Today in Sacramento, radioactive xenon-133 was detected at approximately 0.1 disintegrations per second per cubic meter of air (0.1 Bq/m3; Bq is a becquerel), the EPA says, "which results in a dose rate approximately one-millionth of the dose rate that a person normally receives from rocks, bricks, the sun and other natural background sources. This validates a similar reading of 0.1 Bq/m3, taken from March 16 through 17 in Washington State."
These types of readings remain consistent with our expectations since the onset of this tragedy, and are to be expected in the coming days.... Following the explosion of the Chernobyl plant in Ukraine in 1986 – the worst nuclear accident in world history – air monitoring in the United States also picked up trace amounts of radioactive particles, less than one thousandth of the estimated annual dose from natural sources for a typical person.I don't know about you, but I feel better about all this when they give the actual numbers instead of vague qualifiers like "low."
So how small is a becquerel? According to Wikipedia: "...natural potassium (K40) in a typical human body produces 4,000 disintegrations per second, 4 kBq of activity. The nuclear explosion in Hiroshima (14 kt or 59 TJ) is estimated to have produced 8×1024 Bq (8 YBq, 8 yottabecquerel).