A very high single dose of radiation (acquired within minutes can be more harmful than the same dosage accumulated over time. According to the World Nuclear Association, a single one-sievert dose is likely to cause temporary radiation sickness and lower white blood cell count, but is not fatal. One five-sievert dose would likely kill half of those exposed within a month. At 10 sieverts, death occurs within a few weeks.
The effects of long-term, low-dose radiation are much more difficult to gauge. DNA damage from ionizing radiation can cause mutations that lead to cancer, especially in tissues with high rates of cell division, such as the gastrointestinal tract, reproductive cells and bone marrow. But the increase in cancer risk is so small as to be difficult to determine without studying a very large exposed population of people. As an example, according to Langhorst, 10,000 people exposed to a 0.01-sievert whole-body dose of radiation would potentially increase the total number of cancers in that population by eight. The normal prevalence of cancer, however, would predict 2,000 to 3,300 cancer cases in a population of 10,000, so "how do you see eight excess cancers?" Langhorst asks.
According to Gonzalez, some of the emergency workers at Chernobyl received several sieverts of radiation, and many were working "basically naked" due to the heat, allowing contaminated powder to be absorbed through their skin. In comparison, the Japanese workers are most likely very well-equipped and protected at least from direct skin doses.
The Tokyo Electric Power Co. (TEPCO), the plant's owners, has evacuated most of its workers, but 50 remain at the site to pump cooling seawater into the reactors and prevent more explosions. These workers are likely exposing themselves to high levels of radiation and braving significant health risks. "As a matter of precaution, I would limit the workers' exposure to 0.1 sievert and I would rotate them," Gonzalez says. The workers should be wearing personal detectors that calculate both the rate and total dose of radiation and that set off alarms when maximum doses are reached. "If the dose of the workers start to approach one sievert then the situation is serious," he says.
The thousands of children who became sick in the aftermath of the Chernobyl disaster were not harmed from direct radiation or even from inhalation of radioactive particles, but from drinking milk contaminated with iodine 131. The isotope, released by the Chernobyl explosion, had contaminated the grass on which cows fed, and the radioactive substance accumulated in cows' milk. Parents, unaware of the danger, served contaminated milk to their children. "Certainly this will not happen in Japan," Gonzalez says.
When it comes to radiation exposure, professionals who frequently work with radioactive materials, whether in a hospital or a nuclear power plant, abide by the ALARA principle: "as low as reasonably achievable". Radiation exposure limits are conservatively set well below the levels known to induce radiation sickness or suspected of causing long-term health effects. Temporary exposure to dosages many times these limits, however, is not necessarily dangerous.
News of the U.S. Navy repositioning its warships upwind of the reactor site, the distribution of potassium iodide pills by the Japanese government, and images of officials in hazmat suits using Geiger counters to measure radiation levels among babies may stoke the public's fears—but, for now, these measures are ALARA in action, or "good extra precautions," Gonzalez says. The idea here is to always err on the side of caution.