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Does 50 miles make sense to you and how does radiation spread over farther distances? >> the radioactivity is attached to dust particles and it's just the movement of the dust particles that tells you how the radioactivity moves. again, as you were alluding to earlier in your show, it's the wind. the two big factors that are going to control what happens is how much radioactivity is released and which way the wind is blowing. it really is good news that we're continuing to have offshore winds, because really the radiation is going to get disbursed when it goes over the ocean. and very little w...

Part, we're not detecting radioactivity. you want to know how much exposure have you had. >> a machine doesn't tell you how much exposure you've had three days ago? >> no. think of it like a camera. you can hear it clicking off as it's giving readings here, which are all low. they're significantly low as compared to anything that would trigger an alarm. that measures alpha, beta and gamma rays. this is just measuring the air constantly around us, 13 microrads per hour, and that's, again, a very low number. >> professor, how does radiation spread over distance? if you are -- the u.s. government...

Spread over distance? the obama administration has come forward and said americans should be no closer than 50 miles to this nuclear plant. that is a greater distance than the japanese government has been using the last several days. does 50 miles make sense to you and how does radiation spread over farther distances? >> the radioactivity is actually tached to dust particles and it is the movement of the dust particles that tell you how the radioactivity moves. again, as you were eluding to earlier in the show it is the wind. the two big factors that will control what happens is how much radio...

Running a geiger counter over people. you have a geiger couldn'ter with you. how does that work? does it really tell you anything. >> i don't think this tells you a lot as far as running it over a person's body like that. it is giving you a static image in time, saying right here, this part we are not detecting radioactivity. what you want to know is how much exposure have you had. >> a machine like this doesn't tell you how much exposure you have had three days ago. >> no. think of it like a camera, a static picture in time. you can hear it clicking as it is giving readings here and the readi...

Would be minimal or none at all. but still, of course, the radiation spread will depend on the wind speed and wind direction. >> we want to get more now on this rapidly developing story. stan grant joins us from tokyo. and stan, this is an increasingly fluid situation there in fukushima plant. keep us up to date on what we know right now. >> this really is changing rapidly. we've had the explosions, the fallout, the higher radiation readings. i want to bring you something now about reactor number four. we've been learning this over the last hour. talking about the reactor number four, which wa...