Readers ask: What is the highest intensity of radiation measured so far? What affect will the radiation have on the people of Japan? How many people could it kill?
Science answers: Keep in mind that news outlets have been reporting the levels in two units: millisieverts (mSv), which is one thousandth of a sievert, and microsieverts (μSv), which is one millionth of a sievert. (All numbers that have been reported are per hour.) So if you've been seeing numbers in the thousands, check the units. 3000 μSv/h is equal to 3 mSv/h—equivalent to about 300 chest x-rays, or one head CT scan—which levels at the plant gates have reached at times during this disaster. This New York Times graphic shows how radiation levels around the plant have changed over the past few days with the various explosions that have occurred. The Japanese Atomic Industrial Forum, a nongovernmental organization for nuclear energy, is publishing several updates a day on the plant's status. The most recent update reiterates that levels peaked at 400 mSv next to unit 3 on Tuesday.
The people most at risk from radiation at the Fukushima Daiichi plant are the workers trying to keep things under control. That's because the effect on one's health depends on how near one is to the radioactive sources at the power plant and how long one stays there. Peak radiation levels at the plant were around 400 mSv per hour right in the thick of things, between reactors 2 and 3. According to Peter Burns, former chief executive officer of the Australian Radiation Protection and Nuclear Safety Agency, the limit recommended by the international community for workers dealing with accidents is 100 mSv—so a worker would reach the limit in just 15 minutes. While health effects may not be immediate, that person would also have an increased risk of developing cancer later in life.
Limiting workers' exposure is extremely important; if exposed to that peak radiation for 2½ hours (1000 mSv total dose) a person would start to feel sick. If the person stayed there for 15 hours, they would likely die within a month. But as one goes farther away from a radiation source, the dose one receives per unit time falls off exponentially, which is why the Japanese government has evacuated the area around the plant. Within the 20-kilometer perimeter that has been evacuated, levels peaked at 0.33 mSv per hour, equivalent to three chest x-rays per hour—or about 30 days worth of exposure to background radiation. If one stayed there a day, one's total dose would be about the same as a full body CT scan.