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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