The Chinese Ministry of Environmental Protection has been publishing some data showing tiny amounts of radioactive iodine over many cities in China;  similar small amounts of this iodine-131 have been reported in many cities in America as well, including as far east as Massachusetts. 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 the twice-daily graph that’s being published online as well as in most newspapers (this latest English version is from March 28, 2 days ago):

But here’s the top question on everyone’s mind: what 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, Beijing’s Nutritioneer 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 one millionth (1/1,000,000th) fraction of a sievert (printed as uSv); “millisievert” = 1,000th fraction of a sievert (printed as mSv).

So let’s compare our normal daily dose of radiation (10 microsieverts) to some common items, in increasing order of dose:

• Dental x-ray: 5 microsieverts
• Chest x-ray: 20 microsieverts
• Airplane flight NY-LA: 40 microsieverts
• Mammogram: 3 millisieverts
• Chest CT: 5.8 millisieverts
• Radiation worker annual dose limit: 50 millisieverts
• Lowest one year dose clearly linked to increased cancer risk: 100 millisieverts
• EPA dose limit for workers in emergency situations: 250 millisieverts
• Dose causing radiation poisoning: 400 millisieverts

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

What About In America?

The American system uses the rem classification: 1 rem = 0.01 sievert (10 millisieverts; or 1 sievert = 100 rem)

Here’s that very helpful radiation dose chart; click on it for a closer look: