The US Environmental Protection Agency’s Daily Data Summary continues to say that the “EPA’s RadNet radiation air monitors across the U.S. show typical fluctuations in background radiation levels. The levels detected are far below levels of concern”; also mentioning that, “as the Nuclear Regulatory Commission has said, we do not expect to see radiation at harmful levels reaching the U.S. from damaged Japanese nuclear power plants.” I suppose we will have to take their word for it; I find their RadNet map totally indecipherable, with their Google map offering me gibberish such as “Gamma Energy Range 2 Gross(CPM)” and other such user-unfriendly information. Where’s the real-world context or graphing to keep the raw data in perspective? Where’s the conversion to sieverts, or to rem — to anything that we simple folk can grasp? I find it interesting that here in Beijing, where I currently work, the radiation information is more accessible than in the US. For example, the Chinese Ministry of Environmental Protection has been publishing data twice a day, along with a graph, showing tiny amounts of radioactive iodine over many cities in China. The MEP’s press release from yesterday says “there was no need to take protective measures” as “the levels of radioactive material were below one-hundred-thousandth of the natural background radiation.” Here is their latest graph published online as well as in most newspapers (this latest English version is from March 28):
I will discuss the graph in detail in a minute, but here’s the top question on everyone’s mind: what radiation dose actually is harmless, and at which dose should we worry? So here’s my take on it, and I think the illustration at the bottom of this article is a wonderfully easy way to think about this issue (thanks to Olivia Lee for this image link).
Firstly, I think we all know by now that radiation exists naturally in our environment, from many natural sources such as deep space rays as well as natural radon seeping into our basements. I’ve seen numbers that 10 microsieverts a day is the average normal background dose (a bit higher living in higher altitudes). Annually, we get about 3.65 millisiverts of background radiation exposure. The word “sievert” has been thrown around the news a lot; a sievert is the scientific measure of absorbed radiation. “Microsievert” means 100,000th fraction of a sievert (printed as uSv); “millisievert” = 1,000th fraction of a sievert (printed as mSv). In America, the measurement system is rem and not sievert; one sievert equals 100 rem. The EPA estimated means the average annual radiation dose per person in the U.S. is 620 millirem (6.2 millisieverts).
So let’s compare our normal daily dose of radiation (10 microsieverts) to some common items, in increasing order of dose (data mostly from the EPA):
- Dental x-ray: 5 microsieverts (0.5 millirem)
- Airplane flight NY-LA: 40 microsieverts (4 millirem)
- Chest x-ray: 100 microsieverts (10 millirem)
- Mammogram: 300 microsieverts (30 millirem)
- Chest CT: 5.8 millisieverts (0.58 rem)
- Radiation worker annual dose limit: 50 millisieverts (5 rem)
- Lowest one year dose clearly linked to increased cancer risk: 100 millisieverts (10 rem)
- EPA dose limit for workers in emergency situations: 250 millisieverts (25 rem)
- Dose causing radiation poisoning: 400 millisieverts (40 rem)
Let’s think about this for a minute; a chest x-ray, which most people wouldn’t think twice about, is about 2 days normal exposure. A chest CT gives you over a year’s dose; this is quite a large dose which many doctors now realize could be contributing to new cancers, especially when CTs are done during annual “routine” health physicals. (this is a whole extra topic…)
OK, Let’s Get Back To Japan…
Now let’s use this new info and first go back to that confusing graph above showing readings in China: the graph’s units are in uGy/h; the helpful wikipedia article mentions that 1 sievert = 1Gy. (Technically 1Sv = Gy*w, a weighting factor, but this is where my physics brain hit the wall…) So, this means that the graph is reading microsieverts per hour. All of the graph’s readings are around 0.1 microsievert per hour; multiply that by 24 hours in a day and you have 2.4 microsieverts per day, which as we noted above, is far below the normal daily dose of 10 microsieverts. So yes, it does seem to be true that the current radiation exposure is “harmless”. Of course, no dose of radiation is 100% harmless — even “normal” background radiation is a risk for cancers (did you know that radon inside homes is the #2 natural cause of lung cancer?) Also, I should stress that I am no physics whiz and I may be doing this calculation incorrectly, especially regarding this weighting factor; any expert can feel free to correct me in the comments section below. But the finding is consistent with the reassuring messages from both countries’ agencies.
What About In America?
As I already mentioned, I don’t even see basic information like sieverts or rem from anything on the EPA website; is there another website I am missing that does this? If not, then I think that things can be vastly improved at the EPA’s website, which is supposed to be the final say on such matters. People right now are very anxious and starved for helpful, practical information, and the EPA website is not offering that. They do have a very good collection of pages explaining radiation in general, including information for schools as well as this handy image showing relative doses and health:
However, right now people need more immediate data with simple explanations: I think a visual graph would be enormously helpful which converts their raw data to sieverts or rem, along with a sidebar-type graph to keep it in perspective, similar to the above image or another very helpful (but not “official”) radiation dose chart that I mentioned earlier. You can click on it for a closer look:
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