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  • [♪ INTRO]

  • The termradiationis 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 arekind 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

  • isgiving 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 radiationthey 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 energythe joule

  • the energy needed to lift 100 grams up

  • by one meterwhich 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 secondbasically, 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.