What is a Sievert? Is radiation levels from the Fukushima Daiichi nuclear plant dangerous?

The Japanese authorities say radiation levels from the Fukushima Daiichi nuclear plant climbed to potentially harmful levels for a brief interval on Tuesday.

Residents living within 30km (18 miles) of the plant have been advised to leave the area, or to stay indoors, and try to make their homes airtight.

Experts have stressed that swift action should be able to minimize any impact on human health.

What are the immediate health effects of exposure to radiation?

Exposure to moderate levels of radiation – above one gray – can result in radiation sickness, which produces a range of symptoms.

Nausea and vomiting often begin within hours of exposure, followed by diarrhea, headaches and fever.

After the first round of symptoms, there may be a brief period with no apparent illness, but this may be followed within weeks by new, more serious symptoms.

At higher levels of radiation, all of these symptoms may be immediately apparent, along with widespread – and potentially fatal – damage to internal organs.

Exposure to a radiation dose of four gray will typically kill about half of all healthy adults.

For comparison, radiation therapy for cancer typically involves several doses of between one and seven gray at a time – but these doses are highly controlled, and usually specifically targeted at small areas of the body.

Radiation dose	Effect
Source: World Nuclear Association
2 mSv/yr (millisieverts per year)	Typical background radiation experienced by everyone (average 1.5 mSv in Australia, 3 mSv in North America)
9 mSv/yr	Exposure by airline crew flying New York-Tokyo polar route
20 mSv/yr	Current limit (averaged) for nuclear industry employees
50 mSv/yr	Former routine limit for nuclear industry employees. It is also the dose rate which arises from natural background levels in several places in Iran, India and Europe
100 mSv/yr	Lowest level at which any increase in cancer is clearly evident.
350 mSv/lifetime	Criterion for relocating people after Chernobyl accident
1,000 mSv single dose	Causes (temporary) radiation sickness such as nausea and decreased white blood cell count, but not death. Above this, severity of illness increases with dose
5,000 mSv single dose	Would kill about half those receiving it within a month

How is radiation sickness treated?

The first thing to do is to try to minimise further contamination by removing clothes and shoes, and gently washing the skin with soap and water.

Drugs are available that increase white blood-cell production to counter any damage that may have occurred to the bone marrow, and to reduce the risk of further infections due to immune-system damage.

There are also specific drugs that can help to reduce the damage to internal organs caused by radioactive particles.

How does radiation have an impact on health?

Radioactive materials that decay spontaneously produce ionising radiation, which has the capacity to cause significant damage to the body’s internal chemistry, breaking the chemical bonds between the atoms and molecules that make up our tissues.

The body responds by trying to repair this damage, but sometimes it is too severe or widespread to make repair possible. There is also a danger of mistakes in the natural repair process.

Regions of the body that are most vulnerable to radiation damage include the cells lining the intestine and stomach, and the blood-cell producing cells in the bone marrow.

The extent of the damage caused is dependent on how long people are exposed to radiation, and at what level.

Read the rest at BBC News – Q&A: Health effects of radiation exposure.