"Unless needed there, I would be making immediate arrangements to leave [Tokyo]," said Williams. "Later developments are likely to make it more hazardous to leave the area [right outside the evacuation limit]."
People who receive a single large dose of 1,000 millisieverts of radiation to the total body can expect to experience acute radiation syndrome, characterized by changes in blood count, nausea, vomiting and diarrhea.
Three nuclear plant workers have suffered from acute radiation syndrome, the New York Times reported.
The sievert is a unit of dosage that is used to evaluate the biological effects of radiation.
To put radiation in perspective, a mammogram gives off about .30 millisieverts of radiation and a chest X-ray gives off .04 millisieverts, according to the Environmental Protection Agency. The World Nuclear Association says the lowest level of radiation that is clearly at a carcinogenic level is 100 millisieverts per year. The average American's total radiation exposure equals about nine millisieverts per year.
But even with the potential increased dose of radiation for residents of Fukushima, most doctors said they do not think that the reported levels will lead to a substantial increase in future cancers or other long-term health problems.
They warned, though, that things could change quickly if workers cannot cool the exposed fuel rods.
"Remember that the normal incidence rate of cancer in the U.S., for example, is about 43 percent," said Jerrold Bushberg, director of health physics programs and clinical professor of radiology and radiation oncology. "Even if we assume the linear dose-response is correct and the exposure results in a radiation dose that is 40 [or more] times the normal annual radiation dose from background radiation that we all receive, the increased cancer risk of developing cancer from that exposure would be less than one percent."
Most nuclear power experts have said that Fukushima's plant will not cause as severe of disaster to health as the Chernobyl Nuclear Power Plant accident, which is considered to be the worst nuclear power plant accident in history. The accident caused radioactive fallout that spread throughout the western part of the Soviet Union, and parts of Eastern, Western and Northern Europe, and the area continues to be uninhabited today.
Several studies have shown a significant spike in thyroid cancer in children who lived close to the Chernobyl plant. Other long-range effects associated with the Chernobyl blast included a peak in babies born with Down syndrome and birth and neural tube defects caused during the fetal development in the mother's womb.
"I would agree in general that this is well below Chernobyl in overall exposure risk to the public," said Edward Morse, a nuclear engineer at the University of California, Berkeley. "Chernobyl had nuclear propulsion of its entire core contents shot into the atmosphere after being supercritical... And most importantly, they are receiving the best possible type of human intervention to minimize radiation leakage and exposure to the public."
But Williams said Japan's public health outcomes could be just as bad as Chernobyl's, and perhaps worse.