Reports on the Japanese nuclear situation is complicated and confusing. Having a little better understanding of the units and terms will help. One of the most confusing things is the units. One severt is equal to 100 rads.
MilliSevert is a common term used in the reports. In the United States, Millirad, one thousandth of a rad is more common. A millisevert is equal to 100millirad. Depending where you live you are exposed to radiation in different amounts. There is background radiation from rocks, food, dinner plates, sunlight, televisions (the older ones) and nearly every thing else. You are also exposed to radiation from other things in the workplace, hospital even airplane flights. So figuring out what the average radiation the average American is exposed to complicated. Thanks to the government we have numbers to work with. The Nuclear Regulator Commission (NRC) fact sheet has limits of workplace exposure of 100 millirad to 5000 millirad. The higher number is for people that work with radiation from X-Ray technicians to nuclear industry workers. That is a pretty large range. Unless you happen to know a bunch of X-Ray technicians the are dropping like flies, that 5000 millirad is probably is a realistic number for you to pick, given this emergency.
Most of the reports are using another term very often, microseverts. Micro means a millionth or one divided by one million. That micro also applies to the US unit rad, so a microsevert is equal to 100 microrads (see below). So you should know what the average dose is in microrads which would be 5,000,000 microrads (50,000 microseverts). But that is a yearly dose, a daily maximum average dose would be that divided by 365, or 13,690 microrads (137 microseverts) per day. If the reports say hourly dose, then divide by 24 and you get 570 microrads (5.7 microseverts) per hour. .
Those numbers are a lot higher than you will see in the papers. In the NCR link above, 100 millirads or mrads, the abbreviation for milli, is reported for average folks. So is being an X-ray technician 50 time more dangerous than being a school teacher? You decide for yourself. If you happen to be an outdoor kind of person or love a suntan, you are closer to the X-ray technician, because you are increasing your background exposer. If you are a school teacher in Denver you only get ten times more that the 100 millirads the NCR recommends for non-radiation workers because of a higher background exposure. The average background plus work exposure in the US is 620 millirad per year.
Getting high doses quickly is not good. The report says that low doses, below 10,000 millirad doesn’t seem to be linked to cancer but over 50,000 millirads causes cancer.
Personally, the less radiation the better for me. But I have to be realistic, so I would call 5,000 millirad (50 milliseverts), the maximum I want to be exposed to a year and 10,000 millirad (100 milliseverts) in a short period the maximum before I get too concerned. Then that is just me.
So yearly, 5.0 rad, 5,000 millirad, 50 millisevert, or 50,000 microseverts are all the maximum amount of radiation a person working with nuclear radiation may expect.
Quick (hour to day) dose maximum, 10 rad, 10,000 millirad, 100 millseverts or 100,000 microseverts are all the same amount of radiation and that amount is not proven to cause a significant increase in cancer risk.
Note: Without trying to overly complicate things. The rad is measure of absorbed radiation. The term Severt includes the biological impact of the absorbed radiation which is a measure of equivalent dosage. The conversions listed in the NCR report (millirad to millisevert) are technically incorrect, but a useful simplification. Other units and terms can be used. A variety of conversion programs are available on line. The Rad Pro Calculator is an example.
Update: While I was publishing this article the Nuclear Science and Engineering Department at MIT was posting one of their own. They use Rem as a unit which is more directly converted to Severts. Well worth a read if you are concerned.
As is always advised, double check with other reliable sources, everyone can make mistakes. This article has not been reviewed by Associated Content and the views of this author should not be considered the views of Associated Content or Yahoo!