Radiation destroys cells – the bigger the dose the greater the damage. For workers at the Fukushima Daiichi plant the immediate threat is of radiation sickness.

Doses of up to 400 millisieverts an hour (0.4 sieverts) were reported on Monday, more than 100 times higher than people are ordinarily exposed to in a year. As a result, non-indispensable workers were evacuated.

The speed with which symptoms develop in exposed workers is one indication of the scale of a radiation leak. Nausea, vomiting and diarrhoea are the first signs – within one or two days in mild doses (1-2 sieverts) but in as little as an hour with severe doses (3.5-5.5 sieverts). This is equivalent to 35,000 X-rays.

A couple of symptom-free days may follow before the victim falls ill again with fever, fatigue, loss of appetite and headache. There may also be damage to the skin. This period may last anything from a few hours to months and the victim may suffer seizures or lapse into a coma. About half of those who receive a severe dose will die.

However, the International Atomic Energy Authority stressed that the 400 millisievert reading was "a local value at a single location at a certain point in time". By 6am yesterday morning it had fallen back to 0.6 millisieverts an hour.

Radiation is cumulative – there is no way of ridding the body of it. The only defence is against radioactive iodine, one of the fissile products released into the air at Fukushima, which accumulates in the thyroid gland and can cause thyroid cancer. By dosing victims with iodine tablets, the gland becomes saturated with iodine which stops it absorbing the radioactive sort.

The danger from radiation diminishes with increasing distance from the source. For the local population evacuated beyond the 12-mile exclusion zone, the risk is of a long-term increase in cancer and possible genetic damage from the raised level of radiation.

Exposure to a cumulative dose of 1 sievert would increase the risk of fatal cancers by about 5 per cent. Below 0.1 sieverts (100 millisieverts) an increase in cancer cannot be detected. However, reports suggest that so far the high levels of radiation are confined to, or within the immediate environs of, the damaged plant.

People contaminated by radiation dust on their skin or clothing can spread it to others by touch. It may also enter the food chain if a cloud of radioactive dust carried by the wind falls on pasture and is eaten by animals.

The International Atomic Agency has rated the seriousness of the incident at Fukushima as 4 on a scale of 7, each point of which represents a factor of ten. Chernobyl was a 7, meaning that Fukushima is considered one 1,000th as serious.