The news at Hawaii is good news, but the devil is in the details. Gamma rays & xrays travel at essentially the speed of light, and like light travel in straight lines, bent only a tiny bit by huge gravitational objects, like the sun (general relativity). Gamma rays & x rays from Japan do not make it around the curve of the earth to Hawaii. Beta particles (electrons) and alpha particles (nuclei of helium atoms) don't travel far, and are stopped by things like your clothes. Consequently when you're looking for radioactive stuff at Hawaii, you are essentially talking about radioactive particles, or particles with some radioactive isotopes in them, that are bourne on the prevailing winds & weather systems (often called "fallout"). And typically, 20 or 30 mph is a good brisk wind. The jetstream travels about 100 mph. A thunderstorm can move at 60 mph. The air distance from Tokyo to Honolulu is 3,859 miles. Consequently, when and if a given particle gets to Honolulu from Daiichi is tricky. The prevailing wind from Japan goes to Alaska, so that's where you'd look first for any significant change in background radiation from Daiichi outside of Japan.
As to what you'd look for, let's look at strontium 90, a relatively long-lived radioactive isotope. There are 3 sources of strontium 90 in the US: air testing of atomic weapons in the Cold War, the explosion at Chernobyl, and possible releases to the environment from nuclear plants. If you found some strontium 90, it would be hard to say it came from Daiichi, unless you suddenly found a lot more of it than you are accustomed to seeing.
In fact, that's how it was discovered in the west that there had been a disaster in Chernobyl. The operator of the Swedish Forsmark nuclear power plant thought he was having trouble because his outside radiation measurements were extremely high, and his alarms went off.
When someplace in CA announced that they had seen something surely from Daiichi, they were getting a little speculative in my book, given that the amount was very small, and that the particles, rays & atoms don't exactly arrive with a shipping label from Narita airport.
You can find a little bit of americium (one component of typical radioactive fall out) if someone has broken a smoke detector in the area. You can find other stuff if your lab cleaning lady had cancer treatments for uterine cancer or thyroid cancer recently. And there's always the variable and nasty radioactive components in smoke from your local coal burning power plant, which is where most radioactive components in US air come from.
Article on Coal
http://www.ornl.gov/info/ornlreview/rev26-34/text/colmain.html
Second, although not as well known, releases from coal combustion contain naturally occurring radioactive materials--mainly, uranium and thorium.
and because quite a lot of uranium is found in coal smoke, coal burning plants are the proximate cause of plutonium in the air in the US
Another unrecognized problem is the gradual production of plutonium-239 through the exposure of uranium-238 in coal waste to neutrons from the air. These neutrons are produced primarily by bombardment of oxygen and nitrogen nuclei in the atmosphere by cosmic rays and from spontaneous fission of natural isotopes in soil.
So if you found uranium 238, or plutonium 239, you'd be really hard pressed to say where it came from, until you had a significant increase in total background radiation.
Iodine 131 is the most likely marker of a recent event, because it has a very short half life, being essentially gone from the environment within a year from its release. Its daughter isotope, xenon 131, is also a popular looked for marker. Iodine 131 is very easy to detect, because when it decays, it produces gamma rays and a beta particle. So the assumption that iodine in CA came from Daiichi might be true, but it might also be from Diablo Canyon nuclear reactors or San Onofre nuclear reactors in the LA area. But more likely, it might be from improper disposal of iodine 131 used in treatment of thyroid cancer to make sure that all thyroid tissue is dead. It is also occasional used in some medical imaging. The question again is the amount. In the case of radioactive materials, even a single atom can often be found.
This is why the most important thing to ask is, "How much?"
Lets assume you have 3x the normal amount of iodine 131 in your environment-say in a village near Daiichi or downwind from Daiichi. This would clearly be from Daiichi, since a big percentage change in your background amount has occurred. However, you would not know how dangerous that was until you knew what x was. Three times zero is still zero. Three times one atom is still just three atoms.
In the coming month(s), I can definitely predict that that single atoms, and 1 mrem doses / changes of this or that will be reported everywhere. The issue will be to put that in perspective.
Also there will be considerable "junk science" mixed in with some very good science. A good example of a very long-lived junk science is the Tooth Fairy Project.
http://www.nei.org/resourcesandstat...healtheffectsdispelstoothfairyproject/?page=6
I saw a lesser case of this in my own area in CT where a local anti-nuclear advocate compiled a list of all young children who had died in towns near Millstone Point Nuclear Plant and claimed in the local newspaper that all these children had died due to the nuclear plant. The mistake she made was that she published the names of these children, and was sued by parents, including one family where the father had backed over a child who crawled behind his car when he backed out of his driveway.
Sadly, this is all part of "advocacy science" these days. I don't think you should, as we used to say, "draw your line before you plot your points," that is, form your conclusions first, and then try to defend them by making up data, or editing points that don't agree with your preconceptions out of your data.
