Becquerels and Sieverts, Oh My!

For some reason, the Georgia Straight keeps writing articles about the radiation from the Fukushima Nuclear plant causing us harm here in British Columbia, and the massive conspiracy involving several governments and hundreds of scientists, working to cover it up.

My cynicism makes me suspect the large number of Georgia Straight advertisers selling bogus “Cleansing” and “detoxifying” services may be influencing their editorial decisions. But what is the source of my skepticism? Perhaps it is their dishonest use of technical terms, without explaining what the terms mean.

Look at this story from last week’s edition. There are many scary statistics there: 0.69 Becquerel per Litre (Bq/l) of radiation in Vancouver rainwater, 8.18 Bq/l as an average in Calgary, 13 Bq/l as a spike in Burnaby! But no-where does it put that number into context. I guess it is expected anyone will think any Becquerel in our water is a Becquerel too much!

Part of the problem with radiation is that the science and the numbers are pretty technical and are often really big or really small, so we have a hard time wrapping out minds around them in a physical sense. It is the job of “journalist” to translate this information to the public, not to fear monger by throwing out terns you know your audience doesn’t understand. So let’s talk about Becquerels.

“Becquerel” is the “metric” measure, and it is easy to visualise: 1 Bq is one atom of radioactive material decaying (and therefore releasing one “unit” of radioactivity) per second. That is the smallest possible amount of radiation per second, so we usually think in terms of millions or billions of Bq, which sometimes makes the number cumbersome. The more common unit is the Curie (Ci), which is equal to 37 billion Bq (37,000,000,000Bq = 1 Ci).

So that is the technical meaning, but what do the numbers really mean in the real world? A good way to look at them is to think about the element potassium (K). The world is full of potassium, it is the “K” in NPK, the Nitrogen-Phosphorous-Potassium ratings for fertilizers. However, a small proportion of it (about 0.012%) is a naturally radioactive isotope Potassium 40. You can’t easily separate the two without advanced lab equipment, so it is randomly mixed in with normal potassium and in all chemical terms, acts exactly like “normal” potassium. So of the ~150 grams of potassium in your bones, teeth, and cellular nuclei, about 0.018 grams is Potassium 40. Atoms are really small, so that that 0.018 grams of Potassium 40 represents about 280 quintillion atoms, that is 280,000,000,000,000,000,000 atoms.

Radioactive atoms. In your bones. These are not “toxins” that you can cleanse yourself of, unless you cleanse yourself of all the potassium in your body, which would be very, very bad for your continued existence.

Potassium 40 has a half-life of about a billion years. So over a billion years, about half of those 280 quintillion atoms will decay, releasing a single unit of radiation each. 140 quintillion decays over 1 billion years equals about 4,400 decays per second. So the normal radiation level of a healthy human body from potassium alone is about 4,400Bq. Every second of your adult life you are exposed to 4,400 Bq of radiation from your own cells.

Notably, your body also contains about 13kg of Carbon, about 1partp er trillion of which is Carbon 14, resulting in another 3,700 Bq of exposure. You also get much smaller doses from the iodine, radium, and other trace radioactive substances in your body.

We also take in and excrete radioactive nuclides all day, and they exist (naturally) in our food, especially things like bananas that have lots of minerals (and are therefore good for you!). Banana contain enough potassium to provide about 130Bq per kilogram, and enough carbon for another 100bq.

It stands to reason that 1 kg of drinking water containing 1Bq or radiation is not going to change our environmental exposure to radiation. It is, in effect, much lower than our “Background” exposure.

This story draws some alarm over vanishingly small measurements of airborne radiation:

“The level of iodine-131 in Sidney, B.C., rose to a high of 3.63 millibecquerels per cubic metre in the air on March 20. That’s over 300 times higher than the background level of 0.01 millibecquerels per cubic metre or less.”

This also benefits from a bit of math. 3.6 mBq is 0.0036 Bq, per cubic metre of air. The average person breathes about 11,000L of air a day. That means a person breathing at Sidney would be exposed to 0.04 Bq of radiation per day. That is less than one ten-thousandth of the radiation you are exposed to from your own bones.

Now ask yourself, why does the Georgia Strait reporter never, in his multiple articles on the Fukushima incident, mention what a Becquerel is?

I would be remit to mention that Bequerel are not the entire story. The number you want to calculate when analysing environmental exposure to radiation is the Sievert: which is a measure of dose exposure. That is a relatively complicated measure to take, as it involves the medium delivering the radiation, and the media receiving it. I have already gone on way too long here, so Will not get into that in this post.

Now compare this to . this story in the Georgia Straight which jumps from Becquerel to Sieverts without putting either number in context.

But which is worse? The “Main Stream Media” have reported nothing, so people have no access to better information. Why can’t our media educate and inform?

One comment on “Becquerels and Sieverts, Oh My!

  1. Thank you for an understandable article putting radiation into perspective. I understand after reading it Becquerels and sieverts are not directly comparable.

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