Radiation Levels

A friend of mine who recently moved to Japan asked me about radiation.

Sources of radiation exposure resulting from the Fukushima Daiichi accident to those in the Kanto region could include airborne, water-borne, and food-borne radiation.

Dosage

Biological effects of gamma radiation are denominated in Sieverts (Sv).The effect of gamma radiation is to damage the DNA in our cells, which can interfere with cell function or result in transcription errors. This background gamma radiation is always present in our environment (thanks to radiation from the sun). In fact, if we take a plane flight from New York to LA, that puts us closer to the sun, resulting in an equivalent dosage of 40uSv. This dose chart enables one to visually place dosages in context.

Environmental Quality in Kanto Region

It turns out that air, water, and soil quality are closely monitored by the various municipalities in the Kanto region. Shibuya-ku publishes its test results on this website.

https://www.city.shibuya.tokyo.jp/anzen/tohoku_taiheiyo/radiation_start.html

In Shibuya the sum of the exposures due to air, water, and soil-born contamination depends on the location in the ward, but ranges from one-half to one times the EPA limit for exposure for a single member of the public of 1mSv. It would actually be lower, since none of us spend 24 hours outside. So we can say that the immediate environment is rather safe.

Uptake from Food

However, soil contamination in certain regions of Japan is a real problem, and the question is how much of this gets into the food that we eat. This is hard to estimate because different food products will take up contaminants in different concentrations.

All food that we eat is actually radioactive to some degree, since plants and animals need potassium to perform vital cellular functions. Some 0.012% of this is the radioisotope K40, according to this publication by the Ministry of Health, Labor, and Welfare, resulting in typical radiation levels listed here:

http://www.fsc.go.jp/sonota/emerg/radio_hyoka_kaisetu.pdf

Measuring Concentration

The contaminant of concern from the Fukushima accident is radioactive cesium, which chemically resembles calcium. Both cesium and calcium would be detected in the regular spot-checks on food that Kanagawa Prefecture performs. It does not appear in the 2013 results that food from the prefectures with contaminated soil are higher in radiation than typical. Nevertheless, there could still be hot spots that are not detected by these spot checks.

The Ministry of Health, Labor, and Welfare (MHLW) mandates a maximum radio concentration of 100 Bequerels per kg of radioactive cesium in food. Bequerels are a unit of concentration, not dosage. The actual dosage received depends on how long the cesium stays around – the cesium would be used by our bodies and eventually pass through. I came across one prominent website that mistakenly assumed that the cesium would stay around in our bodies indefinitely, building to ever-increasing concentrations. Not so. An easier way of understanding the meaning of the limit of 100Bq/kg is by comparing it the naturally-occurring radio concentration of beef, which is 100Bq/kg.

Converting to Dosage

Another way is to use the conversion factor that takes the biological half-life in to account. I found a factor of 1.3*10^-5 Sv/Bq on Wikipedia and the MHLW, but could not determine the calculation.

(source: http://www.fsc.go.jp/sonota/emerg/radio_hyoka_kaisetu.pdf)

This means if I daily consumed 1kg of 500Bq/kg food (probably unrealistically high), I would have an annual exposure due to this of 2.36mSv. This is on the low end of what one gets from a session of barium fluoroscopy (to check for stomach tumors, which I took last year Japan get as part of an annual health check-up). I tend to view this as low.

Screening of Food From Affected Areas

Apparently, food from the regions with contaminated soil is being tested, and food with high radio-concentration is not sold. I found a website that talked about testing rice. Rice has a typical radio concentration of 30Bq/kg (due to Potassium). Production from plots containing 100Bq/kg or above are rejected.

I also found information on testing fruits and vegetables, and would assume that other food products are similarly tested.

Conclusion

  • Environmental radiation is closely monitored in the Kanto region, and radiation levels are measurably higher, but within safe levels. In the Kanto region, if you spent 24 hours outside, your exposure would still be <1mSv. Compare this to the 1mSv EPA limit for the general public and the 1.5 – 1.7mSv annual dosage received by flight attendants.
  • The primary route of uptake would be food imported from regions with contaminated soil.
  • Food originating from regions with contaminated soil is monitored against thresholds set by the Ministry of Health, Labor, and Welfare. A diet consisting exclusively of food originating from these regions would have detectably higher radiation, but still on the level of a session of barium flouroscopy, and several times less than a full-body CT scan.

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