
one in two thousand, respectively.
And to put all that in perspective,
the allowed limit for the average
nuclear industry employee is
20 millisieverts per year.
Point is, while radiation exposure
can be bad, it’s also something
that happens to us every day.
And once you understand the lingo
used to describe radiation,
that fact doesn’t seem so scary.
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