Levels "measurable but minuscule" is a complete contradiction of terms as there is no "safe" level of Plutonium - just 1 isotope can kill you.
So what I'd like someone to explain is where exactly all those radioactive isotopes of cesium and plutonium are going to go? They just magically disappear when they reach the US? Somehow they just evaporate over the ocean? The half-life of Cesium is about 30-years and Plutonium 24,000 years.
While they may so "dispersed" in the air as to not set off any warnings on all the geiger counters everywhere - the overall levels are still increased proving the fallout is present and going to land somewhere. And no amount of "monitoring" is going to tell you if you just inhaled that 1 Plutonium isotope that is going to cause lung cancer. Or that 1 stray isotope on that head of lettuce you just ate for lunch or glass of water you just drank. Now really just a game of Russian roulette every time you eat, drink, breathe or otherwise come in contact with this fallout - good luck! No wonder cancer just surpassed heart attacks for the first time as the world's leading cause of death.
And they are still monitoring the fallout present from Chernobyl all across eastern Europe to this day 25-years later - now its America's turn being directly downwind from Japan...
Chernobyl's Lessons for Japan
http://www.reliefweb.int/rw/rwb.nsf/db900sid/ADGO-8EZMKL?OpenDocument