HOW MUCH RADIATION IS DANGEROUS?

Radiation levels remained a huge worry in Japan on Sunday following a spike in radioactivity in water at the Fukushima nuclear power facility.

But there has been no indication that people beyond the plant's immediate vicinity have been exposed to harmful doses.

Operator Tokyo Electric Power Co said radiation levels were over 1,000 millisieverts per hour at Reactor No. 2 and evacuated workers from the turbine building there. Japan's nuclear safety agency has said that as emergency workers, they are allowed to be exposed to 250 millisieverts per year.

Experts say the rise in radioactivity in the water at the reactor does not pose much danger to those outside as long as it is contained safely.

"It depends on where this water's going and what they're doing with it," said Murray Jennex, professor at San Diego State University. "If it's allowed to run off into the ground and stuff, you're getting a concentration in the ground. If it's going into the ocean, you're getting some accumulation in the ocean."

Here are some facts about radiation and the health dangers it poses:

* Radiation is measured using the unit sievert, which quantifies the amount absorbed by human tissues. One sievert is 1,000 millisieverts and 1 million microsieverts.

* People are constantly exposed to some level of natural radiation. They also get exposed to tiny amounts through sitting in airplanes, routine chest or dental x-rays, and larger amounts through medical tests such as CT-scans and MRIs. A single-organ CT scan, for example, gives a radiation dose of about 6,900 microsieverts.

* On Sunday afternoon, radiation levels in central Tokyo were around 0.16 microsieverts per hour. That is a level experts describe as minimal, and just below the global average of naturally occurring background radiation of 0.17-0.39 per hour, a range given by the World Nuclear Association. It is also significantly lower than the cosmic radiation of up to 7 microsieverts per hour experienced on a Tokyo-New York flight.

Below are different levels of massive radiation exposure in a single dose -- all measured in millisieverts -- and their likely effects on humans, as published by the U.S. Environmental Protection Agency:

- 50-100: changes in blood chemistry

- 500: nausea, within hours

- 700: vomiting

- 750: hair loss, within 2-3 weeks

- 900: diarrhoea

- 1,000: haemorrhage

- 4,000: possible death within 2 months, if no treatment

- 10,000: destruction of intestinal lining, internal bleeding and death within 1-2 weeks

- 20,000: damage to the central nervous system and loss of consciousness within minutes, and death within hours or days

Sources: Taiwan Atomic Energy Council, World Nuclear Association, US Department of Transportation, US Environmental Protection Agency.