microsieverts, millisieverts, fukushima and abcnews

Watched an ABC news segment today, reporting the current progress a year after the Fukushima accident. The reporter took some geiger counter readings a little over a mile from the plant (while traveling in a car) and stated that readings were 20 micro sieverts an hour. I wanted to compare that to other more common day radiation doses and also compare how cumulatively damaging that hourly dose would be over the course of a year.

To understand sieverts and it's smaller units of measurements I needed to do some research first.

First, I realized that one sievert can be very damaging; close to lethal if all at once, and still very dangerous - if not mortally so - if over time.

For more common everyday radiation sources, milli and micro prefixes are used to denote much smaller radiation exposure incidents.

Milli denotes a thousandth of a unit or in this case: 1/1000th of a sievert. Mathematically its factor is denoted by 10^-3. The letter m is its symbol.

Micro denotes a millionth of a unit or in this case: 1/1000000 of a siervet. Mathematically its factor is denoted by 10^-6. The letter µ is its symbol.

For an average US user the average yearly radiation dosage is around 3 millisieverts.

How does that equate to the 20 microsieverts an hour the reported recorded near the plant?

For my calculations I used the Microsoft calculator in scientific mode.

To start with the micro measurement I entered 1, hit the Exp button, the +/- button, and then 6. I hit equals to get 0.000001, which is a millionth of 1. I then multiplied by 20 to get 0.00002.

To reach the average US yearly dose it would take 6.25 days near the Fukushima plant.

How much radiation would the 20 microsieverts per hour be if added over the course of a year? I multipled 0.00002 by 24 and then 365 to get 0.1752 of a sievert. To make it easier to read I multipled by 1000 to get a millisievert result.

175.2 millisieverts was the answer, which is quite considerable but shouldn't be lethal over that long a time. US nuclear works were 'classically' allowed up to 50 milliseiverts a year; now 20.

So if you lived near the Fukushima plant for a year you'd get 3-8 times the dose of a nuclear plant worker.