I’ve seen a lot of misleading and incorrect information on the TV when it comes to radiation exposure.

In college I had a co-op job at Diablo Canyon Nuclear Power Plant. That doesn’t make me a nuclear expert, but along with every other new employee at the plant I had to take a series of classes so that I understood the risks of radiation exposure, how to keep from getting irradiated, and what to do if things go Very Wrong. I’ve been reviewing some of that information so that I could give family and friends this quick guide to understanding what they should and should not be concerned about with the partial meltdowns in progress at the Fukushima Daiichi Nuclear Power Plant Complex in Japan. Hopefully after reading this you’ll have a better understanding of whether or not you are in any danger when you hear something on the news or read something on the Internet.

The first thing you need to understand is that there are two very different types of danger from radiation:

• Radiation Exposure
• Radioactive Contamination

Radioactive materials such as Uranium, Caesium-137^[1] , and Iodine-131 emit high energy particles. When these particles hit your body they impart energy to your body. This is what is meant by Radiation Exposure. It’s similar to ^[2] the way a microwave oven imparts energy to a hot dog, or an EasyBake oven bakes a cake, or sitting out in the sun makes your skin tan (or, in my case, burn).

Radioactive Contamination is when radioactive materials actually get into your body, either through your skin or by being inhaled.

When it comes to Radiation Exposure there are three things that determine how much danger you’re in:

• The strength of the source of radiation. (How many high energy particles does it emit per second? What type of particles does it emit? How energetic are those particles?)
• Your distance from the source of radioactivity.
• How much time you spend next to that source while it’s emitting radioactivity.

Strength: Different sources of radiation have different strengths, this is why Uranium, Caesium-137, and Iodine-131 are considered so dangerous, they’re strong sources of radiation, they emit a lot of very high energy particles that can damage human tissue.

Distance: If you’re standing right next to a 100W light bulb it’s very bright. If you’re 1000 yards away it’s dim. If you’re on the other side of town you can’t see it at all. With radioactive materials, the closer you are to a source of radiation the more high energy particles are going to hit your body. As you get farther away fewer particles hit your body. Get far enough enough away from the bulb and you can’t see it. Get far enough away from a radioactive source and there’s no danger.

Time: A chest X-Ray is a very intense burst of radiation, but the amount of time you’re exposed to that radiation is very short, so while there’s no danger from getting a single chest X-Ray, you don’t want to get one every hour of every day for weeks on end. If you stay out in the sun for an hour you might get a tan, stay out in the hot sun all day and you get burned. If you stroll past the Fukushima Daiichi Nuclear Power Plant you’ll get less radiation exposure than if you pull up a chair and sit there for a couple of hours. The amount of time you’re exposed to a radioactive source affects the dose of radiation you receive.

You may hear news people talking about how many “millisieverts of radiation” people are exposed to at Fukushima Daiichi. A millisievert (mSv) is a measurement based on the amount and type of radiation your body would be exposed to over some period of time. Any news story that quotes a sievert number without including the amount of time it takes to get that dosage is just spreading misleading information. If a news person quotes a sievert number without including the duration of the exposure they do not know what they are talking about and you are wasting your time listening to that news source.

Just like other metric measurements there are 1000 millisieverts in one sievert, so if you hear a news commentator mixing up millisieverts and sieverts you can safely change the channel – the commentator has no idea what they’re talking about.

If you hear the commentator talking about “radiation” without making clear whether they’re talking about exposure or contamination chances are the commentator has no idea what they’re talking about.

There is no safe dose of radiation exposure, but there are dose levels that will definitely cause you harm and there are dose levels that will kill you. If you get a dose of 250 mSv within one day you won’t even have any symptoms. Some people are going to show signs of radiation poisoning with a dose of 250-1000 mSv within one day. Anyone exposed to 1000-3000 mSv within one day will show signs of radiation poisoning. 3000-6000 mSv and you’re almost definitely going to die without medical treatment and probably going to die even with medical treatment. 6000-10000 will kill 95% of people exposed, over 10000 mSv within one day will kill anyone.

I saw a report on this morning that said that the radiation level outside of Fukushima Daiichi was 400 millisieverts (mSv). This is a completely meaningless number unless without a time measurement is included. Would someone get a dose of 400 mSv in an hour? In a day? In a minute? It’s not like a chest X-Ray where your exposure is limited to a few seconds, the plant leakage is on-going, so any dose levels have to be considered over some period of time.

According to an IAEA report it was 400 mSv per hour, but it was limited to a single location and during a single point in time on March 15. On that same day at 00:00 UTC the level observed was 11.9 mSv per hour and at 06:00 UTC it was 0.6 mSv per hour. If I had just listened to the TV news report of “400 millisieverts” — without knowing about the other measurements that were made or how long the 400 mSv measurement lasted — I’d be scared shitless. However, in context what this means is that if you were at Fukushima Daiichi standing right between reactor units 3 and 4 all day you’d be having a Bad Day and you’d have a higher risk of cancer for the rest of your life, but you wouldn’t fall over dead. If you were anywhere else on the planet you’d be fine.

In terms of Radiation Exposure, if you’re in Japan near the reactor complex the reactors themselves are the source of radiation you need to worry about.

If you’re not in Japan the source of radiation you need to worry about is the steam and debris being spewed out by the plant. As long as the wind is out to sea most of that debris is going into the Pacific Ocean, but some is going to make it across the ocean. You will hear reports that the amount of radiation from this cloud represents a very low dose, and that there is nothing to worry about. This is half true. In terms of radiation exposure, the cloud is not a major threat. The problem is radioactive contamination.

The contamination is being spread across northern Japan and across the Pacific Ocean. If you get contaminated with radioactive debris, that means that in the spot on your body where that debris enters your skin or gets lodged in your lungs, that one spot is going to continue to get dosed by radiation for a very long time. That is the major concern of contamination – long term exposure to radiation of a single spot on your body. The dosage per hour can be very low, but because the duration is so long (sometimes years if the particle gets stuck in your lungs) and the focus point of the energy is so small the danger is much larger.

The two major things to worry about when it comes to contamination are:

• What type radioactive material did you get on (or in) your body?
• How much radioactive material did you get on (or in) your body?

The type of material will tell you how strong of a source of radiation you’re dealing with. The more material there is, the more high energy particles it will emit per second. Most highly-radioactive materials are also heavy, so they’ll end up in the ocean or in the soil of northern Japan, but some will make it across the ocean. Keeping yourself from getting contaminated with this radioactive debris is what you want to be concerned about.

Before you start freaking out, consider that during the 1950’s the U.S. Army was busy blowing holes in Nevada and New Mexico with nuclear weapons and the fallout from those explosions drifted over the Eastern United States. The debris from those explosions was far, far more dangerous than what’s coming out of Japan these days. Most of the people who lived through the 1950’s did not die from radiation poisoning, and the cancer rate in the United States has been declining for the past several decades.

So yes, there’s a danger, but it’s not huge and it’s probably not going to kill you tomorrow. The farther you are from Fukushima Daiichi the less danger you’re in. Right next to it or just downwind of the plant, lots of danger. Across the ocean, right now, not so much. I would not want to be living in Northern Japan right now, but I’m not too worried about living in Northern California.

However, conditions change. When you watch or read news reports look for terms like “millisieverts per hour”, “contamination” and “fallout” and pay careful attention to where these measurements were made. If the measurement is 400 millisieverts per hour during one particular hour near the reactor in Japan — and you’re in San Francisco — you are not in any danger. If the report says “radioactive cloud headed for Los Angeles” — but gives no measurement of radiation strength or type of contamination present – turn off the TV and go find a news source that has useful news and not useless scaremongering.

And if a report says that a rainstorm is washing radioactive particles out of the sky, stay out of the rain.

The people of Japan have been hit by an earthquake, a tsunami, and now nuclear radiation and contamination. If you want to help, donate to the Red Cross today. Want to do more? One of the first things done for victims of acute radiation poisoning is a blood transfusion, so go to your local Red Cross office and give blood. Your blood donation might be used to help save someone’s life in Japan, but even if it isn’t, it will help save someone.

^[1] “Caesium” is the spelling recommended by the International Union of Pure and Applied Chemistry (IUPAC). The American Chemical Society (ACS) has used the spelling “cesium” since 1921, following Webster’s Third New International Dictionary. I prefer the spelling “caesium”, so that’s what I use.

^[2] I say similar to, and emphasize the words similar to, because they’re not the same, but the similarity to these other forms of (non-ionizing) radiation that people have everyday experience with is one way that people can wrap their heads around this idea. I hope no one misunderstands the words similar to and takes them to mean the same as, because that’s not what similar to means.