Thursday, March 17, 2011


background radiation dosages

I found it frustrating when the news from the disaster at the Fukushima nuclear power plant would mention radiation levels in multiples of average background levels instead of actual numbers.

However, in the past couple of days, during the power plant and government press releases and conferences, as well as the many media reports, there has been a lot of data provided using either microsieverts per hour, or millisieverts per hour. So if you see a number described this way, and are wondering how it relates to the normal radiation levels we receive, read the information below.

I’m mainly using the microsieverts unit of measure, and when there is not a per hour or per year rate qualifier after the unit, then it is a total absorbed dosage amount.

How the US and Japan calculate average annual radiation dosages:

All units are in microsieverts (µSv), and are from documents, not actual measurements I have taken. To convert µSv to millisieverts (mSv), take the microsievert value, and move the decimal point to the left, 3 places. Do the opposite to convert mSv to µSv.

To convert microsieverts to the older millirem (mrem) unit, divide by 10, or to convert mrems to µSv, multiply the mrem number by 10 to get µSv.

Sources of natural radiation (in (µSv/yr) Japan US

1. Cosmic radiation at sea level: 390 260
2. Food (e.g. Potassium): 290 400
3. Radon exposure in air: 1260 2000
4. Terrestial exposure: 480 460

Total avg. natural radiation: 2420 µSv/yr 3120 µSv/yr

I doubt that there is much difference in background levels between Japan and the US at sea level, but the 2420 number for Japan is important since they refer to it often. But in the same document, they mention an additional 1000 µSv/yr for the average citizen for “public spaces” with no further definition.

Additional radiation received from various sources below will likely increase the total amount of radiation that you receive yearly by 2-3 times. Variables that add to our natural background radiation are dependent on the following:

1. Where You Live:

a) Your altitude above sea level increases cosmic ray radiation: add 100 per 1000 ft. elevation.

b) Your terrestrial ground exposure:

For the US, if you live in the Gulf or Atlantic Coast areas, you can reduce the 460 above to 230. However, if you live in the Colorado Plateau, you should add 440 to the 460.

c) What your house is made out of, and what kind of ground it is sitting on. Brick houses, and houses built on rocks with a lot of Radon will add to your annual radiation dose.

d) Your proximity to power plants. Coal plants will generally provide more radiation than a nuclear power plant. Unless the nuclear plant is having issues …

For Japan, if you live in Gifu – Kanagawa, you should add 400 to the 480. For other parts of the world, Wikipedia or Google will probably have approximate data based on your location.

2. How many medical radiological checks you receive per year:

Add these values to the total yearly value, as applicable:

Abdominal X-ray 400
Chest X-ray 60
CT Chest and Abdo 10,000
Dental check-up X-ray 4
Additional radiological: Check with your doctor or radiologist

3. Travel related:

Radiation from flying depends on the altitude, time in air, and radiation levels for block hours over the specific location and route to where you are flying. A chart in dosage to and from many common destinations can be found in table 2 in this document:

Note that they are using millisievert units. To convert to the microsievert units we’ve been using so far, move the decimal point from the millisievert number to the right 3 places for microsieverts.

Flying from Los Angeles to Tokyo, a 12-hour flight, and from Chicago to London, an 8-hour flight, both result in a dosage of 43µSv because the radiation per block hour from Chicago to London is higher than from Los Angeles to Tokyo.

So it’s not simply flight time, but radiation level for block time (time from leaving the airport blocks to touching down), which varies per destination, with a wide variance, over a factor of 20.

Airline crews average an addl. 2200 µSv per year from their occupation.

4. Misc. sources

These all contribute small amounts of additional radiation we receive: weapons test fallout, wearing of old luminous watches, or using certain old camera lenses including some from Leica, Olympus, and Pentax, and probably others where Thorium was used to improve the optical performance, up until the late 1970s or early 1980s.

Be careful to note that if the unit you see is already a rate, like CPM (Counts Per Minute), that is a rate per minute. But if you see an absorption quantity without a rate, that is a total amount absorbed, and if absorbed internally, is cumulative for a long time, depending on the half-life of the radioisotope.

If you see a Geiger counter reading of µSv per hour, you would have to be there for an hour to receive and absorb that the value of that dosage.

If a reading in given in CPM as many Geiger counters provide for, it is common to see that the press has converted it using the ratio of 10 CPM = 0.1 microsievert. So in one minute, at that rate, you would absorb 0.1 microsievert.

However, there is a ~20% margin of error here, depending on whether the test unit measuring CPM was calibrated for Cobalt 60, or Cesium 137. Another factor are the design of Geiger counters, what tube sizes, and sensitivities they provide for. For one vendor, the difference between their lowest end model with lower sensitivity, and their higher end model with 20x larger tubes, and greater sensitivity, the rates can vary up to a factor of three or more, although they do show consistency for trends.

To summarize, if you see that a reading is consistently above around 0.1 µSv/hour, you should try to move away to where it is lower, preferably closer to 0.01 µSv per hour (as it currently is in much of Japan, other than Fukushima), where that is just a bit above the levels before the earthquake.

Always try to minimize your exposure to radiation, but if you’ve been exposed, or it appears that you will likely be exposed to a higher amount of radiation than normal, for a prolonged period of time, schedule a talk with a radiologist that you trust, and be very cautious when reading the news, where in recent days, the units of measurement or their analogies to background radiation have sometimes been in error.

Notes on units. I chose to use microsieverts µSv, but if you have data that you want to add or convert, there are many online calculators for converting units of radiation.

Here are some common ones:

* 1 Sv = 1000 mSv (millisieverts) = 1,000,000 μSv (microsieverts) = 100 rem = 100,000 mrem (millirem)
* 1 mSv = 100 mrem = 0.1 rem
* 1 μSv = 0.1 mrem
* 1 rem = 0.01 Sv = 10 mSv
* 1 mrem = 0.00001 Sv = 0.01 mSv = 10 μSv


Hi I live in Yokohama and hope that you can help me make sense of some of what I have heard. At this page here they measure the amount of radiation in Yokohama but they are doing it in nanogray per hour. How do I change this to microsieverts, and do these numbers indicate a health risk if for example I go on a three hour run or longer? Thanks!
A gray amount, or gray/time rate is equivalent to a sievert amount or sievert/time rate, with gray values specifically relating to absorbed dose by a human.

You can convert gray units from nano to micro at this online calculator:

The data in your link shows that at the time of their measurement for gamma radiation (the "W" multiplier (see below) would be close to 1, and equal to sievert values, but those figures still show values that are 50% higher than the average for the past 20 years.

Note that they gave no data for alpha radiation, where the gray "W" multiplier (see below) can be 20x more if the type of emission is alpha particles, which can be inhaled if outdoors.

While the numbers in your link are low, they may have increased by the time you go for your jog, and they are a tad higher than average.

My opinion is to always minimize radiation when possible. I recommend you talk with your local radiologist or doctor, and then decide if you want to exercise outdoors for a lengthy time with the slightly elevated gamma levels in your area.

From Wikipedia:

The unit gray, measures absorbed radiation which is absorbed into any material. The unit sievert specifically measures absorbed radiation which is absorbed by a person. The equivalent dose to a person is found by multiplying the absorbed dose, in gray, by a weighting factor (W). The weighting factor (sometimes referred to as a quality factor) is determined by a combination of: the radiation type, the tissue absorbing the radiation, and other pertinent factors.[1]

In terms of SI base units:
1 Sv = 1 Gy · W (where Sv=sievert, Gy=gray, W=weighting factor specific to each type of radiation and tissue).

1 Gy = 1 J / kg of absorbing material, therefore:
Sv = 1 J / kg of absorbing material * W
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