We are getting a lot of questions, both at firstname.lastname@example.org and through the Safecast group from concerned people – they all try to understand some aspect of radiation or another. And of course, many people ask similar questions, but without doubt the most often asked question is “Is [something] safe?” or a variation of it. Believe it or not, there is an answer in our radiation FAQ, but people keep asking 😉
Recently someone who was planning to spend a few months in Kyushu asked about the contamination there. He had located some info on the radiation levels, and had concluded that currently the radiation level is higher than before 3/11. He asked if that was because of Caesium and Strontium contamination. In fact, he was comparing two different data sets that had been collected differently. I pointed him to DPNSNNE which allows users to download consistent monitoring data from all over the country and compare the changes over time. This data shows that aerial radiation levels in Kyushu are the same now as they were before the accident. As often happens in forums/mailing lists, once the question was answered, the discussion shifted the topic to “the difference in risk from Radon (naturally occurring) compared to Caesium and Strontium (anthropogenic)”. Immediately, data on half-lives (=time that it takes for the half of a certain amount of a radioactive isotope to decay) of Rn, Cs-137, Cs-134 and Sr-90 was put on the table as “evidence” and difference between half-life and biological half-life was discussed. So the question transformed into:
- Since the (radiological) half-life of Radon is only 4 days, while Cs-134/137 has a biological half-life of 70 days (that is, half of it will be eliminated from the body in 70 days), and Strontium’s is 18 years, doesn’t that mean that radon is less risky than the others?
I had been thinking about this a lot and thought I might be able to explain it by example; an example that turned into this blog post (Here is a good place to thank Azby, Jam and everybody who read through my ramblings and pushed me to publish it).
Yes, it is true that radon (Rn) doesn’t pile up in your body, but you’ll eventually reach some equilibrium and it will stay inside you until you move out of that place. Say you stay 10 years where you are, so for 10 years you’ll be irradiated. Also, a short half-life doesn’t mean “better” or “safer” just because it will disappear relatively quickly. It disappears because it gets transformed into another isotope, emitting alpha and/or beta and/or gamma radiation (depending on the isotope) while it does. So a short half-life just means that this radiation will be emitted sooner, at a higher rate!
Whether your body is adversely affected by that amount of radiation is still an open debate (do a web search for “LNT model”) and defining someone’s personal threshold might be, assuming they have one, yet another debate. But discounting something because it is small (yet persistent) is the wrong approach…
Anyway here is the example… but please don’t take everything literally. Read through, imagine the situation and try to answer quickly all the questions. The setup is simple, so do your best to get “inside” the scene:
Now here are some of the observations you can make:
- Flashes have different intensity at their source (different cameras, different settings)
- Flashes come at random times (e.g. because people take their time to compose)
- Flashes come at random angles (to you), some reflected (off walls, mirrors, etc.), some refracted (bottles, painted glass wall)
- Sometimes pictures are taken without flash (high end DSLR?)
- Sometimes two or more flashes happen at (almost) the same time; your perceived intensity is the sum of all of them
Party goes on…
- You enjoy the first shots (admiring the strange light patterns)
- but it starts to get uncomfortable, and at some point you can hardly see anything when all the flashes have stopped (because somebody snapped a photo of your face from 13.14 cm)
- later your head starts hurting (from too many flashes or from too much alcohol, you wonder)
Party is over…
- You sleep (in a dark room, luckily no more flashes) and the next morning everything is fine (just a hangover lingers)
- The next night you go to a similar party…
- … and the next and the next
OK, now, while still feeling the party, here are some quick questions for you:
- What was the nearest shot of your face? (that was just to wake you up, skip to the next question)
- What is the “single dose” (=strongest flash) that you can take without any adverse effects?
- What is the “single dose” (=wildest party) that you can take without any adverse effects?
- How can you measure or prove that? BTW, what should be considered an “adverse” effect?
- What about your friend? Can YOU tell the above for him/her?
- Do both of you have same single dose threshold?
- What about your kid/parent/grand-parent?
- Can you estimate that for a person you’ve never met/talked with? What if that person is blind?
- How many “single doses” can you take per unit of time without any adverse effects?
- How much “exposure” will lead to permanent damage of your eyes?
- How much “exposure” will lead to permanent damage of your ears (there is sound as well, especially from big flashes)?
- How many parties will it take to permanently damage your liver (from alcohol)? Will that happen sooner than your eyes being damaged from flashes? And your ears?
And the final set of questions:
- If you trip and fall on a staircase and break your neck and die…
- … while going to work, will that be related to the flashes?
- … if you your sight has been damaged (even if you didn’t notice)?
- … if you were having a hangover?
- … if you were thinking about the party last night and all the flashes and were careless?
- … if the staircase was on the exit from the party?
- … if the staircase was on the entrance to another party?
So? Did you manage to get a clear answer quickly to all of the questions? Was it difficult?
Well it was only a simple example involving light (for which we humans have built-in sensors). Now, I’ll leave it up to you to change the example by using radiation 😉
In that same thread, the difference in internal exposure from Caesium/Strontium or Radium was discussed… The argument was that because Cs/Sr have longer half-lives and longer biological half-lives compared to Rn, Cs/Sr would produce a kind of a “bottleneck effect”, meaning that our body would accumulate the radiation without being able to get rid of it.
As a start, the original poster suggested “condition of parity between 0,12 uSv/h of mostly radon and 0,12 uSv/h of mostly caesium and strontium” and that immediately felt like someone saying, “I need to fill up my car, you think 3.5 meters of gasoline will be enough?”. Yes, you were right, internal exposure is not measured (usually) in uSv/h.
And even forgetting that, the answer can be both “yes and no,” since this is comparing apples to oranges…
- Rn-222 is gas, Cs-137/134 is solid (usually in compounds)
- Cs-134 (2 years half-life) and Cs-137 (30 years half-life) have identical biological half-life (and slightly different effective half-life)…
- How should we measure the “same amount” of Cs and Sr?
- By mass, volume, number of atoms?
- In pure form or in a compound?
- In what physical state (a solid block, a solution, a powder, a gas, an aerosol?
- if powder, what particulate size?
- if solution, what is the solvent?
- Which isotope/mixture of isotopes and in what proportions?
Complex enough, right? Then let’s make a “simpler” discussion of half-lives, taking only Cs-137 (30 years) and Cs-134 (2 years) into account.
If you must choose between 2 mg of each to eat, which is “safer”?
Let’s ignore that 2mg of each contains slightly different number of atoms, and assume each has “N” atoms. Biologically they are both equivalent, therefore their biological half-lives are the same (technically speaking, their effective half-lives will be slightly different).
So, roughly speaking, Cs-134 decrease to half (1mg) in 2 years and Cs-137 will be half in 30 years. Does that make Cs-134 the safer choice?
Actually, because of its shorter half-life Cs-134 will emit more radiation per unit time than Cs-137, so assuming they decay in the same way (they actually don’t) Cs-134 is simply more active, it pops faster and produces more radiation per day, resulting in a higher DRE (external gamma dose rate equivalent) from the same “amount” of Cs-134 compared to Cs-137. It simply “burns” faster, delivers more energy per time if you wish. Remember the party example above? Well here we are talking about your threshold again.
Can your body withstand (without damage) 2mg Cs-137 “burning” over 30 years on “low” or is it easier for your body to deal with 2mg Cs-134 burning on “high” for only 2 years?
Now, replace “2mg” with 0.03 ng (nano grams), this is roughly 1 kg of 100Bq/kg food.
Did you change your mind? What do you think of say 1 g of Cs-137 (=3.21TBq)?
Now, looking back at the shorter biological half-life (compared to the radiological half-life), your body will be able to remove most of the Cs-137 and quite a substantial part of the Cs-134 before they decay and cause any trouble, if, AND ONLY IF, that was a “short term” exposure (this is part of the definition of a biological half-life). So if you are ingesting Caesium every day, the picture is quite different – depending on how quickly your body sheds Caesium compared to how much you intake, your Caesium content might be increasing, decreasing or at an equilibrium.
Now, back to Radon… If you constantly live with Radon for a long enough time (compared to Radon’s half-life of 4 days), your body must have reached an equilibrium. If you go to a Radon-free place for long time (usually 10 half-lives, but for humans it is ten effective half-lives) you’ll be practically free of Radon at some point. Coming back you your original place, your body will quickly regain equilibrium.
To sum up, it may be possible to compare Radon exposure to Caesium exposure, but there are so many unknown variables (thousands) that doing so in a scientific manner is close to impossible.
But here are some rules of the thumb:
- The longer you are exposed the more potential damage is caused by radiation
- The higher the decay rates, the more potential damage:
- The more radioactive material (or the more concentrated), the higher the rates
- The shorter the half-life, the higher the rates
- The closer you are (to a point source), the higher the rates
- Effective shielding can reduce both time and rate of exposure
That is why it’s best to minimize exposure times and limit exposure rates at the same time.
So the next question is, if we have 0.12 uSv/h of mostly Radon and 0.12 uSv/h of mostly Caesium and Strontium, wouldn’t the former be less damaging to our health?
Well, without getting too technical, saying “0.12 uSv/h” only indicates the DRE and has nothing to do with how much you’ve ingested/inhaled of anything. Intake is measured in Bq and there is no simple conversion between Bq and uSv/h (trust me!).
Let’s have another example:
There will be a huge difference in the temperature inside the car depending on things like:
- windows open/shut
- car moving/stopped
- air-conditioner on/off (high/low/heat!)
- wet or dry
- windy or calm weather
- sunny or cloudy
- in the sun or in the shade
Despite the fact that all the time it’s 25°C outside… It’s possible to either catch a cold or die from heatstroke in the same car at the same outside temperature. And when you think about all the types of cars (compact/truck, white/black,…) it gets even more complicated.
So the questioner’s reasoning was only partially wrong. I recommend that people dig deeper if they want to make educated guesses about radiation safety. And if they want to do it scientifically… good luck to them, because a lifetime will likely not be enough!
Some people argue whether drinking a glass of red wine per day is good for your health in the long-term or not; others argue that drinking a bottle per day is good/even better/bad; most of us would agree that, say, 10 bottles per day is bad. Yet there are many people who insist that even an occasional glass of wine is bad! Further on, we can discuss coffee, and chocolate, and steaks and what not?
I listen to what most people say, do my own (very complex) analysis (most of the time), and then:
- I drink wine, up to a bottle, yet definitely not every day, but sometimes 1-2-3 glasses every day for several days!
- I drink coffee, sometimes 3-4 cups a day, but sometimes 7-8 a day, and sometimes I don’t drink any at all for a week or two!
- Chocolate, well…
This approach may not work for many people, but this is how I do it for myself.
Actually THIS is the most important thing I am trying to teach people: to think, learn, reason, assess, and act based on your own experience, knowledge, present state, and desires. Out there there are more risks than most of us can imagine, but you’ll never do anything if you just sit and worry about everything. That said, life is hard, read all disclaimers, and shit happens. May the power be with you!
For the record, the discussion threads mentioned above are:
- Background radiation in Fukuoka
- Radiation safety: educated guess (Was: Background radiation in Fukuoka)
I know it was a long read, I hope it was an enjoyable one!