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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. Thanks for watching this episode of SciShow!

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