Doses of radiation received by workers at the plant have not been reported, though it is clear that by this stage many personnel must have received significant exposure. The highest level reported at the plant was a single brief reading of 400 millisieverts/hour, equivalent to two centuries' normal background dose in one hour, but in general readings have been much lower - WNN reports levels below one millisievert (1000 microsievert) per hour this morning.
Radiation sickness typically sets in when a dose of more than 1,000 millisieverts is sustained in a short period but it is highly unlikely that any plant worker has been or will be permitted to reach this stage. Most sufferers recover from radiation sickness at the initial level; the death rate escalates to 50 per cent at a dosage of 4,000 millisievert and exposure of 10,000+ millisievert will typically kill within minutes.
The Japanese authorities' evident willingness to withdraw personnel completely in response to rising radiation levels at the site means that cases of actual radiation sickness are extremely unlikely. Some workers' risk of getting cancer in the long term may be increased by the radiation doses they are and have been receiving, but as cancer is one of the most common causes of death in wealthy countries and only small numbers of people are involved it will be hard for investigators decades from now to attribute any cases directly to this cause.
People who haven't been on the Fukushima site itself aren't subject to either of these risks: so far there is no report of any evacuee sustaining a significant dose and radiation levels beyond the evacuation zone - while measurably up in some areas - are insignificant in a health context.
The primary threat to people outside the immediate vicinity of the plant is radioisotopic iodine. Iodine-131 is produced in significant amounts by the uranium fission chain reaction that powers reactors. Its subsequent decay is one of the primary factors in the residual heating now slowly dying away in the Fukushima cores: as it decays it turns into stable, unthreatening Xenon but it emits ionising radiation as it does so.
Iodine-131 has a half-life of just over eight days, so by the weekend half of what was present in the cores when the quake hit will have disappeared. Eight days later it will be down to a quarter, and within weeks it will almost all be gone no matter where it has travelled to by then.
Pleasingly, there will be almost no iodine remaining in the spent fuel rods at the No 4 reactor pool, so they can't emit any no matter how hot they get. Some radio-iodine, however, has already been carried out of the reactor cores in small amounts along with escaping steam. With one or perhaps two cores open and - perhaps down the road - fully melted, the chances are that this will increase.
Normally the tiny concentrations of iodine that could be encountered - even inside the plant area - would present no significant radiation menace. Unfortunately however, when iodine is taken into the body our metabolisms concentrate it in the thyroid gland and this presents a perceptible cancer risk - thyroid cancer being rare normally. The main route into the body for iodine is ingestion in contaminated water or food, though it can also be inhaled.
The greatest radio-iodine releases ever seen so far occurred during US nuclear weapons tests in Nevada prior to 1970: Americans suffered the iodine equivalent of three Chernobyls during that period, but because they were not told about the hazard they didn't notice it. The bulk of the health effects came from children being fed contaminated milk, children being especially prone to thyroid damage from iodine; in fact they are the main group likely to suffer major effects. Millions of children were also fed contaminated milk following Chernobyl: according to the IAEA the incidence of thyroid cancer among such children and young people rose to one case in 4,500.
In Japan it's to be hoped that nobody will be ingesting radio-iodine in significant quantities: avoiding this should be a simple matter for a country with a proper public health system. The comparatively tiny inhalation threat can be removed by wearing a simple facemask or remaining mostly indoors, and iodine absorption by any route can be blocked by taking iodide pills so as to fill up the body with ordinary iodine and prevent radio-iodine being concentrated in the thyroid.
The Japanese authorities have distributed iodide pills to medical centres around the evacuation zone, but they are not being administered as thus far there is no indication of any need for them: significant levels of iodine have not been detected.
So, to sum up on health effects: it seems likely that even with the situation at Fukushima worsening, health consequences will remain minimal - close to zero - even for workers at the plant. Nobody else, not even evacuees from the area around, need be concerned at the situation: far less anybody in Tokyo or further afield. Foreign governments advising their citizens to leave Japan are indulging in unjustified hysterics.
Public perceptions of the disaster in general continue to remain muddled and panicky, with media often conflating the terrible loss of life caused by the quake itself with the situation at Fukushima. But the two are not related: and it remains just about certain that the harm caused by Fukushima will be insignificant compared not just to the quake and tsunami but compared to industrial accidents in general. One of the most serious health results of Fukushima is likely to be people all around the world making themselves ill by taking iodide pills unneccessarily.
So far from Fukushima proving that nuclear power is dangerous compared to other technologies, it seems to be proving quite the reverse. ®