|University of California at Berkeley|
Both Jonathan Diaz Leon and his team in Seattle and Kai Vetter's team in Berkeley agreed that the amount of radiation recorded was insignificant compared to everyday radiation inside our own bodies
(due mainly to Potassium 40), in the air we breathe (Radon 222) and from cosmic rays from outer space. Both teams measured the number of radioactive particles in air by passing the air through filters, removing the filters and placing them inside sensitive gamma-ray detectors. Because each radio-isotope emits gamma rays with a unique energy, the researchers were able not only to measure the quantity of radioactive fallout but its specific isotopic composition.
In addition to measuring increased radiation in the air, the Berkeley team gathered rainwater and measured its radioactivity as well.
Both teams reported their results in official physics units as Becquerels/liter. One Becquerel equals one atomic decay per second. Both teams independently detected the same pattern of radioactive isotopes in the air. For Berkeley air, the highest results were on the order of 3-4 mBq/liter. For Seattle air, the results were similar but slightly higher.
In addition to measuring the increased radioactivity of both rainwater (3-20 Bq/liter) and air (0.7 - 3.2 mBq/liter), the Berkeley team compared their results to a radiation risk that most people take for granted--air travel. Humans at sea level are shielded to a large extent from cosmic rays by the atmosphere above them but because commercial aircraft fly so high much of this atmospheric shielding is absent. Consequently the cosmic ray exposure of airline passengers and crew can be up to 100 times the exposure on the ground. Cosmic rays however are only a small part (10%) of natural radiation (see pie chart). Roughly speaking, for every three hours you spend in the air you might expect to accumulate an extra day's worth of natural radiation.
For communicating with an anxious public, the Berkeley researchers decided to report the results of their air and rainwater sampling like this:
Fukushima radiation in Berkeley air: According to our measurements, the exposure to the public is very low -- at the highest levels we measured, breathing the air for 2,000 years would increase one's radiation dose by the same amount received by taking a cross-country airplane flight.
Fukushima radiation in Berkeley water: The calculated exposure to the public is so low that the consuming of ~500 liters of this water would only increase dose by the same amount received by taking a cross-country airplane flight.
A good source of information about natural radioactivity (including information about the activity (in Becquerels) of such unlikely radioactive objects as bananas, Brazil nuts and lima beans) may be found here. Another good source for natural radiation (from which the pie chart was taken) and a wonderful photograph of M. Becquerel, the discoverer of radioactivity, can be found here. A remarkable chart prepared by Randall Monroe of XKCD comic fame that provides a fresh perspective on radiation dosages is located here.