Radiation, measured in millisieverts (mSv), can cause radiation sickness in high enough concentrations. A single dose of 1,000 mSv has been known to cause nausea and fatigue among other symptoms; experts say that a single dose of 5,000 mSv would likely kill about half of those receiving it; and that a single dose of 10,000 mSv has, in many cases, been shown to be fatal within a few weeks.
By comparison, most people receive about 2 mSv over the course of a year by way of normal background radiation.
On Tuesday, March 15, Japanese officials ordered people to seal themselves indoors due to radiation measurements of more than 400 mSv per hour, though that measurement soon dropped to 11 mSv per hour.
