About 140,000 people living within 20 miles of the plant were ordered to either evacuate the area or stay indoors and seal their homes. And they reported that radiation levels in Tokyo, about 175 miles south of the nuclear power plant, were about 20 times higher than normal.
So far, only 50 or so of the plant's workers have been exposed to the most dangerous radiation levels. But as those brave workers raced to shut down the critically damaged plant and storage ponds filled with radioactive material that appeared to be on the brink of releasing massive amounts of potentially deadly radiation into the air, nuclear experts warned that the worst of the disaster could be yet to come.
Here are five reasons they say the threat of radiation exposure in Japan may be worse than you thought:
1. It could get worse.
The Fukushima Dai-ichi plant has already suffered a partial meltdown, but nuclear experts say there are warning signs that efforts to control the plant's reactors are failing. And in the case of a total nuclear meltdown, they say the amounts of radioactive material released into the atmosphere would be catastrophic.
"All evidence points to the fact that control is slipping out of their hands," said Robert Alvarez, a nuclear expert and a senior scholar at the Institute for Policy Studies.
Alvarez said the detection of -137 near the crippled plant -- a highly radioactive material that has rendered wide swaths of land near the nuclear disaster at Chernobyl uninhabitable for hundreds of years to come -- was evidence that the situation was becoming increasingly critical.
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2. It's unpredictable.
The radioactive plumes being released by the plant are undoubtedly dangerous, but predicting where they will travel is difficult. Today, winds blew the plumes toward Tokyo, raising radiation levels there, but prevailing winds are forecast to send them away from Japan and out into the Pacific. "A lot will depend on the weather, on which way the wind is blowing," Dr. Thomas Cochran, a senior scientist with the Natural Resources Defense Council, told AOL News today by phone.
And while experts don't agree about the threat the plumes pose to the western United States and Canada, they are careful to note that the ones created by the 1986 Chernobyl disaster -- although very much diluted and less dangerous -- were carried across the entire Northern Hemisphere.
Alvarez, who has been critical of nuclear power, warned that the true impact of radiation from the Fukushima Dai-ichi plant may not be known for decades.
"We have to look at what's happening in Chernobyl right now and be honest about it," he said. "Twenty-five years after the disaster we're seeing that the magnitude of the harm that may have been caused to Russia and Europe is unimaginable. And in Japan, we're looking at the risk of multiple meltdowns. If it gets to that point, it could be worse than Chernobyl. It's just not something you don't want to think about."
3. There's no cure for acute radiation sickness.
By the time the first signs of radiation sickness appear -- nausea, vomiting, reddening of the skin, bleeding gums and hair loss -- the damage, doctors say, has been done. "There's no treatment that reverses the process," Dr. Ira Helfand, the former president of Physicians for Social Responsibility and a nuclear power critic, told AOL News by phone.
At lower levels of exposure, the symptoms of radiation sickness can be treated, but at high levels like those being reported immediately at the site of the Fukushima Dai-ichi plant, radiation is potentially lethal and could cause cancer to develop.
Because radiation disrupts the normal functioning of cells, the threat can be worse for pregnant women and children, since their cells are already rapidly dividing. And while potassium iodide tablets like the ones being given to some Japanese residents and U.S. service members in Japan can block the uptake of radioactive iodine to the thyroid, they are no protection against dangerously high levels of radiation.
Providing the public with a practical sense of the skyrocketing radiation levels near the plant can be difficult. But radiation at Fukushima Dai-ichi has been measured at 400 millisieverts per hour, or 40 rem. The U.S. Nuclear Regulatory Commission, as The Wall Street Journal noted today, has limited the amount of radiation exposure for U.S. nuclear workers to just 5 rem per year.
4. The radiation could enter the food chain.
"The other problem you have here is that in addition to being exposed to these doses [of radiation] outright, is that some of the radioactive products that will fall out of the environment will then get put into the food chain," Alvarez said. He said the major concern is iodine-131, or radioiodine, which is quickly absorbed into milk and other dairy products.
5. Officials may not be entirely forthcoming with the public about the threat level.
Officials are calling for calm. But experts said it's an indicator of how dire the situation at Fukushima Dai-ichi has become that a small group of the plant's employees are working around the clock, exposing themselves to potentially lethal doses of radiation, in the hopes of stopping a total meltdown.
Alvarez said governments around the world should be honest about the potential threat posed by radiation. "The logic is sometimes, well, this is for the greater good because evacuation may prove to be more harmful than telling the truth," he said. "But it's incumbent on the government of Japan and other governments to make sure that the public is aware of the nature of the threat so they can take the necessary precautions."