Background

A Sievert is an attempt to quantify the danger of various radioactivity levels.
Ionising radiation is measured in counts per second (called a becquerel). For example, if a detector picks up 10 bits of ionising radiation every second, then this is 10 becquerel.
However, different types of radiation have different effects on the body. First, they thought it might have something to do with the amount of energy the radiation has – more energetic radiation was worse for you. They came up with a new measurement, which was the amount of energy absorbed per kilogramme of body mass (measured in units called grays).
It did not work all that well, so they decided to multiply by some factor depending on the type of radiation. Beta and Gamma radiation has a factor of 1 and alpha radiation has a factor of 20 – i.e. alpha radiation is deemed to be twenty times worse than beta or gamma radiation. These factors are not measured and are a very rough estimate.
Realising that some parts of the body are more susceptible to radiation, they did try to take this into account, but it all got too confusing, and they decided just to multiply the energy per kilogramme by the weighting factor. That is what we now call the Sievert.

Other Factors

There are many other factors which should be taken into account when assessing health risk:

Internal External Radiation

Is there a difference between getting radiation from a small particle lodged in the lungs compared to the whole body being exposed to the same level of X-rays. The Sievert treats both cases the same, since the radiation dose is averaged across the whole body.

Indeed, the actual concepts of absorbed dose become questionable, and sometimes meaningless, when considering interactions at the cellular and molecular levels.

CERRIE (Government’s Committee Examining Radiation Risks of Internal Emitters) Majority Report Chapter 2 Risks from Internal Emitters

Different Elements

What the chemistry and biology are very important which is why we hear a lot about Iodine 131 which has been associated with thyroid cancer. Different radioactive substances are absorbed by the body in differing amounts and can concentrate in different parts of the body. Iodine is stored in the thyroid glands whereas strontium replaces calcium in bones.

Energy / Amount

The Sievert is measured using the total energy of the radiation. Therefore, low energy radiation emitters (such as tritium) can emit a lot more radiation for the same dose measured in Sieverts. Would multiple low energy emissions have a larger or smaller effect than one high energy emission?

So Is the Sievert Useful?

The measurement is useful if you want to know roughly how long someone can stay with a certain amount of radioactivity before getting acute radioactive poisoning, which causes vomiting, diarrhoea and death. It is not brilliant, but it is the best we have.
It is not so good at determining long term health effects such as the risk from cancer since other factors need to be taken into account.

Milli and Micro Sieverts

The Sievert is a very large dose of radiation, so they

often measure it in thousandths of a Sievert (called a milli sievert- mSv) or a millionth of a Sievert (called a micro sievert – ?Sv).
1 Sv = 1,000 mSv = 1,000,000 ?Sv

Hourly and Yearly Dosage

Almost without exception, the media have confused hourly and yearly dosage.
Normal background radiation is about 2.5mSv per year. You can read in some press reports that the readings of 250mSv per hour (at Fukushima levels of 1000mSv – i.e. 1Sv – have been reported) is 100 times background. THIS IS WRONG.
250mSv per hour = 250 x 24 x 365 = 2,190,000mSv = 876,000 times normal background.
It makes a big difference if you are exposed to the radiation all in one day, or if it is spread out over a year. However, the exact relationship between exposure times and health effects has not been established.

Safety Levels

The safety levels vary from country to country, but is usually at most 20mSv per year. Some Fukushima workers have had this lifted to 250mSv per year. With the 250mSv limit, it would mean that the workers could work for one hour at 250mSv in any one year. With the 20mSv limit, they could only work for less than 5 minutes in that environment.