Relative radiation risk... Recently, we get a lot of radiation values in our news streams and very few have comparison data to estimate: is this normal, elevated, severe or hazardous? Distrust and panic may result. Here, I aim to provide a bit of grounding.
Most simple Geiger counters, as used on this site and in many places, are good for a warning to stay indoors in case of black rain from nuclear fallout. Sensitive as they may be, they are hardly sensitive enough to quickly and reliably detect the small additional activity within the normal background radiation to isolate the contribution from a ground or food sample. It usually takes heavy shielding and good spectrometry to tell that story truthfully.
For example, compare
- Nikko, Japan (2011) from Fukushima, ~ 170 km away
- Munich, Germany (1996), from Chernobyl, ~ 1700 km away
According to this earlier blog post, Nikko city and Munich now have similar radiation levels, around 10,000 - 80,000 Bq/m² from radioactive cesium, Cs-137. Is it significant? Severe?
Compare for yourself. Ground contamination from some 2000 nuclear bomb experiments is about 4000 Bq/m² - everywhere on the planet. To get a feeling for the specific activity not on a surface, but in a mass of matter, here is a reference. According to the UNSCEAR report [UNSCEAR, 1982], the normal range for radium isotopes in earth’s crust is 10 - 50 Bq/kg, whilst the range for the potassium K-40 is 100-700 Bq/kg. Source
The Banana Equivalent Dose leads us to an activity of 77 Bq/kg for human tissue from potassium alone, a mineral that does not accumulate in the body. So, this is a normal low level of activity. It seems that food safety limits like the 300 Bq/l for drinking water (in Japan 200 Bq/l for adults, 100 for children) were established on the basis of not significantly raising the normal low level with daily consumption over a lifetime. By significant I do not mean a factor of 2 or 3, rather 10 or 100. A biochemist or a radiation specialist might use more refined methods to assess relative risks, especially for elements that bioaccumulate.
Radiation is around us.
A bit is OK, usually. I know of no place where it would be zero. At these low normal levels, it may help to visualize it as a dim light in the darkness, whereas in a nuclear bomb it is a blinding light, and in severely contaminated areas an illumination too bright to be comfortable, like the sun that causes skin burns when we stay exposed for too long without protection. The brightness analogy gives us relative importance for the dose rate values in μSv/h, microSieverts per hour. The accumulated dose is simply the rate multiplied by the time of exposure. Compare chart of relative radiation hazard levels.
What would you add here?